Helium detector
BACKGROUND OF THE INVENTION 
     The present invention is directed to detection of a component gas contained in a mixture of gases. More specifically, the invention is a method, and apparatus that implements that method, that detects helium tracer gas for locating leaks in underground telephone cables. 
     Telephone cables, to prevent water from shorting pairs of the wires that make up the cables, are usually enclosed in a sheath and internally pressurized with dry air compressors placed at predetermined locations along the cable. Unfortunately, there are times when the sheath is penetrated, either by chemical degradation (electrolysis), mechanical stresses, or similar phenomena, causing leaks in the sheath and allowing the introduction of water. This, in turn, causes a degradation of any internal insulation, permitting deterioration and shorting of the internal wiring, and degradation of the signals carried by the cable. Repairing this damage can cost substantial sums because it is sometimes necessary to excavate in several points or trench hundreds of yards of cables due to the impossibility of identifying the exact location of the leak. 
     Recent techniques have been developed to more accurately pinpoint the location of leaks that develop in the cable sheath in order to minimize the amount of cable that must be unearthed and replaced. One technique that has been found sufficiently successful to enjoy common use today is one that includes the step of obtaining a preliminary rough location of the leak by taking pressure measurements of the cable. The resolution of this technique can locate the leak within anywhere from 300 feet to 6,000 feet. The cable, at periodic locations, is then pressurized with a helium tracer gas, and a helium detector used to pinpoint the leak more accurately by detecting the helium that permeates to the surface of the ground from the leak. One technique of detection is by use of a helium mass spectrometer. 
     While the use of helium mass spectrometers have been somewhat reliable, mass spectrometers themselves are not without certain problems. Their operation depends upon separation of the helium gas in a vacuum by imparting an electrical charge to the gas sample containing the helium, pushing the sample through a magnetic field, and collecting the hydrogen ions from the result. The electronics, including a supply of high voltage, and the vacuum system make this type of helium detector bulky, complicated and expensive. Since leaks are just as prone to developing in relatively remote areas where access to such heavy equipment can be difficult, the efficacy of this leak-detection technique becomes limited. 
     Helium tracer gas detection is also performed using negative-going combustible gas detectors. These devices, however, lack the sensitivity necessary for use in tracer gas detection, making this type of detection technique less effective than desired. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of quickly detecting the presence of component gas, such as helium, in a sample gas mixture. The method is capable of being implemented in the apparatus aspect of this invention, which is a compact, lightweight, and relatively inexpensive device, capable of being easily carried by one person, even in rugged terrain, and used to quickly detect the component gas being searched. 
     According to the method aspect of the invention, a predetermined sample of a gas mixture is passed through a fixed-phase separator configured to &#34;order,&#34; i.e., relatively position, the individual component gases of the mixture, especially the component gas being searched for, in the separator&#39;s effluent. Preferably, the component gases are ordered on the effluent so that in that ordering the component gas being searched for will exit first from the separator. The rate of change of the thermal conductivity of the effluent is monitored, and, at about the same time, the thermal conductivity is measured (i.e., integrated) over time until the rate of change goes to zero. The measurement obtained provides a relatively accurate indication of the quantity of the searched for component gas contained in the sampled mixture. 
     Apparatus incorporating the invention is disclosed. In the preferred embodiment the method and apparatus of the invention is directed to detection of a tracer gas so that the invention can advantageously be used to assist in detecting leaks in telephone cable sheaths. The apparatus includes a microprocessor-controlled system, including a fixed-phase separator, for sampling a gas mixture to determine if the sample includes the searched for component gas, i.e., helium. The fixed-phase separator is configured to retard those component gases sufficiently to allow any helium gas to be encountered first (in time) in the effluent from the separator. The effluent is passed through a detector chamber that produces a signal indicative of the thermal conductivity of the effluent. The rate of change of the signal is monitored during the first portion of the effluent and, at a predetermined point in time, the signal is integrated over time until the rate of change becomes approximately zero at which point the integration process stops. The integration process develops a measurement that is indicative of the quantity of helium gas, which is displayed to the user. In an alternate embodiment of the invention the integration process is initiated when the rate of change of the effluent&#39;s thermal conductivity exceeds a predetermined level, terminating as before when the rate of change becomes approximately zero. 
     The detector chamber includes a pair of hot wires arranged in a bridge circuit and each mounted in a corresponding cavity of the detector chamber. One hot wire functions as a reference, and is open to the atmosphere; the other has the effluent passed thereacross. A voltage indicative of the imbalance of the bridge is converted to digital information and applied to the microprocessor which, in addition to supervising system operation, produces the measurement indicative of the quantity of gas contained in the sample. 
     A number of advantages are achieved by the present invention. First, since the method disclosed does not require a vacuum or high voltages (of the type used by mass spectometers) the method can be implemented in a compact, lightweight unit that can be easily carried by one person. Accordingly, the invention is particularly advantageous for use in the field for telephone cable leak detection. 
     In addition, the invention provides a relatively accurate indication of the quantity of helium, thereby allowing a user to more accurately pinpoint an underground cable leak in a procedure using helium tracer gas. 
    
    
     These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description, which should be taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a helium detector constructed in accordance with the teachings disclosed herein, illustrating the capability of implementing both the method and apparatus aspects of the invention in a compact unit; 
     FIG. 2 is a block diagram representation of the system incorporating the present invention; 
     FIG. 3 is a schematic representation of the detector chamber and detector circuit used to generate a voltage indicative of the thermal conductivity of the effluent produced by the system of FIG. 1; 
     FIG. 4 is a flow chart illustrating the steps taken by the system of FIG. 2 to measure the amount of helium gas that may be contained in a sampled mixture of gases; 
     FIG. 5 is a diagrammatic representation of the voltage, with respect to time, produced by the comparator of FIG. 3; and 
     FIG. 6 is a flow chart illustrating an alternate method of system operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated in FIG. 1, and designated with the reference numeral 10, is a helium detector constructed in accordance with the teachings set forth herein. The helium detector 10 is a packaged unit measuring approximately 7&#34;×9&#34;×4&#34; and weighing about five pounds. As shown, the helium detector 10 includes a housing 2, containing the component parts of the system. The top surface 4 carries the system&#39;s control panel, including a display 5 and control switches/buttons 6 for initiating measurements, or self-test operation. Attached to ends of the housing 2 is a shoulder strap 7 that facilitates carrying the system. A length of tubing 8 connects the component&#39;s interior of the housing 2 to a nozzle 9. 
     FIG. 2 illustrates in block diagram form the components of the helium detector 10. As shown, the helium detector 10 includes a first two-way valve V1, a second two-way valve V2, interconnected by a pump P and a sample loop 12. The valve V1 has two mutually exclusive inlets 14 and 16, and an outlet port 20 that is connected to the pump P. The inlet 14 is adapted to connect to the tubing 8 (FIG. 1) to receive the gas mixture sample to be investigated for helium content; the inlet 16 is adapted to receive air during an operational cycle that purges the system of the prior sample. The output of the pump P connects to an inlet 22 of the two-way valve V2 via the sample loop 12, which is a length of polyethylene tubing cut to a quantity sufficient to store the desired gas sample. 
     The valve V2 functions to selectively communicate the sample loop 12 to an inlet 28 of a column separator 30 via an outlet port 24, or to an exhaust port 26. An outlet 32 of the column separator 30 connects to a detector chamber 36 via a conduit 38. Effluent from the column separator 30 passes through the detector chamber 36 to an exhaust outlet 40 of the detector chamber. 
     Coupled to the detector chamber 36 is a detection circuit 42 that produces a SENSE signal on signal lines 44. The SENSE signal is a voltage that is indicative of the instantaneous thermal conductivity of the effluent passing through the detector chamber 36. Signal lines 44 couple the SENSE signal to a digital-to-analog (D/A) converter 48 which periodically samples the SENSE signal, producing a digital word for each sample taken. The digitized samples are conducted from the D/A 48 to a microprocessor 50 via an eight-bit data bus 52. 
     The microprocessor 50 (manufactured by Intel Corporation of Sunnyvale, Calif., and sold under the part number 8748) functions to process the received data words in a manner that will be described in greater detail below, ultimately producing information that is displayed to a user by the display 5. 
     The operational functions of microprocessor 50 also include control of the sample procedure--which is discussed below--through operation of the valves V1 and V2 and the pump P via an input/output (I/O) expander 60 (also manufactured by Intel Corporation and sold under the part number 8243). The I/O expander 60 is, in essence, a number of addressable output latches and input buffers, together with multiplexing and control circuitry, that function to form an extension of the I/O ports of the microprocessor 50. Binary data is coupled to the I/O expander 60 via data lines 62, held, and in turn coupled by control lines 64, 66 and 68 to appropriate driver circuitry (not shown) that control operation of the valves V1 and V2 as well as the pump P. Thus, depending upon the binary state placed on the control lines 64-68, the valve V1 will select either input line 14 or input line 16 to communication to the output line 20 of valve V1; the pump P will be placed in a pumping or non-pumping inoperative state; and the valve V2 will operate to select one of the output ports 24 or 26 (but not both) for communication with the input port 22. 
     The detector chamber 36 and detection circuit 42 are illustrated in greater detail in FIG. 3. As shown, the detector chamber 36 includes a pair of cavities 70 and 72, each having ports 74 and 76 formed to communicate the exterior of the detector chamber 36 thereto. The port 74 of the cavity 72 is adapted to connect to the conduit 38 (FIG. 2) to receive the effluent from column separator 30. The port 76 connects to the exhaust line 40. The ports 74 and 76 of the cavity 70 are left open to the atmosphere. 
     Situated in the cavities 70 and 72 are sensors 80 and 82, respectively, each formed from a thin (approximately 1 mil in diameter) platinum wire coated with alumina, and preferably matched to have substantially similar temperature characteristics. The sensors 80, 82 are connected to form a part of a bridge circuit that includes resistors R1, R2 and potentiometer R3, found in the detection circuit 42. The bridge circuit, so formed, is connected by input resistors R4 and R5 to a comparator circuit comprising differential amplifiers 84 and 86. Feedback resistances R6 and R7 set the gain of the amplifiers 84 and 86, while potentiometer R7 functions to balance the gain of the two amplifiers. The outputs of the amplifiers 84 and 86, which produce the SENSE signal, connect to the signal lines 44 that conduct the SENSE signal to the D/A 48 (FIG. 2). 
     As indicated above, the invention of FIGS. 1-3 finds particular applicability as a detector of helium tracer gas used to locate leaks in underground telephone cables. Leak location, using a helium gas tracer, generally proceeds as follows: A section of cable suspected of containing the leak is charged with a helium tracer gas. The ground surface route that generally overlies the suspect (buried) cable is followed, boring holes approximately every eight feet, each hole measuring about six inches in depth and two inches in diameter. A probe is inserted in the hole and the air mixture therein sampled. Using this technique it will be hoped that at least one of the holes will be found to contain a larger concentration of helium tracer gas than the other holes, indicating its close proximity to the leak from which the tracer is escaping. A hole will have a certain amount of helium because the helium will permeate somewhat through the sheath and up to the ground&#39;s surface, but more so at the leak. 
     The present invention fits neatly into this leak location procedure. Thus, the terminus of the probe 9 (FIG. 1) is provided for insertion into the bored holes to conduct a predetermined portion of the gas mixture found in the hole to the helium detection unit 10 to determine its helium content. The helium detector 10 performs a three-part sample/measurement/purge cycle to determine a helium content operation. This operation is illustrated in FIG. 4. A measurement cycle is initiated by activation of a control 6, generating a START signal that is applied to the microprocessor 50 and causing the microprocessor 50 to issue signals via the I/O expander 60 to begin operation of the pump P (FIG. 4--step 90). At the same time, the valves V1 and V2 are operated to place the valve V1 in a position that communicates the inlet 14 to the outlet 20, closing inlet 16. Similarly, and at the same time, valve V2 is in a position that communicates the inlet 22 to the exhaust 26. With the terminus of the probe 9 inserted in a hole (not shown), operation of the pump P will draw a sample of the gas mixture in the hole into the inlet 14, through valve V1 to pump P, into sample loop 12, and ultimately to the exhaust 26 of valve V2. The microprocessor 50 will continue this state of operation for a time (step 92) sufficient to ensure that the sample loop 12 contains a sample of the gaseous mixture taken from the bored hole--typically about 30 seconds. Microprocessor 50 will then issue command signals on the data lines 62 to the I/O expander 60, causing the valve V1 to switch its operating state, communicating inlet 16 to the outlet 20. At the same time, the state of valve V2 is also switched so that the input 22 is communicated to the output 24. The pump P continues to pump, and the sampled air mixture is moved from the sample loop 12 to and through the column separator 30 (step 94). 
     In the preferred embodiment of the invention, the column separator 30 has a packing that is specific to the gas anticipated, i.e., helium. The packing, which is approximately 90% activated carbon and 10% silica gel, functions to separate and order the component gases that make up the sampled gas mixture to produce an effluent containing component gases in separated fashion--with the helium component, if any, located in the initial portions of the effluent. Thus, the effluent that exits the column separator 30 at the output 32 will have the component gases of the sampled gas mixture relatively located in generally an ordered, sequential manner. The helium tracer gas, due to its small molecular size and weight, will precede the other component gases of the sampled air mixture to the detector chamber 36. 
     It should be noted that although the ordering algorithm used here is based on size (&#34;macro-ordering&#34;) other algorithms are possible. For example, ordering of the component gases of the sampled gas mixture could be based upon oxidation-reduction capabilities, charge characteristics, and the like, depending upon the component gas being searched for. To use a different ordering algorithm one may need only change the packing of the column separation 30. 
     The effluent, comprising the separated component gases of the sampled gas mixture, is conducted by the conduit 38 to and through the cavity 72 (FIG. 3) of the detector chamber 36, and out the exhaust line 40. As illustrated in FIG. 3, a voltage V is applied to the bridge circuit containing the sensors 80 and 82, causing the aluminum windings that form the sensors to reach a temperature of approximately 200° C. The coating of alumina tends to concentrate this temperature toward the center of the windings. As the effluent from the column separator 30 passes through the cavity 72, the initial portion of that effluent, in which any helium tracer gas component of the air mixture sample will be located, will cool the sensor 82 by an amount that depends upon the thermal conductivity of the effluent. This causes a change in resistance of the sensor 82, relative to that of sensor 80, and an imbalance in the bridge circuit containing the sensors 80 and 82. The imbalance so produced is a measure of the thermal conductivity of the effluent, and will be directly related to the quantity of helium tracer gas in the effluent. The imbalance is amplified by the amplifiers 84 and 86, forming the SENSE signal that is communicated to the D/A 48 by signal lines 44. 
     The SENSE signal is converted to a digital representation thereof by the D/A 48, and monitored by the microprocessor 50. This monitoring process, in essence, examines the rate of change of the SENSE signal during that period of time that the effluent from the column separator 30 is expected to contain the helium tracer gas, i.e., the initial portion (FIG. 4--step 96). A predetermined time (t.sub.φ) after step 94 is initiated the microprocessor 50 samples the SENSE signal, obtaining and temporarily storing a baseline value of S.sub.φ (step 97). At the same time the microprocessor 50 begins an integration process (step 98) that integrates the SENSE signal with respect to time. The integration process continues until the rate of change of the SENSE signal, which is continuously monitored during the integration process, drops to approximately zero or integration has been performed for a predetermined time (t 2 ), whichever occurs first (step 100), at which time the integration process terminates. If the integration terminated normally (i.e., when the rate of change of the thermal conductivity of the effluent dropped to zero), the resultant measure obtained by the integration process will be a relatively good indication of the quantity of the helium tracer gas component of the sampled gas mixture. When the conditions of step 100 are met, i.e., the rate of change drops to zero or the integration process times out, the operation then proceeds to the purge portion of the cycle (steps 102, 104). 
     Before continuing, the integration aspect of the measurement procedure bears further examination. Referring to FIG. 5 there is illustrated a curve 110 that generally represents the SENSE signal, i.e., the measured thermal conductivity of the first portion of the effluent passing through the cavity 72 of detector chamber 36 relative to the atmosphere in cavity 70, with that first portion containing a helium tracer component. The curve 90 is a plot of the SENSE signal (S) with respect to time (t). 
     At the completion of the time period indicated in FIG. 5 as t 0 , the microprocessor commences the integration process of the SENSE signal, continuing to monitor the rate of change ds/dt (steps 98 and 100, FIG. 4). At point 114 the rate of change becomes zero, indicating the peak of the curve 110, at which time the integration process stops. The result obtained is a measurement, indicated by the cross-hatched portion of the curve 110, that is indicative of the quantity of helium tracer gas component in the sampled air mixture. This measure is adjusted (i.e., multiplied by 2 and scaled) to form a representation that is transferred to the display 54 where it is latched and displayed to a user. 
     The measurement cycle of FIG. 4 is completed by a purge of the system. Thus, at the step 102, the valves V1 and V2 and pump P remain in a state that continues to draw air from the atmosphere and through the column separator 30 to clear it of any extraneous helium for the next measurement cycle. At predetermined intervals the SENSE signal is sampled, and compared to the baseline amount (S.sub.φ) obtained in step 97. If the present sample is within a certain range (L) of the baseline amount, the purge cycle is complete; if not, step 102 continues. 
     FIG. 6 illustrates an alternative sequence of the steps that can be taken in accordance with the present invention. The steps of FIG. 6 labeled 90, 92, 94, 102 and 104 are identical to those of FIG. 4. However, the steps 96, 97, 98 and 100 of FIG. 4 are replaced with steps 96A, 98A and 100A in FIG. 6. The alternate method proceeds, up to step 94, as explained above with reference to FIG. 4, and, although not shown in FIG. 6, the baseline amount (S 0 ) is obtained. At step 96A, the rate of change of the SENSE signal commences to be monitored. When that rate of change exceeds a predetermined amount (K), the integration process is initiated (step 98A). Integration of the SENSE signal proceeds until the rate of change of the SENSE signal is approximately zero (step 100A), at which time the purge cycle is entered at step 102. If the rate of change of the SENSE signal never exceeds K, the integration steps 98A, 100A are never entered, and system operation proceeds directly from 96A to step 102. Steps 102 and 104 are the same as those described with respect to FIG. 4. 
     In summary, there has been disclosed a method and apparatus for detecting a helium tracer gas that is simple and quick to use, inexpensive to manufacture, and adapted for field use. While a full and complete disclosure of the preferred and alternate embodiments of the invention are set forth, various modifications, alternate constructions and equivalents may be employed without departing from the true spirit and scope of the invention. For example, it will be evident to those skilled in this art that the configuration of the column separator 30 can be tailored to locating and measuring gases other than helium. One reconfiguration would be to use &#34;polar packing&#34; to effect gas separation by relative surface charge on the molecules that make up the component gases contained in a mixture, retarding certain gases relative to others so that the searched for gas will appear preferably first in time in the effluent. Separation can also position the searched for gas further down the line in the effluent and changes in the rate of change of a monitored physical property used to count when that portion of the effluent containing the searched for gas is encountered. This technique, however, requires the presence in the effluent of the component gases that precede the searched for gas by the ordering established by the column separator 30. For this reason it is preferred that the ordering performed by the column separator place the searched for gas at the initial portion of the effluent. 
     Therefore, this disclosure and illustrations should not be construed as limiting the scope of the invention, which is defined by the appended claims. 
     Set forth below in Table I is a source code listing of the microprocessor program that causes the system to operate in the manner above-described with reference to FIG. 4. ##SPC1##