Electronic thermometer
United States Patent Georgi [451 Apr. 15, 1975 ELECTRONIC THERMOMETER Heinz W. Georgi, La Jolla, Calif.  
 Assignee: Ivac Corporation, San Diego, Calif.  
 Filed: Sept. 8, 1972 Appl. No.: 287,341  
 Related U.S. Application Data Inventor:  
 Primary ExaminerRichard C. Queisser Assistant Examiner-Denis E. Corr Attorney, Agent, or FirmFulwider, Patton, Rieber, Lee &amp; Utecht [57] ABSTRACT An electronic method and apparatus for measuring temperatures by means of a thermistor in one arm of an electrical bridge, bridge output being varied by the on and off duty cycle of a shunting impedance selectively switched into and out of the balancing arm of the bridge in accordance with the relative states of a counting display register and a cyclically scanned counting register, the display register state being altered by gated pulses, under the control of bridge output, to provide a digital indication of measured temperature. Bridges of both the nulling and non-nulling types are disclosed using either a dual ramp integrator or conventional detector acting upon bridge output to provide an impedance measurement subsystem independent of reference supply voltage and also enabling non-linear analog to digital conversion to compensate for a nonlinear thermistor temperature vs. thermistor resistance characteristics, without sacrificing measurement sensitivity. Anticipation circuitry is provided for monitoring and correlating the display register pulses with the time vs. temperature response characteristic of the thermistor, to selectively alter bridge balance and display register state so as to provide an advance indication of the anticipated temperature at which the thermistor will finally stabilize. Either the particular display register pulse rates searched for, or the magnitude of the anticipation correction introduced, is varied as a function of the elapsed monitoring period from a prescribed reference point in a given measurement cycle. Additionally, a safeguard subsystem requiring a prescribed pulse period sequence for proper completion of the measurement cycle minimizes false readings which might otherwise be provided by improper measurement technique of operating personnel.  
 62 Claims, 17 Drawing Figures 22 MtJN/TMED 21 r SIGNAL Alida/Fawn) iEIIESEA/T/NG [J3 SUESVSTEM rsnpiurues r- 1 I f 2; 24 1 22 1 MW/ T0! Ma/v/ me I R475 17; 514x550 rm: elm/6! WMEASt/FEMiA/l j 575N114 c ya 5 .5! I I l 6 2; i 2 2i l I I oar/ ar l 1 Mill/IEMEA/T Y a; couscr/o/v I I l 27 l 1 l |VJE  51/500420 f summers/w PATENTEEAPR 1 SENS SHEET 2 or kmwilll sum u or 9 PATENTEDAPR 1 5197s Nu K E BRQQ PATENTEUAPR 1 55975 saw 9 or 9 ELECTRONIC THERMOMETER CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of my copending application Ser. No. 45,990, filed June 15, 1970, now US. Pat. No. 3,702,076 entitled Electronic Thermometer. The latter parent application is assigned to the same assignee as the present application. All of the disclosure in the parent application Ser. No. 45,990 is specifically incorporated by reference in this application.  
 BACKGROUND OF THE INVENTION This invention relates generally to improvements in temperature measurement methods and apparatus and, more particularly, to a new and improved electronic thermometer system enabling very rapid, accurate, reliable and easily read temperature measurements.  
  lt is common practice in the medical arts, as in hospitals and in doctors offices, to measure the body temperature of a patient by means ofa glass bulb thermometer incorporating a heat responsive mercury columm which expands and contracts a calibrated temperature scale. Typically, the glass thermometer is inserted into the patient, either orally or rectally, and subsequently removed after a sufficient time interval has passed to enable the temperature of the thermometer to stabilize at the body temperature of the patient. This time interval is usually of the order of two to four minutes. After a sufficient period of time has passed, the thermometer is removed from the patient and is subsequently read by appropriate medical personnel.  
  It will be apparent from the foregoing that conventional temperature measurement procedures using glass bulb thermometers and the like are prone to a number of significant deficiencies. Temperature measurement is rather slow and, for patients who cannot be relied upon (by virtue of age or infirmity) to properly retain the thermometer for the necessary period of insertion in the body, may necessitate the physical presence of medical personnel during a relatively long measurement cycle, thus wasting valuable time. Furthermore. glass bulb thermometers are not as quick and easy to read and. hence, measurements are prone to human error, particularly when made under poor lighting conditions or read by harried personnel.  
  Various attempts have been made by the prior art to minimize or eliminate the aforedescribed deficiencies of the glass bulb thermometer by using appropriate temperature sensing probes which are designed to operate in conjunction with direct reading electrical thermometer instrumentation, typically employing an output galvanometer having an indicator needle moving along a calibrated scale. However, such probes and electrical thermometers have typically proven to be just as slow in making temperature measurements as glass bulb thermometers and, at best, output measurements have been only slightly easier to read.  
  Hence, those concerned with the development and use of thermometer apparatus in the medical field have long recognized the need for improved temperature measuring devices which result in accurate, reliable, more rapidly obtained and easily read measurements. An electronic thermometer satisfying all of these requirements is disclosed in the aforementioned copending application Ser. No. 45.990. This electronic thermometer provides a temperature measurement output as a direct digital display and further employs a novel anticipation technique to provide an advance indication of the temperature at which a thermistor probe will finally stabilize. The anticipation technique used causes the state of the digital measuring means to be altered during a measurement cycle by a single value representing a fixed temperature differential, the magnitude of the correction value and the duration of the temperature measurement cycle being correlated as a function of the rate of change of the temperature being measured.  
  Basically, the electronic thermometer described in parent application Ser. No. 45,990 includes a temperature-responsive transducer in one arm of an electrical bridge network, the bridge including a balancing arm having a variable impedance, the impedance being selectively varied under the control of a digital counter indicating temperature, the counter being continually counted up by electrical impulses so long as the transducer temperature exceeds the temperature represented by the bridge balance impedance. The time period between successive impulses to the counter, i.e., the pulse period of pulse rate, is correlated with the time vs. temperature characteristic of the temperature responsive transducer to selectively alter bridge balance, and, hence, the state of the counter, so that the counter will rapidly count up to the anticipated temperature at which the transducer will finally stabilize. The latter is accomplished substantially sooner than actual stabilization of the transducer at its final temperature. The final temperature registered in the counter is appropriately indicated by a digital display unit connected to receive the counter output, the digital display providing an easily read output indication of temperature.  
  An electronic thermometer embodying the various features of the invention set forth in patent application Ser. No. 45,990 may include a thermistor as a temperature-responsive transducer in the measurement arm of a Wheatstone bridge, the balancing arm of the bridge having a bank of parallel resistances, each resistance being selectively inserted into the bridge balancing arm under the control of its own switch, all of these switches being in turn controlled by various counting states registered in the digital counter indicating temperature, the counter being counted up by clock pulses which are gated on only when the thermistor temperature exceeds the equivalent temperature represented by the resistance in the bridge balancing arm. Since the thermistor approaches its final stable temperature asymptotically, the last increments of temperature change occur very slowly, whereas the major portion of the temperature change, in stabilizing the thermistor at the temperature of the environment, occurs relatively rapidly. In this regard, the time period between clock pulses gated to the counter is correlated with the rate of change of the thermistor temperature to anticipate the remaining temperature differential between the actual thermistor temperature and the final thermistor temperature, and to alter the balancing arm of the bridge accordingly so that the counter registers the an ticipated final temperature long before the thermistor would normally actually stabilize at such a final temperature. This results in a much more rapid, yet accurate and reliable temperature measurement.  
  Correlation of the time period between clock pulses, or pulse rate, passing through the counter, with the temperature vs. time characteristic of the thermistor, and altering the state of balance of the bridge, may be accomplished in any of several ways. In one embodiment of the electronic thermometer set forth in application Ser. No. 45,990, an additional resistance shunts the bridge balancing arm so that the balancing arm and the counter are offset by the equivalent of a predetermined temperature differential, i.e., the counter is driven to a higher counter state than would ordinarily be dictated by the actual thermistor temperature, in order to compensate for the additional resistance shunting the balancing arm. Hence, the temperature indicated by the counter display leads the actual temperature of the thermistor by the predetermined temperature differential. It is readily ascertained empirically, for any given thermistor probe, how the rate of change of temperature varies with time, and the latter is correlated with the time period between pulses passed to the counter to determine when the actual thermistor temperature differs from its final stable temperature by the aforedescribed predetermined temperature differential between the counter state and the temperature represented by the bridge balancing arm resistance. In this connection, the pulse period for pulses incrementing the counter is monitored and, when the pulse period reaches the proper magnitude, the pulses to the counter are gated off to freeze the counter and its associated display at an indication representing the anticipated final temperature of the thermistor.  
  In another embodiment described in application Ser. No. 45,990, a specified time of measurement is selected, e.g., fifteen seconds. At that point, voltage which is a function of the remaining temperature differential between the actual thermistor temperature and its ancitipated final temperature is inserted into the bridge balancing arm to deliberately unbalance the bridge and to force the counter to rapidly count up to V the state representing the final anticipated temperature.  
  The temperature anticipation method described in connection with the aforementioned electronic thermometer system applies a fixed correction to the temperature measured by the thermistor probe, under the assumption that the heating of the probe occurs with essentially the same temperature vs. time characteristic each and every time a temperature is taken. However, variations in personnel measurement techniques, probe time constants. and even the variations in thermal response characteristic of biological tissue from one patient to another may cause variations in the final temperature vs. time function which affect the accuracy of the temperature readings obtained. In this regard, the use of a fixed correction is intended for an idealized case where substantially optimum technique is employed in taking temperatures.  
  For example, the optimum measurement technique may call for insertion of the thermistor probe into the patients mouth and maintaining the probe tip in constant Contact with the tissue under the tongue while sliding the probe tip along the tongue for five or six seconds so that the probe tip is continually exposed to fresh tissue during this time interval. Otherwise, a draw down phenomenon may occur wherein the probe tip cools down the tissue excessively so that the time constant for arriving at the final temperature measurement is different from the expected time constant for the idealized case. In addition, since counter pulse rate is monitored to either vary the magnitude of the correction or determine when the measurement cycle should be terminated, it will be apparent that loss of probe contact with the tissue early in the temperature measurement cycle may result in an unduly long pulse period, which causes the anticipation circuitry to believe it has reached the searched for portion of the time vs. temperature characteristic and, therefore, prematurely terminate the measurement cycle. In this case, a low temperature reading would result.  
  The aforedescribed requisites for optimum technique cannot reliably be obtained by untrained personnel, with a consequent requirement for relatively timeconsuming and costly training effort necessary to insure proper usage by appropriate medical personnel.  
  Accordingly, those concerned with the development and use of temperature measurement methods and apparatus have recognized the desirability for further improvement in temperature measurement systems enabling enhanced accuracy, with even greater reliability,  
 and with less dependence upon the use of optimum technique by personnel making such measurements. In addition, there has been a desire for improved electronic means for implementing such temperature measurement systems, characterized by greater accuracy, reliability, economy, simplicity, enhanced linearity of response, stability and suitability for implementation by modern electronic manufacturing methods, such as MOS (metal oxide semiconductor) technology. The present invention fulfills all of these needs.  
 SUMMARY OF THE INVENTION Briefly, and in general terms, the present invention provides a new and improved electronic thermometer enabling a temperature measurement output as a direct digital display and, further, capable of providing, substantially independent of operating personnel technique, an accurate and reliable final temperature measurement output indication prior to actual stabilization of the thermometer input at the anticipated final temperature indicated. In addition, the present invention provides improvements in maximum reading analog-todigital converters, utilizing either nulling or non-nulling bridges, and further provides means for compensating for nonlinear termistor response to provide an output which is a linear function of temperature even though the resistance of the thermistor is a non-linear function of temperature.  
  Basically, the present invention includes an improved electronic method and apparatus for measuring temperatures by means of a temperature responsive transducer in an electronic network having an output which controls the state of a digital counter indicating temperature, the counter being continually counted up by electrical impulses so long as the transducer temperature exceeds the temperature represented by a prescribed state of the transducer network, the latter state being under the control of the counter so that the temperature indicated by the counter leads the transducer temperature by a predetermined temperature differential. The rate at which the temperature being measured is changing is directly related to the pulse period or time between successive impulses to the counter and is correlated with the time vs. temperature characteristic of the temperature responsive transducer to control the state of the counter so that the counter will rapidly count up to provide an advance indication of the anticipated temperature at which the transducer will finally stabilize. The final temperature registered on the counter is appropriately indicated by a digital display unit connected to receive the counter output, the digital display providing an easily read output indication of temperature.  
  In accordance with the invention, a variable correction subsystem and a pulse period sequence safeguard subsystem are utilized to make temperature measurements less dependent on operator measurement techniques, even by untrained personnel. In this connection, the pulses driving the display counter are monitored, and the particular pulse rate searched for is selectively altered during a measurement cycle as a function of the elapsed monitoring period for the measurement cycle. Alternatively, if the pulse rate searched for is held constant, then the magnitude of the correction increment added to the display counter by the anticipation network is a function not only of rate of change of temperature but also of elapsed monitoring period for the measurement cycle.  
  In addition, in order to compensate for possible loss of tissue contact during a measurement cycle, the temperature measurement system will not certify the final output indication unless the indication is made after certain pulse sequence conditions have been met. In this regard, a plurality of successively increasing pulse periods, in excess ofa prescribed pulse interval, are required before the measurement cycle is terminated. This prevents any error due to loss of tissue contact where the period of loss contact exceeds the termination pulse period interval searched for and would thus cause a corresponding false termination of the measurement cycle. Hence. the safeguard network is a resetting subsystem requiring a prescribed sequence of pulse periods for proper completion of the measurement cycle and thereby minimizes false readings which might otherwise be provided by poor measurement technique of operating personnel.  
  By way of example, the safeguard subsystem. in accordance with the invention, searches for two time periods in sequence, a first pulse period somewhat shorter than the pulse period normally indicating termination of the measurement cycle, followed by a second pulse period at least equal to the termination determining pulse period duration. The search for the second pulse period does not begin until the first pulse period has occurred, i.e., occurrence of the first prescribed pulse period is a condition precedent to the search for the second pulse period. If a pulse period shorter than the first pulse period occurs after the first pulse period has been detected, the safeguard subsystem is reset and again searches for the first pulse period rather than the second pulse period, thereby again requiring a double pulse period sequence of prescribed magnitudes as though the first pulse period had never been detected at all during the measurement cycle.  
  In addition, since the magnitude of the anticipation correction is a function not only of rate of change of measured temperature, but also of elapsed time from a prescribed reference point in a given measurement cycle, the magnitude of the particular pulse periods searched for is increased with increasing elapsed time. For example, the initial pair of pulse periods searched for might be 1.0 seconds followed by 1.3 seconds. If it takes longer than 13 seconds, but less than 23 seconds to complete the measurement cycle, the pulse period sequence searched for is increased to 1.5 seconds and 2.0 seconds, respectively. If it takes longer than 23 seconds to make a temperature measurement then the pulse period sequence searched for by the safeguard subsystem is 2.0 seconds and 2.6 seconds.  
  The first and second pulse periods are selected sufficiently close to each other in magnitude so that measurement errors are minimal in the event of the very remote possibility that tissue contact is lost as a point in time immediately after the first pulse period requirement has been met.  
  The present invention also includes a bridge network for lineralizing the response characteristics of the temperature responsive transducer, without reducing sensitivity, by proper selection of the magnitudes and ratio of magnitudes of the bridge impedances, in a nonlinear, maximum reading analog-to-digital conversion system. Moreover, the transducer input network, including the bridge and bridge detector, essentially provides an impedance measurement subsystem independent of reference supply voltage. The bridge may be of the nulling or non-nulling types, using either a dual ramp integrator or conventional detector, depending upon the speed of response desired in correlating transducer input changes with indicated output.  
  More specifically, and in a presently preferred embodiment, by way of example and not necessarily by way of limitation, an electronic thermometer embodying the various features of the invention may include a thermistor as the temperature responsive transducer in the measurement arm ofa bridge network of essentially the Wheatstone type, the balancing arm of the bridge including a shunt resistance selectively switched into and out of the bridge so that the bridge output is varied by the on and off duty cycle of the latter resistance, the resistance being switched into and out of the balancing arm of the bridge in accordance with the relative states of a constantly compared counting display register and a cyclically scanned counting register, the display register being counted up by pulses which are gated on only when the thermistor temperature exceeds the equivalent temperature represented by the average resistance in the bridge balancing arm, the latter resistance average being varied by the on and off duty cycle of the balance arm shunt resistance.  
  Since the thermistor approaches its final stable temperature asymptotically, it will be apparent that the last increments of temperature change occur very slowly, whereas the major portion of the temperature change in stabilizing the thermistor at the temperature of the environment, occurs relatively rapidly. Hence, the time period between the pulses gated to the display register is correlated with the rate of change of the thermistor temperature to anticipate the remaining temperature differential between the actual thermistor temperature and the final thermistor temperature, and to alter the balancing arm of the bridge network accordingly so that the display register indicates the anticipated final temperature long before the thermistor would normally actually stabilize at its final temperature.  
  This alteration of bridge balance is accomplished by additional anticipation correction resistance included in the bridge balance arm so that the balancing arm and the display register are normally out of phase by the equivalent of a predetermined temperature differential, i.e., the display register is driven to a higher counter state than would ordinarily be dictated by the actual thermistor temperature, in order to compensate for the additional resistance of the balancing arm. Hence, the  
 . temperature indicated by the display register leads the actual temperature of the thermistor by the predetermined temperature differential or anticipation correction.  
  The time intervals between pulses passed to the display register are monitored and, when the time intervals reach the proper magnitudes, further pulses to the display register are gated off to freeze the register and its associated display at an indication representing the anticipated final temperature of the thermistor.  
  Alternatively, the relationship between the compared states of the display register and the scanning register can be altered. thereby altering the on and off duty cycle of the shunting resistance and the balancing arm of the bridge, to provide the desired anticipation correction.  
  If desired, an ordinary null detector may be used with the bridge so that out of null conditions in a prescribed direction can be sensed and used to control the gating of pulses which drive the display register. In a presently preferred embodiment, a non-nulling bridge is utilizied with bridge output directed to a dual ramp integrator which evaluates the differences between the thermistor temperature and the count in the display register on a cycle by cycle basis. each cycle referring to a complete counter scanning cycle of the continuously running scanning register. This arrangement also provides electrical output essentially independent of the reference voltage supply.  
  The states of the display register and the cyclical scanning register are constantly compared, and the on and off duty cycle of the shunting resistance in the balancing arm of the bridge network is varied in accordance with the relative states of the two registers. For  
 example, the latter resistance is switched into the bridge between the time that the scanning register is at some selected reference state, such as zero, and the time that the scanning register is equal to the display register. When the count in the scanning register exceeds the count in the display register, the shunt resistance is switched out of the bridge network until the scanning register again arrives at the prescribed zero or other reference state.  
  An interval timer, which is reset by each pulse driving the display register. measures the intervals between pulses and, through appropriate decoding, indicates when the pulses periods required by the safeguard subsystem have been achieved, so that an appropriate control subsystem can appraise the pulse sequence and determine whether or not the measurement cycle should be terminated.  
  An elapsed timer, incremented every time the scanning register completes a scanning cycle, keeps track of the elapsed monitoring period from a prescribed reference point in a given measurement cycle. For example, the initial reset state of the thermometer system may be an output indication of 94F, and, hence, the reference point at which the elapsed timer would begin to keep track of the monitoring period in any given measurement cycle would be the first display register counting pulse indicating a temperature in excess of 94F, e.g., 94.lF. The state of the elapsed timer is decoded and either modifies the counter modulus of the interval timer or alters the decoding for the interval timer so that, when the elaspsed timer reaches certain prescribed states, such as l3 seconds and 23 seconds, the pulse periods sequences provided by the output of the interval timer decoding to the control subsystem are correspondingly altered. In this way, the anticipation correction is a function not only of rate of change but also of elapsed time. Of course, rather than changing the magnitudes of the pulse period sequences, the magnitudes of the aniticipation correction resistance in the bridge network can be altered instead, to provide the compensating correction for elapsed time.  
  In accordance with the invention, means are also taught for adapting digital electronic thermometers, such as that set forth in copending patent application Ser. No. 45,990, so that they may embody the advantageous features of the pulse sequence safeguard subsystem and the variable anticipation correction as a function of elapsed time.  
  Hence, the electronic method and apparatus of the present invention supplements the improvements of the measurement systems set forth in copending patent application Ser. No. 45,990 in satisfying a long existing nedd in the medical arts for. a thermometer capable of making accurate, reliable and easily read temperature measurements much more rapidly than has heretofore been feasible with the thermometers of the prior art and, further, in a manner which is essentially independent of the measurement techniques employed by various operating personnel.  
  The above and other objects and advantages of this invention will be apparent from the following more detailed description when taken in conjunction with the accompanying drawings illustrative embodiments.  
 DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an overall system in which some of the basic concepts of the temperature measurement method and apparatus of the present invention are embodied;  
  FIG. 2 is a combined block diagram and electrical schematic of one embodiment of an electronic thermometer in accordance with the present invention, with particular emphasis upon the analog to digital conversion system;  
  FIG. 3 is a block diagram of the anticipation subsystem, with emphasis of the pulse interval timing and elapsed monitoring period timing, suitable for use with the system of FIG. 2;  
  FIG. 4 is a block diagram of a control subsystem suitable for use with the system of FIGS. 2 and 3;  
  FIGS. 5a, 5b and 5c are wave forms applicable to various portions of the electrical circuitry illustrated in FIG. 2, FIG. 5a being directed to electrical output ramps of the dual ramp integrator, while FIGS. 5b and 5c show that state of a switch controlling the duty cycle of a resistance shunting the balancing arm of a bridge under various ramp output conditions;  
  FIG. 6 is a flow chart for an algorithm describing the handling of data in practicing the invention with the electronic thermometer system set forth in FIGS. 2, 3 and 4;  
  FIG. 7 is a flow chart illustrating an algorithm for varying the magnitudes of the pulse intervals in the pulse period sequence of the safeguard subsystem;  
  FIG. 8 is a combined block diagram and electrical schematic of a portion of the electronic thermometer set forth in FIG. 2, but modified to introduce anticipation correction increments of different magnitudes, rather than variations in the magnitude of the pulse intervals searched for;  
  FIG. 9 is a flow chart illustrating an algorithm for handling temperature measurement data in accordance with the embodiment of the thermometer illustrated in FIG. 8;  
  FIG. 10 is a block diagram of an adapter subsystem for modifying digital thermometers to incorporate the pulse period sequence safeguard feature and variable correction with elapsed monitoring period features of the present invention;  
  FIGS. 11a through lld are wave forms applicable to various portions of the electrical subsystem illustrated in FIG. 10; and  
  FIG. 12 is a combined block diagram and electrical schematic for another embodiment of an electronic thermometer in accordance with the present invention and particularly suited. for use with the adapter subsystem in FIG. 10.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 off the drawings, there is shown a new and improved system for temperature measurement embodying features of the present invention.  
  An appropriate signal source provides a signal, representing the temperature being monitored, over line 21 to a suitable measurement subsystem 22 and simultaneously directs the signal toa subsystem 23 for monitoring the rate of change of the signal. The source of the signal may be a thermistor or other temperature responsive transducer and may include appropriate networks for shaping the response characteristic and converting the signal from analog to digital form. While the signal may remain in analog form for the practice of some features of the invention, the presently preferred embodiments of the invention contemplate use of signals in primarily digital form.  
  The measurement subsystem 22 amy include a counter, a plurality of counters operating a concert, or any other measurement network capable of suitably responding to the monitored signal representing temperature.  
  The measurement subsystem 22 provides an output over line 24 indicating a starting reference point for the monitoring period in the temperature measurement cycle. By way of example, in preferred embodiments of the invention, this temperature is selected as 94F and, accordingly, output is provided over line 24 only when the signal over line 21 to the measurement subsystem 22 indicates that the temperature has exceeded the referenced 94F. The latter event activates a subsystem 25 for monitoring elapsed time in the measurement cycle.  
  The figure of 94F as a starting reference point in a measurement cycle is not critical, but is used by way of practical example in the ensuing discussion. It will be apparent that other initial temperature settings may be selected, and that the temperature may be expressed in any convenient scale of temperature units, such as degrees Centrigrade instead of degrees Fahrenheit.  
  The rate of change monitoring subsystem 23 provides .1 conditioning output over line 26 to an output measurement correction subsystem 27 which is also responsive to the state of the elapsed time monitoring subsystem 25, and indicated schematically by the line 28, to provide an anticipation correction increment over line 29 to the measurement subsystem 22 so that the measurement subsystem will provide an advance indication of the anticipated temperature at which the temperature responsive transducer will finally stabilize. The output of the measurement subsystem 22 is appropriately directed over line 30 to any suitable output display subsystem 31, preferably in an easily read digital format. Hence, the output measurement correction subsystem 27 is conditioned as a function of the rate of change of the monitored signal and also a function of elapsed time from the occurrence of a starting reference level in any given measurement cycle.  
  Instead of effecting the output measurement correction subsystem 27 directly, the elapsed time measurement subsystem 25 may act upon the rate of change monitoring subsystem 23, as indicated by the dashed line 32, so that the particular rate of change searched for is altered as a function of elapsed time, and the latter rate of change is, in turn, used to condition the time of insertion of the output measurement correction into the measurement subsystem 22 or alternatively, if the correction is already inserted into the measurement subsystem 22, the time when the measurement cycle is terminated. In this latter regard, the rate of change monitoring subsystem may direct an input over dashed line 33 to the measurement subsystem 22.  
  While the output measurement correction subsystem 27 is shown in FIG. I as directing an input over line 29 to the measurement subsystem 22, it will be appreciated that the correction subsystem can, instead. direct its output to the signal source subsystem 20, and this alternative approach is indicated schematically by the dashed line 38 in FIG. 1. The only difference between the two approahes shown is the point of insertion of the anticipation correction in the overall system.  
  A safeguard subsystem 37 conditions the measurement subsystem 22 over line 34 and prevent false termination of the measurement cycle which might otherwise occur in the event of a detection of a rate of change calling for termination and induced not by the normal temperature vs. time characteristic of the temperature transcuder but instead by a measure cycle interruption. Such an interruption might occur, for example, if a thermometer probe loses contact with the tissue of the patient whose temperature is being taken. When contact is re-established with the tissue, the measurement cycle will normally resume, unless the hiatus in the measurement cycle is sufficient to cause the rate of change subsystem 23 and measurement subsystem 22 to believe that the proper termination point in the measurement cycle has been reached. If this occurred, the output display 31 would indicate a final out put represented by an erroneously low temperature reading. In order to prevent this type of error, the safe guard subsystem receives an input over line 35 from the rate of change subsystem 23 and prevents termination of the measurement cycle by the measurement subsystem 22 until certain prescribed conditions regarding successive signal rate of change samplings have first been met. In digital form, the technique employed by the safeguard subsystem 37 requires a predetermined pulse period sequence to occur before enabling the measurement subsystem 22 to terminate the measurement cycle.  
  In a presently preferred embodiment, the safeguard subsystem 37 requires a pair of sequential pulse periods (or pulse rates) detected by the rate of change subsystem 23, wherein the second pulse period is of a magnitude which would normally call for termination of the measurement cycle, and the first pulse period is of a magnitude somewhat shorter than the termination pulse period. For example, if the termination pulse period is 1.3 seconds, the requirement for the first pulse period might be an interval of 1.0 seconds or more. A time interval between signal pulses equal to or greater than the first pulse period searched for is required before any search for the second pulse period is even initiated. If a, pulse interval shorter than the required first pulse period occurs after the first pulse period requirement has been met, as would occur in the case of loss of tissue contact for some time followed by reacquisition of tissue contact, the safeguard subsystem 37 will be reset and again require a double pulse period sequence, as though the first pulse period requirement had never been met during the measurement cycle.  
  The first and second pulse periods are selected sufficiently close to each other in magitude so that measurement errors are minimal in the event of the very remote possibility that tissue contact is lost at a point in time immediately after the first pulse period requirement has been met.  
  Since the pulse p&#39;eriod sequence required by the safeguard subsystem 37 will vary as a function of elapsed time in those instances where the output measurement correction is a single fixed increment, an input is shown to the safeguard subsystem 37 over dashed line 36 from the elapsed time measurement subsystem 25.  
  Hence, it will be apparent that the temperature measurement method and apparatus schematically depicted by the overall system embodiment of FIG. 1 provides an advance indication of final temperature by introducing an output measurement correction whose magni tude, or time of introduction, or whose magnitude coupled with the selected time of termination of the measurement cycle, is a function both of the rate of change of the parameter being monitored and the elapsed time from a prescribed reference point in the measurement cycle. The desired anticipatory output indication may be provided by varying the actual magnitude of the correction introduced, or altering the particular signal rate of change searched for in determining when the measurement cycle should be terminated, or a combination of both expedients may be employed if desired. In addition, premature false termination is avoided by the requirement ofa required rate of change sequence before detected rate of change calling for termination will be recognized as such.  
  While the various monitoring, measurement, correction and safeguard subsystems are shown as separate entities in FIG. 1, for purposes ofillustrating their functional inter-relationship in acting upon measurement data, the various subsystems may be combined wherever feasible or desired for purposes of specific system implementation.  
  By way of clarification with regard to the various digital electronic systems described herein, all of the apparatus systems disclosed in FIGS. 2 through 4, for purposes of setting forth illustrative embodiments, are synchronous systems operating on a conventional clock system with clock inputs to all of the flip-flops, registers and counters, with all gates being set on the previous clock pulse, while flip-flops are normally set on the next clock pulse. For purposes of simplicity, the conventional clock inputs have been omitted from the drawings. It will also be appreciated that, while conventional synchronous logic is employed in the illustrated embodiments, those of ordinary skill in the art can readily provide equivalent logic in asynchronous form without departing from the invention.  
  Referring now to FIG. 2 of the drawings, there is shown a generalized system for one embodiment of an electronic thermometer incorporating the novel features of the present invention. The thermometer of FIG. 2 includes a basic anticipatory measurement system incorporating an improved maximum reading analog to digital converter and bridge network, including a dual ramp integrator as the bridge network detector, for a rapid cycle by cycle analysis of the indicated output temperature of the thermometer relative to the temperature being measured.  
 The thermometer system shown in FIG. 2 is also ca- ,pable of being operated in either an extended mode,  
 wherein the indicated temperature is the same as the temperature of the thermistor, or in a rapid anticipation mode wherein the measured temperature indicated is an,extrapolated higher termperature rather than the actual thermistor temperature.  
  The system of FIG. 2 includes a first counting register 40, hereinafter referred to as the display register, and a second counting register 41, hereinafter referred to as the scanning register.  
  The scanning register 41 is continuously counted on a cyclical basis by a high frequency clock 42 over line 43, typical clock frequency being 20,000 Hz. The display register 40 is counted up by appropriate incrementing pulses received over line 44 at a frequency no greater than the complete cycling frequency of the scanning register 41, typically Hz.  
  The output of the display register 40 is appropriately directed to a display decoding and multiplexing subsystem 45 of conventional design, the output of the latter subsystem being, in turn, fed to an appropriate output digital display unit 46 so that measured temperatures are displayed directly in easily read form.  
  Basically, the display register 40 and scanning register 41 cooperate with each other to modulate the state of balance of a bridge network 47, including a thermistor 48, to selectively increment the display register so that it counts up as the thermistor temperature increases.  
  The display register 40 and scanning register 41 may be any digital counters known in the art. For convenience, however, in a presently preferred embodiment of the invention, each of these registers 40 and 41 comprise a pair of decades and a divide by two flip-flop to provide each register with a 200 count capacity. The reason for this is the desire to cover the temperature range between 94F and 108F in 0. 1 intervals, requiring a total of counts. Hence, a 200 count binary coded decimal register, using two decades and a single flip-flop, is a very simple, easy and economical digital counting means for providing the desired count capacity of 140 steps between 94 F and 108 F. However, any other counters, such as conventional binary counters, with the desired count capacity may be used.  
  The bridge network 47 includes a bridge having resistances R R R and R electrically connected in a dc. Wheatstone bridge configuration, with the electrical output of the bridge available at terminals 49, 50. Resistance R and R are reference resistances, while resistance R is the resistance of a thermistor whose impedance varies with temperature. By way of example, the thermistor 48 is presumed to have a negative temperature coefficient, so that its resistance R decreases with increasing thermistor temperature. However, it is to be understood that the same system may be employed for positive temperature coefficient transducers, wherein resistance increases with increasing temperature, the only difference being in whether resistance must be added or removed to restore balance in the bridge with changing temperature.  
  R is the basic resistance in the balancing arm of the bridge, the total resistance in the bridge balancing arm being cyclically varied by varying the on and off duty cycle of a shunt resistance R which is electrically connected in parallel with the resistance R whenever a switch 51 is closed. The switch 51 is typically a solid state switch in the form of a bipolar or field effect transistor which is turned on or off by the presence or absence of an enabling input over line 52 from a switch flip-flop 53.  
  The switch flip-flop 53, switch 51 and, hence, the duty cycle of the resistance R is controlled by the constantly compared states of the display register 40 and scanning register 41 to modulate the state of balance of the bridge network 47 by varying the period of time that the resistance R is connected into the balance arm of the bridge. In this regard, the ratio of closing to open time for the switch 51 and, hence, the ratio of time in to time out for the resistance R is representative of the count in the display register 40. Hence, the net resistance in the bridge balancing arm is selectively varied by the state of the display register 40, in a manner to be hereinafter described. so as to reduce the electrical output of the bridge network and thereby correlate the count in the display register with the resistance R representing the temperature being measured.  
  In a presently preferred embodiment of the invention, the resistance R is made of higher resistance magnitude than that normally required to balance the bridge, so that the balancing arm of the bridge and the display register 40 are out of phase by the equivalent of a predetermined temperature differential, i.e., the display register is driven to a higher counter state than would ordinarily be dictated by the actual thermistor temperature, in order to compensate for the additional resistance in the balancing arm. Hence, the temperature stored in the display register 40 and indicated by the digital display 46 leads the actual temperature of the thermistor 48 by the predetermined temperature differential. It can be readily ascertained empirically, for any given thermistor probe, how the rate of change of temperature varies with time, and the latter is correlated with the time period between incrementing pulses passed over line 44 to the display register 40 to determine when the actual thermistor temperature differs from its final stable temperature by the aforedescribed predetermined temperature differential. In this connection, the time interval between incrementing pulses passed to the display register 40 is monitored over line 52 by a suitable anticipation subsystem 53 and, when the pulse periods reach the proper magnitudes, a suitable control subsystem 54 is informed by the anticipation subsystem over line 55, the latter control subsystem then gating off further counting pulses and freezing the display register 40 and its associated display 46 at an indication representing the anticipated final temperature of the thermistor 48.  
  As previously indicated, the on-off duty cycle of the resistance R is controlled by the switch 51 which, in turn, is under the control of the switch flip-flop 53. The switch 51 is closed, to insert the shunt resistance R into the balancing arm of the bridge whenever the flipflop 53 is in its 1 state, and the switch is opened to disconnect the resistance R from the bridge whenever the flip-flop is in its 0 state. The state of the switch flip-flop 53 is, in turn, controlled by the states of the display register 40 and the scanning register 41. In this regard, the state of the scanning register 41 is decoded by a decoder 56 which decodes out a preselected reference state of the scanning register for control purposes. Normally, the reference state is the zero count of each cycle of the scanning register 41 and, hence. the decoder 56 is, in such instances, referred to as a zero detector.  
  The zero detector 56 provides one enabling input over line 57 to an AND gate 58. The gate 58 also receives an input over line 59 from a threshold detector 60 which evaluates the level of output, on a cycle by cycle basis, from a dual ramp integrator in the bridge network 47.  
  Assuming conditions are such that the threshold detector 60 provides an enabling input to the gate 58, then, each time the scanning register 41 counts to its zero state to begin a new scanning cycle, the output of the zero detector 56 over line 57 will generate an output over line 61 from the gate 58 to set the switch flipflopp 53 to its 1 state and thereby connect the resistance R into the bridge balancing arm.  
  A coincidence detector 62 constantly compares the states of the display register 40 and the scanning register 41 and, each time the two registers are equal, the detector 62 generates a coincidence output over line 63 to set the switch flip-flop 53 into its 0 state, and thereby disconnect the resistance R from the balancing arm of the bridge.  
  Hence, the switch flip-flop 53 gets set to its 1 state every time the scanning register reaches zero, thus gating the switch 51 on. The flip-flop 53 gets reset to its 0 state every time the scanning register has the same count as the display register, thus gating the switch 51 off. The switch 51 is thereby periodically turned on and off, once for every full count cycle (200 counts) of the scanning register 41. The on-off duty cycle of the switch 51 and, hence, the ratio of time in to time out of the resistor R,- depends on the count in the display register 40, the higher the count, the longer the on period and the shorter the off period.  
  Thus, the switch flip-flop 53, switch 51 and shunt resistance R in the balancing arm of the bridge network 47 are modulated by the relative state of the display register 40 and scanning register 41 so that the resistance R is in the bridge when the scanning register count is between zero and the display register count, the resistance being connected out of the bridge whenever the scanning register has a count higher than that of the display register. In this way, the electrical output of the bridge network 47 varies in accordance with the count in the display register 40.  
 The output of the dual ramp integrator 65 is directed over line 68 to the threshold detector 60 which monitors the state of the ramp output from the integrator and thereby selectively enables the gate 58 over line 59 for control of the switch flip-flop 53.  
  The output of the threshold detector 60 is also directed over line 70 as input to the control subsystem 54. The latter control subsystem also receives input over line 71 from the zero detector 56 and, further, receives input from the anticipation subsystem 53 over line 55. The control subsystem evaluates all of these inputs, in a manner to be subsequently described in detail, and thereby provides an output over line 72 which selectively enables an AND gate 73 to pass incrementing pulses via line 44 to the display register 40. These incrementing pulses essentially occur as a result of the output of the zero detector 56 which is also provided as input over line 74 to the AND gate 73. Each time the zero detector provides an output level when the scanning register 41 is in the 0 state, the latter decoded state, which lasts for a single clock pulse interval, is used to pulse the display register 40 through the AND gate 73. Alternatively, the output of the gate 73 can be used as an enabling input to a second AND gate (not shown) to selectively gate clock pulses into the display When the thermometer is initially turned on, a reset start subsystem 78 generates a pulse or brief duration d.c. level over line 79 which resets the display register 40 to a count of 040 representing 94F.,.resets the scanning register 41 to zero, sets the switch flip-flip 53 to the 0 state, and also directs appropriate resetting input to the anticipation subsystem 53 and control subsystem 54.  
  The thermometer system shown in FIG. 2 always starts initially in the anticipation mode of measurement and remains in the anticipation mode if the resistance R of the thermistor 48 indicates a lower thermistor temperature at the beginning of the measurement cycle than the initial setting of the net resistance in the balancing arm of the bridge network 47.  
  Since the reset subsystem 78 initially sets the display register 40 to 94F., and assuming an anticipation correction temperature differential of lF., the thermometer system will remain in the anticipation mode only if the initial temperature of the thermistor is less than 93F., as is usually the case. However, in the event the thermistor 48 is initially at a temperature in excess of 93F., at the beginning of a measurement cycle, then the control subsystem 54 closes a normally open switch 81, via line 82, to insert an additional extended mode resistance R in parallel with the resistance of the balancing arm in the bridge network 47.  
  The switch 81 is typically of the same type as the switch 51 and is referred to as an extended mode switch&#34; since the insertion of shunt resistance R into the balancing arm of the bridge reduces the total balancing arm resistance, and thereby cancels out the initial added resistance included in R as previously indicated, for anticipation purposes. Hence, the bridge network 47 and the display register 40 are no longer offset by the equivalent ofa lF. temperature differential once the resistance R has been connected into the bridge. In this regard, the count registered in the display register 40 is lF. lower than would ordinarily be dictated by the anticipation mode of operation, in order to compensate for the additional shunt resistance R in parallel with the bridge balancing arm. Hence, the temperature indicated by the counter display 46, when the thermometer is in the extended mode of operation, no longer leads the thermistor temperature by an anticipation temperature differential, but rather displays the actual temperature of the thermistor 48.  
  The manner in which the dual ramp integrator 65 and threshold detector operate to indicate the relative states of the thermistor temperature 48 and the count &#39;registered in the display register 40, to determine whether or not additional count pulses should be gated to increment the display register 40, is best understood by reference to the waveforms of FIG. 5.  
  The upper waveform in FIG. 5a represents the output voltage of the integrator across the integrating capacitor 67. The middle waveform diagram in FIG. 519 indicates the state of the switch 51 which connects the shunt resistance R into and out of the bridge balancing arm, as the switch state varies in accordance with the dashed line ramps in FIG. 5a. FIG. 50 is a waveform similar to FIG. 5b, but for the solid line ramps of FIG. 50.  
  Referring to FIG. 5, with the switch 51 open and the resistor R disconnected from the bridge, and with the thermistor 48 at a temperature in excess of 93F, the bridge network 47 is unbalanced in such a direction that the output of the operational amplifier 66 is at one defined region of saturation shown as a positive saturation level 80. The switch 51, together with the resistance R always causes the bridge to be unbalanced in either one direction or the other in a consistent manner for the normal measurement range of the system. In other words, when the switch 51 is open, the bridge is unbalanced in such a direction that the output of the operational amplifier 66 always attempts to go to positive saturation. On the other hand, when the switch 51 is closed, and the resistance R is connected into the bridge, and the output of the operational amplifier 66 seeks a negative saturation level. Hence, the switch 51 and resistance R always introduce a known imbalance into the bridge, while the thermistor 48, by virtue of its changing temperature, introduces a variable imbalance into the bridge which manifests itself as a shift in the slope of the output voltage ramps from the integrator 65.  
  Thus, the switch 51 and resistance R determine the relative durations of the negative going and positive going output voltage ramps from the integrator 65, whereas the temperature of the thermistor 48, by virtue of the variation in the resistance R varies the slope of the ramps.  
  When the switch 51 has been opened for some time, the operational amplifier 65 is at its positive saturation point. When the switch 51 is closed, at the time when the scanning register goes through zero, the bridge is unbalanced in such a way that the integrator 65 generates a negative going ramp having a slope determined by the temperature of the thermistor 48 which sets the actual input voltage on the positive input of the operational amplifier 66. The duration of the negative going ramp is determined by the presently displayed count in the display register 40, since that count determines how long the switch 51 is closed. When the scanning register 41 is counted up to the same count as the display register 40, the switch 51 is opened again, which causes the resistance R to be disconnected from the bridge, and unbalances the bridge in the opposite direction, whereby a positive going voltage ramp is generated whose slope is also dependent on the temperature of the thermistor 48.  
  Depending upon whether or not the output voltage ramp of the integrator 65 has returned to the positive saturation level 80, the next time that the scanning register goes through zero, a decision is made as to whether or not the count is high enough in the display register 40. If the ramp has returned to the positive saturation level when the scanning register is zero, the count in the display register is too low, i.e., the time period for the negative going ramp has not been long enough since, the longer the switch 51 is closed, the more negative the first ramp will go, and, of course, the longer it will take for the positive ramp to come back up to the positive saturation level. If the time period is shorter for the first ramp than it should be, so that the second positive going ramp has returned to the positive saturation level when the scanning register is zero, an appropriate signal will be generated within the control subsystem 54 over line 72 to gate 73 to enable the output pulse from the zero detector 56 to add one count into the display register 40. At the same time, since the scanning register is passing through zero, the switch 51 is again closed to generate a new ramp which will be on for one more clock pulse than the intial ramp for the previous counting cycle, and it will take one more clock count for the scanning register 41 to come up to equality with the display register 40.  
  As soon as the display register 40 has reached a count which is slightly larger than the actual temperature represented by the bridge network, the ramp output of the integrator 65 will not have returned to its positive saturation point at the time that the scanning register 41 goes through zero. This failure to return to the positive saturation point is recognized by the threshold detector 60 which advises the control subsystem 54 accordingly so that gate 73 blocks the Zero detector pulse over line 74 and prevents any additional counts from being entered into the display regsiter 40, thus holding the display register at the last temperature indicated. In addition, the failure of the ramp to return to positive saturation as the scanning register goes through zero, causes the zero detector pulse over line 57 to be blocked by the gate 58, whereby the switch flip-flop 53 is maintained in a state so that a new ramp cannot be generated until the next point in time where the scanning register goes through zero and the ramp has returned to positive saturation.  
  Referring now to FIG. a of the drawings, assume that the output of the integrator 65 starts out at its positive saturation level 80. At the time the scanning register goes through zero, the switch 51 is closed and the output voltage of the operational amplifier 66 goes negatively in a linear fashion indicated by the linear ramp 81. When the scanning register count equals the display register count, the switch 51 is opened again and the output voltage of the integrator follows the linear ramp 82 back up to the positive saturation level before the next time that the scanning register goes through zero. The return to the positive saturation level indicates that the display register count is too low, and, hence, an additional count is added to the display register. At the same time, the switch 51 is closed again to generate another negative going ramp 83, which will be longer, by one clock pulse period, than the duration of the previous ramp 81. When the scanning register 41 equals the display register 40 again, a new positive ramp 84 is generated to bring the output voltage up to the saturation level 80. The state of the switch 51 for these cycles is illustrated&#39;by the waveform of FIG. 5b.  
  Again, referring to FIG. 5a, assume that the temperature indicated by the display register 40 has been higher than the actual temperature represented by the bridge network 47. In that case, a negative going ramp 85 would be generated which will reach a more negative voltage level during the on period of the switch 51 than the ramp 81 previously discussed and, accordingly, the positive going ramp 86 generated when the switch 51 is opened, will not have reached the positive saturation level 80 the next time the scanning register goes through zero, thereby preventing the display register 40 from being incremented by the system of FIG. 2. At the same time, referring to the lower waveform of FIG. 50 for the switch 51, since the ramp 86 does not bring the output ofthe integrator 65 back up to positive saturation at the time when the scanning register goes through zero, the switch 51 is held open and is not again closed until the ramp has, in fact, returned to positive saturation. After the latter return to positive saturation, the very next time the scanning register goes through zero, a new negative ramp 87, followed by its associated positive going ramp 88 can be generated. It will be apparent from FIG. 2, that the switch 51 is held open because of the failure of the switch flip-flop 53 to be set to the 1 state, as a result of the threshold detector 60 having gated off the zero detector pulse by failing to enable AND gate 58.  
  The dual ramp integration approach to evaluation of the state of balance of a bridge enables very rapid, cycle by cycle, analysis of the relative states of the display register 40 and the temperature of the thermistor 48. In addition, since the entire evaluation by the threshold detector 60 takes place over a single cycle of the scanning register 41, or 0.01 seconds, the threshold detector need be stable for only one integration perior at a time. Hence, the detector 60 can be of relatively simple circuit design, with freedom to shift with temperature on a long term basis, without effecting the accuracy of the evaluation system.  
  Referring again to FIG. 2, it has previously been indicated that the count in the display register 40 leads the actual temperature of the thermistor 48, when the thermometer is operating in the anticipation mode, due to an increase in the magnitude of the R resistance. However, it will be appreciated that other techniques may be employed in the system of FIG. 2 to introduce the desired anticipation of temperature. For example, instead of decoding out the zero state of the scanning register 41, the switching flip-flop 53 and switch 51 can be activated at a different reference state of the scanning register. If the decoder 56 decodes out the 005 state of the scanning register 41, then at F. offset would be introduced into the thermometer system to cause the display register 40 to lead the temperature of 5 the thermistor 48 by that temperature differential. Similarly, if the decoder 56 decodes out the 010 state of the scanning register as the reference state for switching the resistance R,- into the bridge, then a l.OF. anticipation temperature differential would be introduced into the thermometer system.  
  Referring now to FIG. 3 of the drawings. the anticipation subsystem 53 includes a pair of digital timers, an elapsed timer 90 and a pulse interval timer 91. The elapsed timer 90 is a counting register which counts time elapsed during a monitoring period which starts from a predetermined reference point in each measurement cycle, e.g., when the display register 40 receives its first counting pulse to increment the display register above its starting point of 94F.  
  The electrical state of the elapsed timer 90 is directed over a plurality of lines 92 to a suitable decoder 93 having a pair of output lines 94, 95 which go true&#34; in various combinations depending upon the elapsed monitoring period registered by the time 90. For example, in a preferred embodiment, the output of the decoder 93 over both lines 94 and 95 is false whenever the elapsed time in the monitoring period is less than 13 seconds. When the elapsed time is 13 seconds or more, but less than 23 seconds, line 94 will go true, while line 95 will remain false. When the elapsed time is 23 seconds or more, both lines 94 and 95 will be true,  
  Elapsed timer 90 is clocked by the 0.01 second cycle increments of the scanning register 41 each time the scanning register goes through zero, but only if the prescribed reference point conditions have first been met, e.g., the conditions for feeding a first counting pulse to the display register 40. The latter determination is made in the control subsystem 54 and, hence, elapsed timer 90 is shown in FIG. 3 as being incremented by pulses over line 96 from gate 122 of the control subsystem. Similarly, the control subsystem 54 counts up the pulse interval timer 91 over line 97.  
  Both the elapsed timer 90 and the. pulse interval timer 91 are initially set to zero by the reset subsystem 78 over lines 98, 99, respectively.  
  The pulse interval timer is a counting register which counts the pulse interval between each pair of pulses which increment the display register 40. In this regard, the pulse interval timer 91 is reset over line 52 each time a pulse adds a count to the display register.  
  The electrical state of the pulse interval timer 91 is directed over a plurality of lines 102 to a conventional decoder 103 which decodes out a pair of pulse periods T, and T as they occur, over output lines 104, 105, respectively, which are directed to the control subsystem cycle 54 for evaluation to determine whether or not the measurement cycle should be terminated. The outputs T and T are reset on each display register incrementing pulse, just as is the pulse interval timer 91. In this regard, in accordance with a safeguard technique, a prescribed pulse period sequence, e.g., a pair of pulse periods of prescribed magnitudes is required for proper termination of a measurement cycle.  
  In accordance with the invention, a variable anticipation correction and a pulse period sequence safeguard technique are utilized to make the temperature measurement less dependent on operating techniques. In this connection, the pulses over line 52 are monitored by the interval timer 91 and the particular pulse rate searched for is selectively altered during a measurement cycle as a function of the elapsed monitoring period for the measurement cycle, as measured by the elapsed timer 90. Alternatively, if the pulse rate searched for is held constant, then the magnitude of the correction increment added by the anticipation network is a function not only of rate of change of temperature but also of elapsed monitoring period for the measurement cycle. In addition, in order to compensate for possible loss of tissue contact during a measurement cycle, the temperature measurement system will not certify the final output indication unless the indication is made after certain pulse sequence conditions have been met. In this regard, a plurality of successively increasing pulse periods T and T in excess of a prescribed pulse interval, are required before the measurement cycle is terminated. This prevents any error due to loss of tissue contact where the period of loss contact exceeds the termination pulse period interval searched for and would thus cause a corresponding false termination of the measurement cycle. Hence, the  
 safeguard network is a resetting subsystem requiring a prescribed sequence of pulse periods for proper completion of the measurement cycle and thereby minimizes false readings which might otherwise be provided by poor measurement technique of operating personnel.  
  The safeguard subsystem, partially illustrated in FIG. 3, searches for two time periods in sequence, a first pulse period T, somewhat shorter than the pulse period normally indicating termination of the measurement cycle, followed by a second pulse period T at least equal to the termination determining pulse period duration. The search for the second pulse period T does not begin until the first pulse period T has occurred. If a pulse period shorter than the first pulse period T, occurs after the first pulse period has been detected, the safeguard subsystem is reset and again searches for the first pulse period T rather than the second pulse period T thereby again requiring a double pulse period sequence of prescribed magnitudes as though the first pulse period T, had not been detected at all during the measurement cycle.  
  In addition, since the magnitude of the anticipation correction is a function not only of rate of change of measured temperature, but also of elapsed &#39;time from the prescribed reference point in a given measurement cycle, the magnitude of the particular pulse periods T, and T searched for is increased with increasing elapsed time.  
  As previously indicated, in accordance with the invention, the magnitudes of the particular pulse periods searched for, assuming a fixed anticipation correction temperature differential, are varied as a function of elapsed time. Hence, the pulse periods T, and T decoded out of the decoder 103 over lines 104, 105 must change as a function of the time registered by the elapsed timer 90. This may be implemented in any one of several conventional approaches well known in the art, By way of example, the outputs 94 and 95 from the elapsed timer decoder 93 are directed as inputs to the pulse interval timer 91, which is essentially a resettable digital counter, to change the counter modulus at preselected times in the measurement cycle. For example,