Abstract:
A method of, and apparatus for, determining leakage in a valve or the like which is not dependent on bulk modulus of the fluid is described. The test apparatus includes a pressure transducer which measures the change in pressure along a length of pipe (XY) from the valve under test to a second valve for a period of time. The effect of bulk modulus of the test fluid is eliminated by placing a volume of gas between the pressure transducer and the section XY of pipe. Leakage is determined by measuring the change in pressure at the transducer over a period of time after the valve under test is closed.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to leak detectors and, in particular, to liquid or fluid-based leak detectors for testing for leaks in hydraulic or liquid-carrying valves, and the like. 
     Leak detection equipment is used in a variety of industries to determine whether products are properly manufactured and assembled. Leak detection equipment is used to test individual products for presence of leaks which would degrade the performance of the product during its useful life. Not all leaks are, however, fatal to the performance of the product and a maximum acceptable leak rate is often established. 
     The object of leak testing is to measure the rate of leakage and to determine whether the measured rate is less than the maximum acceptable leakage rate. Any product leaking at a rate less than the maximum acceptable leakage rate meets the performance specifications relating to the product. 
     A leak is defined as the escape or entry of a gas or liquid into sealed enclosure. Leaks may result from material defects due to holes or porosity or process deficiencies such as sealing and joining problems. The majority of leaks are not simple circular holes in thin walls exhibiting predictable performance but more often are comprised of multiple variable leak paths that tend to be unique to a particular test part subjected to a specific set of conditions. Leaks of these types often have low leakage rates that are difficult to measure. 
     The detection of a leak from a test part under pressure is difficult to measure when the anticipated rate of leakage from the part is low or the available time for the measurement is relatively brief. High levels of pressurization can produce sufficient adiabatic heating effects which must be dissipated before any accurate measurement can be made. The magnitude of this problem is a direct function of the mass of the pressure gas; thus, the volume enclosed by the test part and the pressure of the test gas contained within the test part may effect the magnitude and severity of the adiabatic heating effects. 
     Bubble testing is the most prevalent method of leak testing in the industry. It comprises pressurizing the part to be tested, submerging the part in a water bath, and looking for a stream of bubbles. Although leaks as small as 0.05 standard cubic centimeters per minute (sccm) can be detected by this method, this method suffers from major disadvantages. It is relatively slow; it demands continuous operator attention and it usually requires drying the tested part before the tested part can continue in the manufacturing process. Determination of the amount of leakage is a difficult task. 
     Helium mass spectrometer leak detection is the most common method used to detect very small leaks as low as 10 −11  standard cubic centimeters per second (sccs). The part under test is either pressurized internally or externally with helium or a mixture of helium and air. The helium being leaked is drawn into a very low vacuum and introduced into a mass spectrometer tuned for helium. The mass spectrometer output is proportional to the number of helium ions, which is a direct measure of the rate of leakage. Helium leak detection equipment is very expensive and can require long test times for particular parts. 
     Another method of leak detection is the pressure decay method. In the pressure decay method the part to be tested is pressurized to a pressure determined by a supply pressure. Once pressurized, the part under test is sealed to maintain the pressure therein. A pressure sensor is attached to the part which measures the internal pressure of the part of the test. If a leak is present, the pressure of the part will begin to decay at a rate determined by the size of the leak and the volume of the part. A test operator can determine the relative size of the leak by reading the pressure at the end of the test time and comparing it to a predetermined value. 
     Pressure sensors used in the pressure decay method are typically gauge pressure sensors having a reference to atmospheric pressure. When the gauge pressure sensor is used at normal test pressures, the pressure change resulting from the leak test is a very small portion of the total range on the sensor, since the gauge pressure sensor measures pressure difference between the part pressure and atmosphere. Consequently, the signal from the sensor is relatively small. In order to obtain a usable reading with this system, it is often necessary to extend the test time, particularly, if large parts having small leaks are involved. In some cases this may result in unacceptably long test times. 
     Another method uses a mass flow leak sensor rather than a gauge pressure sensor. In precision mass flow leak testing, a mass flow leak sensor couples the test part to a non-leaking reference volume usually having substantially the same volume as the test part. Then the reference volume and the test part are pressurized to the same pressure. Both the reference volume and the test part are sealed off from the pressure supply. If a leak is present in the test part, the mass flow leak sensor measures the flow and the equalization of pressure between the leaking test part and the sealed reference volume. The difference in pressure causes gas to flow from the reference volume to the test part at a rate proportionate to the leak rate. 
     An alternative method to leak detection measurement using a deferential pressure sensor encloses the test part in a sealed bell jar. Leakage from the test part increases the internal pressure in the internal space within the bell jar and exterior to the test part. The increase in pressure relative to a reference pressure is measured by a transducer and converted to an equivalent leakage rate. 
     This method, however, is less sensitive to the effects of high test pressures and is not sufficiently accurate to detect low leakage rate particularly when short test times are employed. This is because the internal free volume of a bell jar can be large. Because the differential pressure rate involves the measurement of pressure at two different times and the time interval is a function of transducer sensitivity, this method is often not adequate in critical applications. These measurements cannot be made until sufficient time has lapsed to develop a differential pressure. 
     U.S. Pat. No. 5,546,789, Leakage Detection System, assigned to the assignee of the present invention, employs a sealed test fixture or bell jar which surrounds the test part and is connected to a reference pressurized reservoir. A reference bias flow is introduced into the bell jar to establish a floor or offset where any deviation measured from the introduced bias flow indicates an anomaly (or leak). The test part is pressurized and the flow between the bell jar and the reference pressurized reservoir is measured by a mass flow leak sensor. The measured flow is used to determine the leakage of the test part by taking into account the previously introduced reference bias flow. This method provides accurate measurements of leaks having low flow rates occurring over short periods of time. 
     While the foregoing method provides accurate measurements of leaks using a mass flow technique, in some situations the mass flow technique may not be used. For example, accurate measurement of the seat leakage of hydraulic valves is a difficult to achieve. Seat leakage occurs at very small flow rates compared to the flow rate of fluid when the valve is opened. Detection systems suitable for accurate measurements in the range of typical leakage flows are usually not robust enough to withstand the full flow of fluid when the valve is open for purging, for example. Typically, the ratio of full flow to leakage flow is approximately 250:1 or greater. 
     For this reason, most hydraulic leakage detection systems utilize pressure, rather than mass flow, to measure leakage. A typical pressure measurement system employs a pressure source (which may be liquid or gas), which applies pressure to the valve under test, a test pressure sensor between the source and the valve under test, a second valve and a leakage pressure transducer between the two valves. Seat leakage is quantified by measuring the change in pressure in the pipe section between the two valves after the valve under test is closed. To measure leakage pressure, the two valves are closed. Pressure is then measured in the section for a period of time and is a function of the leakage from the valve under test. 
     This techniques has an important drawback. The relationship between the pressure increase and the leakage is a function of the compressibility of the test fluid in the pipe section between the valves. Fluid compressibility is often expressed as bulk modulus, E, of the test fluid and is the ratio of the change in pressure to the percent change in volume, E=dP/(dV/V). Solving for dP=E(dV/V). 
     Bulk modulus is not constant in most practical applications; it is nonlinear in most ranges. The bulk modulus for most fluids is a function of temperature, fluid pressure and fluid purity (many fluids are hygroscopic in that they absorb water from the air). Also, bulk modulus is dependent on the proportion of entrapped or entrained air in the fluid. Even if temperature, pressure and fluid purity can be controlled, eliminating air from the test fluid is almost impossible in test situations. Since new valves for test are repeatedly being introduced into the test system, there is sometimes insufficient time to purge the entrapped air between tests. Even if additional time is taken to purge the system between tests, some air in the system, such as air in fittings, pipe threads, dead end or blind passages, cannot be purged. 
     There is a need for a leakage detection system and method which is independent of the bulk modulus of the test fluid. 
     SUMMARY OF THE INVENTION 
     A method of, and apparatus for, determining leakage in a valve or the like which is not dependent on bulk modulus of the fluid is described. A valve under test is provided for measurement of leakage in a test apparatus. The test apparatus includes a pressure transducer which measures the change in pressure along a length of pipe (XY) from the valve under test to a downstream or purge valve for a period of time. The effect of bulk modulus of the test fluid is eliminated by placing the pressure transducer in pressure communication with a volume of gas. To measure leakage of a valve under test, the volume of gas in pressure communication with the pressure transducer is maintained at atmospheric pressure as long as the valve under test and the downstream valve is open. When the valve under test and the downstream valves are closed, the pressure is first measured. Then the pressure is measured after a predetermined period of time. The pressure difference is a measure of the leakage of the valve under test. 
     In an alternate embodiment, the pressure transducer is placed between two volumes of gas. The first volume of gas (the test volume) is placed between the test section and one side of the pressure transducer. A second or reference volume of gas is placed on the other side of the pressure transducer. A reference vent valve is provided between the pressure transducer and the second or reference volume of gas such that the reference volume is maintained at whatever atmospheric pressure is present when the reference valve is closed. 
     The air test volume, is constructed so that the leakage test fluid in a line section XY pressurizes the air in the air test volume much like an air over oil accumulator. The air volume inside the air test volume is approximately equal to the air volume in the reference volume. Both volumes are many times greater than the maximum possible air volume that may be present in a test fluid section XY. 
     The sequence of operation is generally as follows. During the initial phase, the valve under test is opened. For convenience, the pipe test section XY is located between the valve under test and a downstream or purge valve. The valve under test and the purge valve are opened. Full rated flow from the valve under test takes place for the section XY through the purge valve. The air test volume is held at atmospheric. 
     Next the test leak sequence is started. First, the valve under test is closed. Then the purge valve is closed. Pressure is measured at the pressure transducer. If a reference pressure volume is present, the pressure in the reference volume is fixed at the atmospheric pressure existing at the start of the leak test. After a predetermined period of time, pressure is measured at the pressure transducer. Pressure in the air test volume increases as a function of the leakage from the valve under test. 
     If air test volume is designated V t , the relationship between the pressure increase in volume V t  and the leakage is expressed as follows: 
     
       
         P Vt =P XY    
       
     
     
       
           P   Vt   V   t   =MRT =constant ( K )  
       
     
     
       
           dP   Vt   =P   Vtf   −P   Vti (final pressure−initial pressure)  
       
     
     
       
           d   Vt   =V   tf   −V   ti (final air volume−initial air volume)  
       
     
     
       
         
           P 
           Vti 
           V 
           ti 
           =P 
           Vtf 
           V 
           tf  
         
       
     
     
       
           dP   Vt   =−P   Vti   dV   t /( V   ti   +dV   t )  
       
     
     
       
           V   XY   +V   t =constant  
       
     
     
       
           dV   XY   =Lt (change in volume due to leakage)  
       
     
     
       
           dV   t   =−dV   XY (change in air volume)  
       
     
     This is where: 
     P Vt =Pressure in air test volume, 
     P XY =Pressure in pipe segment XY, 
     f=final condition, 
     i=initial condition, 
     M=Mass of air in the test reservoir, assumed to be a constant, 
     R=Gas Law constant for air, 
     T=Absolute temperature of air in the test, reservoir (assumed to be constant during testing), 
     L=Leakage in cc/min, and 
     t=Test time in minutes. 
     The differential pressure transducer has one side at atmosphere and the other slightly above atmosphere, resulting in an output signal proportional to the pressure differential representing leakage at the end of an interval, such as 30 seconds. The introduction of the reservoir V t  with an air volume many times greater than any likely volume of air in a pipe segment XY during leak testing changes the detection principle from dependency on the characteristics of bulk modulus to dependency solely on Boyle&#39;s gas law. The resulting low liquid flow leak detection system is significantly more accurate and repeatable than prior systems utilizing test fluid compressibility. 
     The principle aspect of the present invention is to provide a highly accurate liquid leak testing apparatus and method. 
     It is another aspect of the present invention to provide a highly repeatable liquid leak testing apparatus and method. 
     Other aspects and advantages of this invention will become apparent to one of ordinary skill in the art upon a perusal of the following specification and claims in like of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of leak detection apparatus; 
     FIG. 2 is a perspective view of the test connector shown in FIG. 1; 
     FIG. 3 is a schematic of the electrical and pneumatic circuits of the test apparatus shown in FIG. 1; 
     FIGS. 4 a - 4   b  is a schematic of the hydraulic circuit of the test apparatus shown in FIG. 1; 
     FIGS. 5 a - 5   c  is a flow chart of a test sequence for testing a valve; 
     FIGS. 6 a - 6   c  is a flow chart of a zero calibration sequence; and 
     FIGS. 7 a - 7   c  is a flow chart of a span calibration sequence. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and especially to FIG. 1, a leak detection apparatus according to the invention is shown therein and indicated by reference numeral  10 . Leak detection apparatus  10  includes a test connector or fixture  20  for receipt of a valve to be tested which is enclosed in a chamber  122 . When a test is to be made, door  18  is closed and locked providing a sealing to chamber  122 . This protects the operator in the event of a malfunction of the test device. Operation of the test apparatus is controlled by computer  12  which includes a keyboard  16  for receipt of user commands and a display  14  for displaying graphics and text messages and information. 
     Referring to FIG. 2, test connector or fixture  20  includes a housing or nest  24  for receiving the valve under test (not shown). Energizing coil  26  receives electric power from line  28  and opens and closes the valve under test within the housing. 
     Referring to FIG. 3, control of the apparatus for leakage detection  10  is accomplished through computer  12 . Computer  12  includes standard user interface elements: touch screen interface and display  14 , keyboard  16 , hard disk  17 , and floppy disk  19 . Electrical signals M from leakage pressure sensor  220 , L from pressure gauge  250 , K from leak fluid temperature sensor  350 , J from test fluid temperature sensor  338  and H from test pressure sensor  348  are input to multiplexer  90 , which provides the analog signals to analog/digital converter card  92 , which provide digital inputs to computer  12 . 
     I/O module board  300  is couples control signals from computer  12  via digital I/O card  34  to the various control elements in apparatus  10 . When part energizing coil relay  32  receives a signal from I/O board  300 , power from coil power supply  31  is enabled energizing part energizing coil  26 . 
     Start button  52  provides a start input to the computer via I/O board  300 . I/O board  300  also provides control signals from computer  12  to door latch  94 , signal G to fixture test valve  342 , signal F to fixture unvent valve  346 , signal E to pressure enable valve  340 . I/O board  300  provides signals A, B, C, D to solenoid valves  62 . Signal A enables air supply  64  to test valve  318 . Signal B enables air supply  64  to verify vent valve  256 . Signal C enables air supply  64  to verify enable valve  254 . Signal D enables air supply  64  to test valve  316 , reference valve  230  and purge valve  240 . 
     Programmable logic controller (PLC)  48  receives signals from part position sensor  41 , part clamped sensor  43 , guard door closed sensor  45 , guard door lock  50  and provides ready to test signal  54  to board  300 . PLC  48  receives computer ready signal  56 , test in progress signal  58  and test complete signal  60  from board  300 . These signals are used in the test sequences (described below with reference to FIGS. 5,  6  and  7 ). 
     Test fluid is provided in a tank  200 . Temperature of the tank fluid is detected by tank fluid sensor  302 , which provides a tank temperature signal to temperature controller  84 . Temperature controller  84  controls tank heater  310  and cooling water control  88  which are used to maintain temperature of the test fluid at a constant level during tests. 
     Fluid level of the test fluid in the tank is also monitored and maintained. Pump motor  66  pumps fluid from fluid tank  200  through the test lines. Pump control  68 , responsive to inputs from motor start button  72 , motor stop button  70 , low level sensor  74 , emergency stop button  76 , low level sensor  80 , and over temperature sensor  78 , controls operation of pump motor  66 . 
     Referring to FIGS. 4 a - 4   b,  fluid is provided from fluid tank  200 . The fluid used should be the type of fluid that will be flowing against the valve under test in its actual use. For example, the fluid may be hydraulic brake fluid when brake valves are being tested. To minimize the effects of bulk modulus changes due to temperature changes of the test fluid, several sensors and a heater are provided. Tank heater  310  maintains the fluid in the tank at a substantially constant temperature. Tank fluid temperature sensor  302  detects the fluid temperature and provides a temperature signal to temperature controller  84 . Over temperature sensor  304  is a safety device which detects if the fluid temperature reaches a predetermined value. Over temperature sensor  304  provides its output to the temperature controller  84 . Preferably, when this sensor detects the over temperature, the tank heater  310  is turned off. Two low level sensors  306 ,  308  detect the level of the fluid in the tank. Fluid may be drained or added to the tank from valve  312 . Fluid is provided to the test system through output pipe or line  320 A and fluid returns through return pipes or lines  320 B,  320 C. 
     Valve  192  is located in line  320 A and is manually operable to provide isolation between the test circuit and the fluid tank  200 . Pump motor  66  drives pump  190  which provides fluid from tank  200  via line  320 A to the test circuit during test. Test pressure source  330  provides initial fluid pressure to valve under test  210 . Cooling is provided to maintain temperature and pressure substantially constant and is provided by cooling water control  88  which controls cooling water in  196  and cooling water out  198  through device  194 . 
     Test fluid temperature is taken at sensor  338  and provided as signal J to multiplexer  90 . Pressure is enabled at valve  340  by signal E from I/O module board  300 . Pressure gauge  350  detects pressure in the source and provides a signal L to multiplexer  90 . Test pressure sensor  348  provides the test pressure signal H to multiplexer  90 . I/O board  300  enables fixture test valve  342  via signal G, which when enabled provides the fluid pressure to valve under test  210 . When testing is complete, I/O board  300  enables fixture unvent valve  346  via signal F. 
     The output of valve under test  210  is point X of the XY segment of pipe or line  212  through which test measurements are made. The XY line ends at point Y, which is prior to relief vent valve  230 . A pressure transducer  220  is disposed between the valve under test  210  and relief valve  220 . Two gas volumes, V r    218  and V t    216 , provide the air over accumulator effect for eliminating the effect of bulk modulus of the fluid in line XY. The volume of air in  218  and  216  should be much larger than the anticipated volume of air in the fluid from leakage. Air at a volume of 20 cc, for example, may be used. 
     When valve under test is fully opened for full fluid flow, test pressure is maintained and verified by a series of valves which are controlled through I/O board  300 . Valves  254  and  256 , in conjunction with verify pressure valve  258 , are used to maintain test pressure through line  211  and are controlled through signals C and B, respectively. Similarly, valve  252  is used to maintain test pressure in line  212 . 
     Test enable valve  318  is located upstream of the pressure transducer  220  and is enabled by signal A from I/O board  300 . Purge valve  240 , reference valve  230  and test vent valve  316  are enabled by signal D from I/O board  300 . Operation of the various valves during test measurement is described with respect to test sequence  500  below. 
     A test sequence  500  for detecting leaks in hydraulic valves is described with reference to FIGS. 5 a - 5   c.  To begin the test sequence  500 , the operator inserts a valve to be tested  210  into nest  24  of test fixture  20  (step  502 ). The operator then pushes a part clamp fully in (step  504 ) to insure the valve is properly inserted into the fixture  20 . Next the operator places the energizing coil  26  over the valve under test  210  (step  506 ). The operator must slide door  18  into a closed position (step  508 ) and presses the start button  52  (step  510 ). 
     In step  512 , several tests are made before the computer  12  will allow the test to begin. The part position sensor  41  must indicate the valve under test is properly located, the part clamp sensor  43  must indicate the valve under test is properly held in place and the guard door closed sensor  45  must indicate the door is properly closed. If any of these conditions is not satisfied, the valve under test  210  is not ready to be tested and the routine branches to step  558 . 
     If all conditions are satisfied, a “test in progress” message  58  is displayed on the display  14  (step  514 ). In step  516 , the “test complete” signal  60  is set as false. In step  518 , the guard door  18  is locked. In step  520  the pressure enable valve in energized (closed). In step  522 , the operator views the system pressure on a pressure gauge and adjusts the pressure until the desired test pressure is obtained. Once the desired pressure is obtained, the operator presses the start button again (step  524 ). 
     At this point, the part energizing coil  26  is energized, opening the valve under test (step  526 ). Next the fixture downstream valve is opened (step  528 ) and the fixture unvent valve is closed (step  530 ). The operator waits for the system to purge itself; test fluid flows through the valve under test  210 , down the XY pipe  212 , through the downstream valve and back to the fluid tank (step  532 ). After the end of the purge time, the part energizing coil is de-energized closing the valve under test  210  (step  534 ) and the test enable valve  246  is energized (opened) (step  536 ). The system waits for the dwell time in step  538 . If the test pressure is not within limits (step  540 ), the routine branches to step  558 . 
     If test pressure is within limits, the test vent valve  244 , reference vent valve  230  and purge valve  240  are energized (closed) (step  542 ). The system waits a period of time for the system to stabilize and valve transients to subside (step  544 ). Then the system starts the leakage pressure test recording in step  546 , waits a test time in step  548  and records the leakage pressure in step  550 . 
     In step  552 , leakage pressure start is subtracted from leakage pressure end to obtain a delta pressure (AP) for the test time. In step  554  valve leakage L in cubic centimeters per minute (cc/min) is calculated, where L=(ΔP−offset ) gain. Gain and offset are determined from the Zero Calibration sequence  600  and Span Calibration sequence  700 , respectively. Next the routine compares valve leakage L to the upper and lower limits defined for the particular valve and displays a message on display  14  advising the operator of the results and status of the test (step  556 ). In most cases, the routine will display qualitative results: Accept, Upper reject, Lower reject, rather than quantitative results. 
     In step  558 , all valves are de-energized. The “test in progress” signal is set false (step  560 ), the “test complete” signal is true (step  562 ) and the guard door  18  is unlocked (step  564 ). The operator then opens the guard door  18 , removes the part energizing coil  22  (step  568 ), pulls the part clamp out (step  570 ) and removes the valve under test (step  572 ). If step  558  were reached as a result of a failure of any of the preceding tests, the operator would reinstall the valve or other item requiring attention and restart the test sequence  500 . 
     Offset is calculated in the zero calibration sequence  600 . Referring to FIGS. 6 a - 6   c,  to begin the zero calibration sequence  600 , the operator inserts a master valve into nest  24  of test fixture  20  (step  602 ). The operator then pushes the part clamp fully in (step  604 ) to insure the valve is properly inserted into the fixture  20 . Next the operator places the energizing coil  22  over the master valve (step  606 ). The operator must slide door  18  into a closed position (step  608 ). The operator then selects “zero calibration” mode on computer  12  (step  610 ) and presses the start button (step  612 ). 
     In step  614 , several tests are made before the computer  12  will allow the test to begin. The part position sensor must indicate the master valve is properly located, the part clamp sensor must indicate the master valve is properly held in place and the guard door closed sensor must indicate the door is properly closed. If any of these conditions is not satisfied, the master valve is not ready to be tested and the routine branches to step  658 . 
     If all conditions are satisfied, a “test in progress” message is displayed on the display  14  (step  616 ). In step  618 , the “test complete” signal is set as false. In step  620 , the guard door  18  is locked. In step  622  the pressure enable valve in energized (closed). In step  624 , the operator views the system pressure on a pressure gauge and adjusts the pressure until the desired test pressure is obtained. Once the desired pressure is obtained, the operator presses the start button again (step  626 ). 
     At this point, the part energizing coil is energized, opening the master valve (step  628 ). Next the fixture test valve is opened (step  630 ) and the fixture unvent valve is closed (step  632 ). The operator waits for the system to purge itself; test fluid flows through the master valve, down the XY pipe  212 , through the purge or downstream valve  240  and back to the fluid tank (step  634 ). After the end of the purge time, the part energizing coil is de-energized closing the master valve (step  636 ) and the test enable valve  246  is energized (opened) (step  638 ). The system waits for the dwell time in step  640 . If the test pressure is not within limits (step  642 ), the routine branches to step  658 . 
     If test pressure is within limits, the test vent valve  244 , reference vent valve  230  and purge valve  240  are energized (closed) (step  644 ). The system waits a period of time for the system to stabilize and valve transients to subside (step  646 ). Then the system starts the leakage pressure test recording in step  648 , waits a test time in step  650  and records the leakage pressure in step  652 . 
     In step  654 , leakage pressure start is subtracted from leakage pressure end to obtain a delta pressure (ΔP) for the test time. In step  656  the measured ΔP is the offset and stored in memory for use during test sequence  500 . 
     In step  658 , all valves are de-energized. The “test in progress” signal is set false (step  660 ), the “test complete” signal is true (step  662 ) and the guard door  18  is unlocked (step  664 ). The operator then opens the guard door  18 , removes the part energizing coil  22  (step  668 ), pulls the part clamp out (step  670 ) and removes the valve under test (step  672 ). If step  658  were reached as a result of a failure of any of the preceding tests, the operator would reinstall the master valve or other item requiring attention and restart the zero calibration sequence  600 . 
     Gain is calculated in the span calibration sequence  700  shown in FIGS. 7 a - 7   c.  To begin the span calibration sequence  700 , the operator inserts a master valve into nest  24  of test fixture  20  (step  702 ). The operator then pushes a part clamp fully in (step  704 ) to insure the valve is properly inserted into the fixture  20 . Next the operator places the energizing coil  22  over the master valve (step  706 ). The operator must slide door  18  into a closed position (step  708 ), selects the “span calibration” mode on computer  12  (step  710 ), connects a device which supplies gas flow at a rate equal to the calibrated (cal) value stored in the routine to a calibration port (step  712 ) and presses the start button (step  714 ). 
     In step  716 , several tests are made before the computer will allow the test to begin. The part position sensor must indicate the master valve is properly located, the part clamp sensor must indicate the master valve is properly held in place and the guard door closed sensor must indicate the door is properly closed. If any of these conditions is not satisfied, the master valve is not ready to be tested and the routine branches to step  760 . 
     If all conditions are satisfied, a “test in progress” message is displayed on the display  14  (step  718 ). In step  720 , the “test complete” signal is set as false. In step  722 , the guard door  18  is locked. In step  724  the pressure enable valve in energized (closed). In step  726 , the operator views the system pressure on a pressure gauge and adjusts the pressure until the desired test pressure is obtained. Once the desired pressure is obtained, the operator presses the start button again (step  728 ). 
     At this point, the part energizing coil is energized, opening the master valve (step  730 ). Next the fixture test valve is opened (step  732 ) and the fixture unvent valve is closed (step  724 ). The operator waits for the system to purge itself; test fluid flows through the master valve, down the XY pipe  212 , through the downstream valve and back to the fluid tank (step  736 ). After the end of the purge time, the part energizing coil is de-energized closing the master valve (step  738 ) and the test enable valve  246  is energized (opened) (step  740 ). The system waits for the dwell time in step  742 . If the test pressure is not within limits (step  744 ), the routine branches to step  760 . 
     If test pressure is within limits, the test vent valve  244 , reference vent valve  230  and purge valve  240  are energized (closed) (step  746 ). The system waits a period of time for the system to stabilize and valve transients to subside (step  748 ). Then the system starts the leakage pressure test recording in step  750 , waits a test time in step  752  and records the leakage pressure in step  754 . 
     In step  756 , leakage pressure start is subtracted from leakage pressure end to obtain a delta pressure (ΔP) for the test time. In step  758  gain=cal value/(ΔP−offset) is calculated and stored for use in test sequence  500 . 
     In step  760 , all valves are de-energized. The “test in progress” signal is set false (step  762 ), the “test complete” signal is true (step  764 ) and the guard door  18  is unlocked (step  766 ). The operator then opens the guard door  18  (step  768 ), removes the part energizing coil  22  (step  770 ), pulls the part clamp out (step  772 ) and removes the valve under test (step  774 ). In step  776  the operator removes the device from the cal port. If step  760  were reached as a result of a failure of any of the preceding tests, the operator would reinstall the master valve or other item requiring attention and restart the span calibration sequence  700 . 
     EXAMPLE 
     Test results using the leakage detection apparatus  10  produced the following results. 
     
       
         P Vt =14.7 psia, V t =20 cc,  
       
     
     
       
         L=0.05 c/min, t=0.5 minutes  
       
     
     
       
           dV   XY =0.05×0.5=0.025 cc  
       
     
     
       
           dV   t   =−dV   XY =−0.025 cc  
       
     
     
       
           dP   Vt   =−P   Vti   dV   t /( V   ti   +dV   t )=(−14.7×(−0.025))/(20+(−0.025))=0.0184 psia  
       
     
     
       
           P   Vtf =0.0184+14.7=14.7184 psia.  
       
     
     The differential pressure transducer has one side at atmosphere (assumed to be 14.7 psia) and the other at 14.7184 psia resulting in an output signal proportional to 0.0184 psia representing the leakage at the end of a 30 second test interval. 
     While there have been illustrated and described particular embodiments of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.