Method and apparatus for zero drift compensation in a leak detector using a trace gas detector circuit

A method and apparatus are disclosed for automatically compensating for the effects of variations in ambient trace gas levels or other causes of drift in trace gas leak detection testing, by generating a stored reference signal corresponding to the trace gas sensor output signal, freezing the reference signal at the beginning of each test and summing the output signal with the frozen reference signal to create a compensated output signal which is then compared with a preset threshold signal to trigger a reject during a given test. The uncompensated output signal is also continuously compared with a preset signal value and the test disabled if the output signal exceeds the preset signal at the beginning of each test.

FIELD OF THE INVENTION 
This invention concerns testing and more particularly leak testing using a 
trace gas detector to measure trace gas concentrations during successive 
tests conducted as is done on each of a number of items during production 
of manufactured items. 
BACKGROUND DISCUSSION 
It is sometimes necessary to test manufactured items for very minute leaks 
through portions of the item defining at least in part, a fluid tight 
cavity, such as the walls of fuel tanks and the rims of automotive wheels. 
If such testing is done on every item, or a substantial proportion of the 
items produced, high volume production rate testing is necessary, with 
only a minimum test interval desirable. When testing for very minute 
leaks, achieving reliable detection in a short test interval is difficult. 
A method heretofore employed is tract gas leak detection, in which the item 
is placed in a test fixture and a pressure differential applied across the 
portions of the item defining the fluid cavity, so that a trace gas 
containing fluid will be caused to pass through any leaks. Trace gas 
containing fluid passing through the item portion is collected, and passed 
over a trace gas detector of a type able to detect very minute 
concentrations of trace gas, with any increase in trace gas concentrations 
during the time the item is subjected to the differential pressure 
measured by the detector to detect leakage through the item portions. 
Examples of such method are disclosed in U.S. Pat. No. 4,584,877 and 
copending application Ser. No. 873,518 filed on May 23, 1986. 
Typically, leaks must be of a predetermined magnitude in order to require 
failure of a tested item, so that the magnitude of detected levels of 
trace gas concentrations are relied on to measure the size of any leaks. 
Typically, the detected trace gas concentration levels are compared with a 
preselected threshold level corresponding to leakage of an unacceptable 
size. 
Since many such tests are usually conducted successively over extended 
periods of time with the same apparatus, a problem has occurred in 
maintaining reasonably precise correlation between the detected trace gas 
concentration and the size of the leaks in the tested item. 
This problem typically is caused by increases in the ambient levels of 
trace gas in the vicinity of the trace gas detector, as some trace gas 
escapes incidental to each test and causes a concentration increase in the 
air about the area of the test. Despite efforts to purge the test 
apparatus and shield the sensor from stray trace gas sources, this 
increase in ambient levels causes the trace gas sensor to read out an 
indication not only of increased trace gas concentration due to test item 
leakage, but the increase in ambient levels thereby creating error in the 
test results. 
Another cause of such degradation of correlation of test results is drift 
in the electronic circuiting used to process the trace gas detector output 
signal. The phenomenon is herein referred to as the "zero drift". 
Since the leak detection method depends on reasonably precise correlation 
between the detected concentration level of trace gas and the leak, 
accuracy of the test is compromised by such zero drift. 
The time demands of production testing render impractical the calibration 
of equipment to zero prior to each test, and the skill level necessary for 
performance of the testing would be increased if such calibration were 
necessary. 
Accordingly, it is the object of the present invention to provide a method 
and apparatus to automatically and reliably compensate for zero drift in a 
trace gas detector used for successive leak tests. 
SUMMARY OF THE INVENTION 
The present invention automatically compensates for test-to-test zero drift 
of a trace gas detector used in successive leak tests, by a circuit which 
maintains a reference signal in memory of a signal value corresponding to 
the output signal of the detector. At the beginning of each test interval, 
the reference signal is frozen in memory and used as a zero reference in a 
threshold level summing amplifier which also directly receives the 
detector output signal. 
During the leak test, the trace gas detector output signal is compared with 
the reference signal value in memory at the beginning of each test cycle, 
so that any drift in the "zero level" i.e., the level of the detector 
output signal in the absence of a leak, is automatically compensated for 
to obtain a zero drift compensated detector signal. 
The circuit according to the present invention compares the compensated 
output signal to the detector output signal at the moment of the test, so 
that when the compensated signal reaches a present threshold value, a 
"reject" indication is triggered. 
The circuit also continuously compares the uncompensated detector output 
signal to a preset value, and generates a "test fault" output whenever the 
uncompensated signal exceeds a preset level, to abort further testing, as 
when ambient concentrations of trace gas reach excessive levels likely to 
interfere with reliability of the test results.

DETAILED DESCRIPTION 
FIG. 1 illustrates a typical leak test apparatus of the type with which the 
present invention is utilized, as described in further detail in the 
abovereferenced patent and patent application. 
This includes a fixture 2 adapted to receive a test item 1, here 
illustrated as an automobile fuel tank, with upper and lower bowl shaped 
housing halves 3 configured to enclose the fuel tank 1 with a clearance 
space therebetween when the housing halves 3 are brought together. 
A hermetically sealed chamber 7 is established in the clearance space by 
sealing the housing halves 3 tegether. 
A mixture containing trace gas is supplied via line 5 from a source of gas 
mixture 4 and connected as detailed in U.S. Pat. No. 4,584,877 so as to 
pressurize the interior cavity of fuel tank 1. 
The clearance space 7 is evacuated as by a vacuum pump 12 connected so as 
to create a pressure differential tending to induce flow out of the 
interior cavity of fuel tank 1 through the portion of the fuel tank 
defining the cavity into the clearance space 7, and thence out through 
line 9. If any leakage does occur, a trace gas detector 8 is arranged to 
receive the outlfow from the clearance space 7 and thereby sense the trace 
gas in the mixture escaping through any leaks. 
Another trace gas leak testing arrangement is shown in Ser. No. 873,518 
referenced above, for testing of wheel rims. 
Referring to the FIG. 2, the circuit 10 according to the present invention 
is adapted to process analog electrical signals received from a detector 
8, which for comercially available trace gas detectors, may typically 
range up to 10 V dc output. 
The detector output signals are passed through an inverter/limiter 14, as 
shown to prevent malfunctioning of an up/down counter to be described 
included in the circuit 10. 
The detector output signal is transmitted to output line 16 of the 
limiter-inverter 14. 
Branch line 18 causes the signal to be applied to one input of a count 
control comparator 20. 
The output 22 of count control comparator 20 is connected to an up/down 
count control terminal 24 of a 12-bit up/down counter 26. 
Up/down counter 26 counts clock pulses received over line 28 from the 
output of an AND gate 30. 
AND gate 30 has one input connected to a clock pulse source 32, and the 
other input to the output of an OR gate 34 received over line 36. 
OR gate 34 has one input connected to a manual switch 38 via a "debounce 
circuit" 40 such as a flip-flop, and line 42. 
A manual switch 38 is provided to enable selective testing of the circuit 
10, which normally is operated by signals from an overall control system 
for the testing apparatus designated as a machine controller 46. 
Manual switch 38 has two positions. The first is the "hold" position in 
which no output is received by OR gate 34. The second, "sample" position 
applies a signal to OR gate 34, while the other input of OR gate 34 is 
connected to line 44 to receive an external signal from the machine 
controller 46. 
Thus, if either manual switch 38 or machine controller 46 apply a signal to 
OR gate 34, a signal is applied to AND gate 30, and clock pulses from 
source 32 are gated into the up/down counter 26, and which are counted up 
or down depending on the signal received from comparator 20. 
The machine controller 46 will normally apply a signal to OR gate, except 
during a leak test allowing clock pulses to be counted into counter 26. At 
the time of a leak test, OR gate 34 will no longer produce a signal, and 
gating of clock pulses from source 32 into counter 26 will be 
discontinued. 
The parallel count in the up/down counter 26 is continuously read out into 
a 12 bit D/A converter 48 to convert the count to a corresponding analog 
signal on output 50, applied to the other input of comparator 20. 
Thus, if the output signal from detector 8 is higher than the analog signal 
corresponding to the count in the up/down counter 26, an output 22 causes 
a count-up of pulses, and if lower, a count-down of pulses received on 
line 28 to equalize these signals. A count-down LED 52 and count-up LED 54 
connected via inverter 56 may provide a visual indication of the direction 
of counting into counter 26. 
Accordingly, memory means are provided to continuously maintain a reference 
signal on line 50 corresponding to the output signals of detector 8. 
When the manual switch is in the hold condition, and the system controller 
46 does not send a "hold" signal, to OR gate 34 clock pulses are no longer 
transmitted to the up/down counter 26, which freezes or fixes the 
reference signal on line 50 out of D/A converter 48. This fixing of the 
reference signal is normally done during the time a leak measurement is 
made. 
During the time when switch 38 is in the "sample" position, or the machine 
controller 46 is applying a signal on line 44, as will occur at all other 
times line 50 will be equal in magnitude. The output reference signal on 
line 50 is connected to the substracting input of a summing amplifier 58 
with the detector output signal applied to the adding input 60. 
The output of summing amplifier 58 is the input on line 60 minus the input 
on line 50 and if one is the inversion of the other, the output on line 62 
will be equal to zero. Thus, a compensated signal is generated at the 
output 62 with any zero drift occurring from test-to-test automatically 
offsetting the output 62. 
The compensated signal is applied via line 62 to one input of one or more 
comparators 66a, 66b, or 66c, the other input of each being a 
predetermined constant voltage from adjustable potentiometers 68a, 68b, or 
68c respectively. 
Each reference signal may correspond to a calibrated threshold value 
selected for a particular item. For example, for different size fuel tanks 
or wheel rims, different leak rates may be acceptable. 
If the compensated signal exceeds these predetermined threshold levels, the 
output of comparators 68a, 66b or 66c connected via power transistors 70a, 
70b, 70c, respectively, trigger a suitable indicator or other system 
component 72a, 72b, 72c respectively to produce a "reject" of the tested 
part. 
Circuit 10 also includes a comparator 74 having an input receiving the 
uncompensated detector output signal, and also a fixed reference signal 
from adjustable potentiometer 76. 
If the uncompensated detector signal exceeds the preset reference signal, 
as reflecting an unacceptably high concentration of trace gas prior to 
start of the test, the output of comparator 74 transmitted via power 
transistors 78 causes triggering of an "abort" indicator or test disable 
system component 80. 
Accordingly, it can be appreciated that a simple but reliable apparatus and 
method have been provided to affect the effects of drift in the "zero" or 
initial detector signal value prior to the conduct of the test, to thereby 
enable a practical production application of trace gas leak detection 
methods.