Safety system for heating conduit
This application is a continuation of application Ser. No. 547,238, filed Oct. 31, 1983 now abandoned. 
    
    
     TECHNICAL FIELD 
     The present invention concerns a novel process for detecting if a temperature sensor is correctly located in a heating conduit at a point that is remote from the heater. 
     BACKGROUND ART 
     In U.S. Pat. No. 4,303,601, entitled &#34;Ventilator-Humidifier,&#34; there is disclosed a system for providing humidified air or other gas to a patient. Air or other gas is directed to the humidifier where the gas is saturated by the humidifier. The saturated gas is fed through tubing which extends to the patient. The airway temperature of the humidified gas (at the airway opening adjacent the patient&#39;s mouth) is different from the temperature of the gas adjacent to the humidifier. A temperature probe is provided adjacent the airway opening for feedback control of the temperature. 
     It is important that the temperature probe be located correctly adjacent the airway opening. Because the temperature probe is located in the tubing at a point that is remote from the heater, it is desirable to detect if the temperature probe is correctly located prior to the time that the delivered gas reaches an undesirable high temperature. 
     Although the illustrative embodiment concerns the detection of the presence of a temperature probe in the air delivery tubing connected to a feedback-controlled heated humidifier that is used for delivering humidified gases to a patient, it is to be understood that the present invention is applicable to the detection of any temperature sensor located in a heating conduit at a point that is remote from the heater. 
     Therefore, it is an object of the present invention to provide a process for detecting if the temperature sensor is correctly located in a heating conduit at a point that is remote from the heater, before the heated fluid, e.g., gas, reaches an undesirable high temperature. 
     Another object of the present invention is to provide a system which is operational to detect if the temperature sensor has been removed from a heating conduit during operation of the system. 
     Other objects and advantages will become apparent as the description proceeds. 
     DISCLOSURE OF THE INVENTION 
     In accordance with the present invention, a process is provided for detecting if a temperature sensor is correctly located in a heating conduit at a point that is remote from the heater. The process comprises the steps of applying a predetermined amount of power to the heater for a predetermined amount of time, observing the change in the temperature sensed by the temperature sensor and providing a signal that the temperature sensor is located correctly if the temperature sensed by the temperature sensor has risen at least a predetermined amount. 
     In an illustrative embodiment, if during observation the temperature sensed has not risen at least a predetermined amount, then a second predetermined amount of power is applied for a second predetermined amount of time. The change in the temperature sensed by the temperature sensor is again observed. If the temperature sensed by the temperature sensor has risen at least a predetermined amount, a signal is generated indicating that the temperature sensor is located correctly. If the temperature sensed has not risen at least a predetermined amount after the second application of power, an alarm signal is provided. 
     In an illustrative embodiment, means are provided for detecting if the temperature sensed is at least a second predetermined amount below a selected set point. If the temperature sensed is at least a second predetermined amount below the selected set point, then a timer is set for a selected amount of time. The change in the temperature sensed during the selected amount of time is observed and an alarm signal is provided if the temperature sensed has not risen at least a third predetermined amount during the selected amount of time. 
     In an illustrative embodiment, the second predetermined amount is between 3° C. and 4° C. and the third predetermined amount is about 1° C. 
     In an illustrative embodiment, the sensed temperature is stored. Means are provided for detecting if the temperature sensed has dropped at least a fourth predetermined amount within a third predetermined time period. If the temperature sensed has dropped at least a fourth predetermined amount within the third predetermined time period, then it is determined if the temperature drop is maintained or exceeded during a fourth predetermined time period after the temperature drop is detected. An alarm signal is provided if the temperature drop is maintained or exceeded during the fourth predetermined time period. 
     In an illustrative embodiment, it is determined whether the probe temperature has reached a first predetermined temperature. If the probe temperature has reached the first predetermined temperature, it is thereafter determined if the probe temperature has fallen to a second predetermined temperature, with the second predetermined temperature being a lower temperature than the first predetermined temperature. If the probe temperature has fallen to the second predetermined temperature, an alarm signal is provided. However, if the temperature has not reached the first predetermined temperature, then power is continued to be applied to the heater to increase the temperature and there is a continuing determination if the probe temperature has reached the first predetermined temperature. In this manner, once the first predetermined temperature is reached an alarm signal will be provided if the probe temperature falls to the second predetermined temperature that is below the first predetermined temperature. The difference between the first predetermined temperature and the second predetermined temperature gives sufficient hysteresis to prevent nuisance alarms. 
    
    
     A more detailed explanation of the invention is provided in the following description and claims, and is illustrated in the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a humidifier temperature control system of the type with which the process of the present invention is intended to operate. 
     FIG. 2A is a time versus power graph of the power applied to the heater in accordance with the process of the present invention. 
     FIG. 2B is a time versus temperature graph of the temperature detected by the temperature probe, in coordination with the time versus power graph of FIG. 2A. 
     FIG. 3 is a time versus temperature graph showing one possible temperature sensing operation by the temperature probe. 
     FIG. 4 is a time versus temperature graph showing another possible temperature sensing operation by the temperature probe. 
     FIG. 5 is a time versus temperature graph showing a possible sensed probe temperature. 
     FIG. 6 is a time versus temperature graph showing further temperature sensing operation by the temperature probe. 
     FIG. 7A and FIG. 7B, when taken together, form a flow diagram of an impulse process for detecting if a temperature sensor is correctly located in the heating conduit. 
     FIG. 8 is a flow diagram illustrating another process in accordance with the principles of the present invention. 
     FIG. 9 is a flow diagram illustrating still another process according to the principles of the present invention. 
     FIG. 10 is a flow diagram illustrating an additional process according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT 
     Referring to FIG. 1, a humidifier 10 with a heater has a humidified gas outlet tube 12 coupled to a patient 14 with tube 12 typically terminating at a tracheal tube. Humidifier 10 is supplied with appropriate liquid, such as sterile water, from liquid source 16, and with appropriate gas, such as an oxygen-air mixture, from gas source 17. The temperature of the humidified gas adjacent the patient&#39;s mouth and nose, preferably the &#34;airway&#34; temperature, is sensed by means of a thermistor 18 which provides a signal via line 20 to power control system 22. The power control system 22 includes an analog to digital converter which receives an analog signal from the thermistor 18 which provides a signal via line 20 to power control system 22. The power control system 22 includes an analog signal from the thermistor 18 and provides pulses proportional to the analog signal to a microprocessor. 
     The voltage supply 24 for the humidifier-heater is controlled in accordance with the power control system 22. In the illustrative embodiment, the humidifier uses a fixed resistance heater 28 and the power control system 22 is operative to actuate a heater control timer 26. Timer 26 applies the supply voltage 24 to the heater 28 for a fractional portion of a selected sensing period. In this manner, the fixed resistance heater within humidifier 10 is effectively operating for only a fraction of the sensing period, with a fraction determined by a microprocessor which reads the airway temperature sensed by thermistor 18. 
     A detailed description of the microprocessor-based control system for the heater 28 of humidifier 10 is disclosed in United States application Ser. No. 383,113, filed May 28, 1982, entitled &#34;Temperature Controlled Process,&#34; and assigned to the assignee of the present invention. 
     In accordance with the present invention, a process is provided for detecting if thermistor 18 is correctly located in the airway tube 12. In the specification and drawings, the term &#34;POA&#34; will sometimes be used to represent the testing accomplished by the process of the present invention, with &#34;POA&#34; meaning &#34;probe out of airway&#34; and thereby referring to the detection of the temperature probe being out of the airway tube. 
     Now referring to FIG. 2A, this Figure shows the power on line 28&#39; (FIG. 1) being applied to the humidifier-heater over a period of time. Once the power is turned on, for ambient temperatures of 30° C. and below, one or two pulses of 100 percent power are applied to heater 28 in order to verify that the remote temperature probe 18 is correctly placed in the airway 12. It can be seen that when the power commences, 30 seconds of full power is applied to heater 28, followed by a 2.5 minute of observation to determine whether there has been a 1° C. rise in airway temperature. If a 1° C. (or greater) rise in airway temperature is detected, the power control system 22 operates to provide the normal warm-up curve, designated &#34;power control start-up&#34; in FIG. 2A. On the other hand, if the 1° C. criterion is not satisfied at the end of the 2.5 minute interval, 45 seconds of full power is applied, followed by an 8.2 minute period of observation to determine if there has been a 1° C. (or greater) rise. 
     The temperature curve is illustrated in FIG. 2B. When power is turned on initially, the probe is at ambient temperature. In FIG. 2B, during the first 2.5 minute observation interval, there has not been a 1° C. rise and thus the 45-second application of power is utilized. Prior to the end of the 8.2 minute observation interval, the probe detects a 1° C. rise and the power control start-up commences. 
     Two applications of power are used because of the differences between the response of pediatric airway tubing and adult airway tubing. The 30-second power application is sufficient to detect the temperature probe in place in the pediatric circuit for flow rates of 4 to 25 LPM without producing an unacceptable rise in gas temperature, and the combination of the 30-second and 45-second applications is sufficient to detect the probe in place in the adult airway tube under flow conditions from 4 to 60 LPM. 
     It can be seen that once the 1° rise of temperature is detected during one of the observational intervals, a signal is provided to the power control system indicating that the temperature probe is located correctly and the power control system then commences its start-up procedure. If the temperature sensed has not risen at least 1° C. during one of the observation intervals, an alarm signal is provided indicating a POA (probe out of airway) condition. Another process for detecting if the temperature sensor is correctly located in the airway tube is illustrated in FIGS. 3 and 4. Referring to FIGS. 3 and 4, it can be seen that a temperature set point T s  is 37° C. When the probe temperature is 4° C. or more below this set point, a probe alarm will be activated if the probe temperature does not rise at least 1° C. in each 10-minute interval. In FIG. 3, the ambient temperature is 25° C. The first portion of the graph (designated &#34;impulse&#34;) corresponds to the impulse test of FIGS. 2A and 2B. Once the impulse test is passed, so long as the probe temperature is 4° C. or more below set point (i.e., 33° C. or below) the probe temperature will be observed every 10 minutes to determine whether there has been at least a 1° C. rise in temperature. It can be seen that in the second interval, there has been a 1° C. rise within the 10-minute time interval and thus the probe alarm will not be activated. The 1° C. rise per 10-minute time period has continued and the temperature finally reached 37° C. without the probe alarm being activated in FIG. 3. 
     In FIG. 4, the ambient temperature is 31° C. and it can be seen that the temperature has risen at a rate of at least 1° C. per 10-minute time interval, thereby preventing the probe alarm from being activated. 
     Thus if the temperature sensed is more than a predetermined amount (e.g., 4° C.) below a selected set point (e.g., 37° C.), then a timer is set for a selected amount of time (e.g., 10 minutes). The change in the temperature is sensed during the selected amount of time and an alarm signal is provided if the temperature sensed has not risen at least a second predetermined amount (e.g., 1° C.) during the selected amount of time. 
     If the temperature sensed has risen at least 1° C. within the selected 10-minute time interval, then the sensed temperature is stored and the process is repeated for another 10-minute time interval until the temperature is within 4° from the set point. 
     Another process is utilized for detecting if the temperature sensor is correctly located in the airway tube, comprising the detection of the probe temperature decreasing 2° C. or more in a 20-second time period. To this end, the sensed temperature is stored and means are provided for detecting if the temperature sensed has dropped at least a first predetermined amount (e.g., 2° C.) within a first predetermined time period (e.g., 20 seconds). If the temperature sensed has dropped at least the first predetermined amount within the first predetermined time period, then means are provided for determining if the temperature drop is maintained or exceeded during a second predetermined time period (e.g., 20 seconds) after the temperature drop is detected. An alarm signal is provided only if the temperature drop is maintained or exceeded during the second predetermined time period. 
     The aforementioned process is illustrated in FIG. 5 in which the airway temperature is 37° C. During a first predetermined time period of 20 seconds, a 2° C. drop is detected. However, if the probe alarm will not be activated but instead the probe temperature will be detected for another 20-second time period. As shown in the full line, since the 2° C. drop was not maintained during the second 20-second time period, an alarm is not provided. However, as shown in the dashed line representation, if the 2° C. drop were maintained during the second time period, the alarm would be activated. 
     Previously in connection with FIGS. 3 and 4, it was discussed how the probe temperature will be observed every 10 minutes to determine whether there has been at least a 1° C. rise in temperature. So long as the probe temperature is below a set point and there has been a 1° rise within a 10-minute time interval, the probe alarm will not be activated. However, this routine requires that a first 10-minute interval be observed and even if the rise does not occur during that first 10-minute interval, a second 10-minute interval must be observed. In FIG. 6, another routine is disclosed for determining if the probe is out of the airway, but without requiring a 10-minute observation time in order to make the determination. Referring to FIG. 6, the full line representation shows power being applied to the heater and the airway temperature increases. Once the airway temperature reaches a first predetermined temperature, for example, 27° C., a determination is made whether the airway temperature thereafter falls to a second predetermined temperature, for example, 23° C., with the second predetermined temperature being lower than the first predetermined temperature in order to provide sufficient hysteresis to prevent nuisance alarms. In FIG. 6, the full line representation shows normal operation wherein the power is applied and the airway temperature reaches its normal heated temperature of 37° C. The dashed line representation shows an abnormal condition, such as the probe being out of the airway. In this manner, it is seen that the temperature begins to drop after it has reached 27° C. and it continues to drop until a low temperature alarm is provided at the second predetermined temperature which is 23° C. in this illustrative embodiment. 
     By using the aforementioned routine during warm-up, an extensive amount of time is not required to determine if the probe is out of the airway. There is simply a determination as to whether the temperature has dropped below a second predetermined temperature which is below the first predetermined temperature at which the routine begins. 
     Referring to FIGS. 7A and 7B, a flow diagram is provided for illustrating the steps of the process of the present invention relating to the impulse test of FIGS. 2A and 2B. 
     In FIG. 8, a flow diagram is provided for illustrating the steps of the process of the present invention relating to the process of FIGS. 3 and 4, in which a probe alarm will be activated if the probe temperature does not rise at least a predetermined amount within a selected time period. 
     In FIG. 9, a flow diagram is provided for illustrating the steps of the process of the present invention in connection with the FIG. 5 embodiment, in which the probe alarm will be activated if the probe temperature decreases a predetermined amount or more within a selected time period and the decrease is maintained during a second selected time period. 
     In FIG. 10, a flow diagram is provided illustrating the steps of the process of the present invention in connection with the FIG. 6 embodiment, in which the probe alarm will be activated if the probe temperature falls to a second predetermined temperature that is lower than a first predetermined temperature which is reached shortly after warm-up. Although in the FIGS. 6 and 10 embodiment the first predetermined temperature is 27° C. and the second predetermined temperature is 23° C., it is understood that these temperatures are examples only and may be varied as desired. 
     The process steps described and illustrated in the flow diagrams of FIGS. 7A, 7B, 8, 9 and 10 can be achieved using copyrighted computer programs, ©1983 by Travenol Laboratories, Inc., which are presented in detail below. Although illustrative embodiments of the invention are described herein, it is to be understood that various modifications and substitutions may be made by those skilled in the art without departing from the spirit and scope of the present invention. 
     The following assembly language problems and hexidecimal code comprises register definitions embodied in a 2K×8 E PROM operable with a MOSTEK F8 microprocessor: ##SPC1## 
     The following are the memory definitions embodied in the E PROM: ##SPC2## 
     The following assembly language program and hexidecimal code embodied in the E PROM is the impulse routine of FIGS. 2A , 2B and 7: ##SPC3## 
     The following assembly language program and hexidecimal code embodied in the E PROM is the routine of FIGS. 3, 4 and 8: ##SPC4## 
     The following assembly language program embodied in the E PROM is the decreasing temperature routine of FIGS. 5 and 9: ##SPC5## 
     The following assembly language program and hexidecimal code embodied in the E PROM is the temperature drop routine of FIGS. 6 and 10: ##SPC6##