Method of and apparatus for compensating for temperature in leak testing

A method and apparatus for testing for leaks in a test item which has a temperature different from ambient temperature in which the test item is sealed at atmospheric pressure and the pressure of the sealed air in the test item is measured. The rate of change of the pressure of the sealing air due to the non-ambient temperature of the test item is calculated for providing a temperature correction signal proportional to the temperature of the test item and the temperature correction signal is stored. The test item is then subjected to a leak test pressure, any change in the pressure of the pressurized test item is measured and the stored correction signal is applied to the pressure measurement for determining the extent of any leak in the test item while compensating for the temperature of the test item. The temperature correction signal is obtained by differentiating the pressure measurement of the sealed test item in ambient air at ambient pressure, is stored, is integrated, and is inverted and applied to the pressure measurement obtained during the normal pressure decay test time.

BACKGROUND OF THE INVENTION 
As described in U.S. Pat. No. 3,800,586, pressure decay leak testing is a 
way of determining leakage in a test item by pressurizing the test item to 
a given pressure, closing off the pressure source, and monitoring the 
decay in pressure which is a function of leakage. However, there are some 
leak testing applications in which the part to be tested is not at ambient 
temperature or the temperature of the testing fluid, but has a temperature 
different from ambient temperature. If this non-ambient test item is 
pressurized with air at ambient temperature, the test item will tend to 
heat or cool the air causing pressure changes due to temperature 
differences thereby causing an error in the pressure decay measurement. 
SUMMARY 
The present invention is directed to a method and an apparatus for testing 
leaks in a test item and for compensating for leak rate pressure 
measurement errors incurred when testing hot or cold test items. 
Yet a further object of the present invention is the provision of a method 
and apparatus for compensating for the non-ambient temperature of test 
items during leak testing in which the test item is sealed with ambient 
air at atmospheric pressure and the pressure of the sealed air is 
measured. The rate of change of the pressure sealed in the test item is 
calculated for providing a temperature correction signal proportional to 
the temperature of the test item and the correction signal is stored. The 
test item is then pressurized and the decay in pressure is monitored which 
is a function of leakage. The temperature correction signal is applied to 
the pressure measurement for determining the extent of any leak in the 
test item as well as compensating for the temperature of the test item. 
Yet a further object of the present invention is the provision of the 
method and apparatus for testing leaks in a test item having a temperature 
different than ambient air temperature which includes means for sealing 
the test item in ambient air at atmospheric pressure. Transducer means 
measure the pressure in the test item and differential means are connected 
to the transducer output for calculating the rate of change of the 
pressure of the air sealing the test item for providing a temperature 
correction due to the temperature of the test item. Storage means store 
the temperature correction signal. Means are provided for pressurizing the 
test item to a given test pressure. The transducer measures the 
pressurized air and an integrator and inverter are connected between the 
storing mans and the transducer for applying the temperature correction 
signal to the transducer measurement while the transducer monitors the 
pressurized item for a leak. 
Other and further objects, features and advantages will be apparent from 
the following description of a presently preferred embodiment of the 
invention, given for the purpose of disclosure, and taken in conjunction 
with the accompanying drawings where like character references designate 
like parts throughout the several views.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
While the temperature compensation feature of the present invention will be 
described, for purposes of illustration only, in combination with a model 
310 pressure decay leak tester of Uson Corporation, it is understood that 
the present invention may be used for other types of leak detection. 
Referring now to the drawings, particularly to FIG. 1, the test item 10 
which is to be tested is connected to a line 12 and a conventional 
pressure transducer 20 which measures any change in pressure in the test 
item 10. If desired, a valve 13 normally opened to the atmosphere may be 
connected to line 12 to insure that the air sealed in test item 10 is not 
pressurized when sealed. After sealing, the valve 13 is closed. Initially, 
a normally closed solenoid valve 18 connected to line 12 isolates the test 
item 10 trapping ambient temperature air at atmospheric pressure in the 
test item 10 whereby the pressure transducer 20 may measure the change in 
pressure of the sealed test item 10. In this step of the operation, the 
pressure transducer 20 will measure pressure changes in the ambient 
temperature of the trapped air caused by the temperature of the test item 
10 if the temperature of the test item 10 is different from the ambient 
temperature of the air or other testing gas. That is, if the test item 10 
is hotter or cooler than the ambient air the test item will tend to heat 
or cool, respectively, the trapped air and the pressure measurement of the 
transducer 20 is indicative of the temperature of the test item 10. In 
addition, the test line 12 is connected through solenoid valve 18 to a 
pressurized gas from a suitable gas testing supply such as air or a vacuum 
source 14 through a pressure regulator 16. When the solenoid valve 18 is 
opened, the test item 10 is subjected to a pressurized gas or vacuum, the 
valve 18 is closed and the transducer 20 then measures any pressure 
changes in the test item 10 which is an indication and measurement of the 
extent of a leak, if any, in the test item 10. As will be more fully 
explained hereinafter, the pressure measurement initially made while the 
test item 10 is sealed under ambient air at atmospheric pressure is 
combined with the second measurement by the transducer 20 of the 
pressurized air for determining the extent of any leak in the test item 
while compensating for the temperature of the test item 10. 
Referring now to FIG. 2 the dotted outline 21 is directed to a leak tester 
as generally disclosed in U.S. Pat. No. 3,800,586 and the dotted outline 
indicated by the reference numeral 23 generally indicates the automatic 
temperature compensation circuit of the present invention which 
compensates leak rate measurement errors incurred when the test item 10 
has a temperature different from the ambient temperature of the testing 
gas. The pressure transducer 20 continuously measures the trapped pressure 
in the test item 10 and its output is transmitted to a chopper amplifier 
22. Initially, the test is made by connecting the test item 10 to the line 
12 with the solenoid valve 18 in the closed position and with valve 13 
being moved from the open position to the closed position after the 
connection is made. The test is started by the actuation, either manually 
or automatically, of the temperature sense timer 100 and the amplifier 
zero logic 102. Actuation of the timer 100 turns on switch 104 and 
actuation of the zero logic 102 extends a zero command through line 45 to 
the zero circuit 52 to in turn send a signal through a memory amplifier 54 
and a memory retaining means such as capacitor 56 for zeroing the 
amplifier 22 and releasing it to measure any pressure changes in the test 
item 10 by the pressure transducer 20. Any pressure changes in the trapped 
ambient air in the test item 10 is an indicia of the temperature of the 
test item 10 and the measured changing pressure is transmitted to a 
differentiator 106 which calculates the rate of pressure change which is 
then transmitted through the open switch 104 to storage means 108. That 
is, the pressure rate of change calculation provides a temperature 
correction signal proportional to the temperature of the test item and 
thus latches or stores a voltage that is proportional to the pressure rate 
of change in the storage 108. After the pressure rate of change is 
measured, the temperature sense timer 100 turns off, closing switch 104 
and sends a signal through line 110 to start the basic leak test cycle. 
Starting of the test cycle actuates the test control logic 50 which in 
turns actuates the solenoid timer 36, actuates solenoid relay 38 for 
opening the normally closed solenoid valve 18 to allow an air supply or 
vacuum to pressurize the test item 10. At the same time, the test control 
logic 50 inhibits through line 40 the actuation of the indicator light 28 
and 30. 
After predetermined length of time sufficient to pressurize test item 10, 
the solenoid timer 36 deactuates the solenoid valve 18 trapping the 
pressurized air in the test item 10. If desired, a delay timer 46 may be 
provided to delay the start of the test measurement to allow for pressure 
stabilization in the test item 10. 
The pressure transducer 20 measures the pressured air in the test item 12 
and transmits the measurement to the amplifier 22. The delay timer 46 
actuates through line 45 and to zero circuit 52 the zeroing memory circuit 
54. The zeroing and memory circuit 52 and 54 is used to establish an 
artificial zero reference against which pressure deviation in the test 
item 10 may be measured. If desired, the compensation circuit of U.S. Pat. 
No. 3,800,586 may be included to compensate for adiabatic heating when 
pressurizing the test item. 
When the zero circuit 52 has reached zero, it actuates the test timer 58 by 
means of the zero confirm signal line 47 through line 49 which enables the 
accept reject logic circuit 24. The logic circuit 24 is a go no-go logic 
circuit which determines whether the output from the amplifier 22 is above 
or below a predetermined amount which is determined by adjusting a set 
point 26. 
The test timer 58 in addition to actuating the logic 24 also actuates 
switch 110 which enables the integrator 112 whereby the previously stored 
pressure rate of change temperature signal stored in the storage means 108 
is converted back to a changing voltage, and inverted by inverter 114 and 
by line 116 is applied to the input of the chopper amplifier 22 along with 
the pressure transducer 20 measurement. Therefore, the temperature 
correction signal is applied to the pressure transducer measurement to 
null and compensate for the non-ambient temperature effect of the test 
item 10 on the pressurized air during the leak test. Gain adjustment 
attenuator 109 may be provided to adjust the gain of the correcting signal 
to properly compensate the measurement. 
The input to the amplifier 22 from the pressure transducer 20 and the 
inverter 114 is compared with the initial pressure now stored in the 
memory capacitor 56 and the pressure decline from this initial pressure is 
transmitted by the amplifier 22 to the logic circuit 24 for the test 
period set by the timer 58 to determine possible leakage of the test item 
10 while automatically compensating for the temperature of the test item 
10. That is, the logic circuit 24, after the predetermined test interval, 
measures the amount of pressure change in the test item 10 by means of the 
transducer 20 and amplifier 22, and if the amount of pressure change is 
below the set point, signal light 28 lights up indicating the test 
specimen is acceptable. If the pressure change which is measured by the 
logic circuit 24 is above the test point, the circuit 24 actuates light 30 
indicating that the test item 10 has an unacceptable leak and is therefore 
rejected. 
Meter 70 may be used to measure percent pressure and meter 72 provides a 
reading of the change in pressure of the test item 10. 
While the detailed circuits of the components of the automatic temperature 
compensation circuit 23 shown in block form in FIG. 2 are generally 
textbook circuits, for purposes of fuller illustration, the circuits are 
shown in greater detail in schematic form in FIG. 3. 
The present invention, therefore, is well adapted to carry out the objects 
and attain the ends and advantages mentioned as well as others inherent 
therein. While a presently preferred embodiment of the invention is given 
for the purpose of disclosure, numerous changes in the details of 
construction and arrangement of parts may be made which will readily 
suggest themselves to those skilled in the art and which are encompassed 
within the spirit of the invention and the scope of the appended claims.