Abstract:
A method of determining the accuracy and repeatability of leak testing instrumentation comprises the following steps: providing a two chamber vessel having an access port and a flow controlling reference orifice associated with each chamber and a third reference orifice communicating between the two chambers, providing a leak testing device and connecting such leak testing device first to one of such ports, pressurizing the associated chamber and, with the associated orifice open, observing and recording the pressure measured by the leak testing device under test as a function of time. The second test repeats this activity with the other chamber and the other orifice. A third test is undertaken with the third orifice open. One of the chambers is smaller and incorporates a smaller orifice and the other chamber is larger and incorporates a larger orifice thus achieving leak testing under different conditions.

Description:
FIELD 
     The present disclosure relates to leak testing instrumentation and more particularly to a method of objectively testing leak testing instrumentation, determining its performance and accepting or rejecting same. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
     Leak testing of industrial, commercial and consumer devices and components is a sophisticated and necessary adjunct to manufacture. Broadly speaking, the testing is part of quality control and its results can be critical not only to the successful manufacture of components but also to their performance and achieving their intended service life. That a given hydraulic pump, solenoid valve, hydraulic cylinder, or complex hydraulic device such as a vehicle transmission operates properly and provides the desired output can often depend upon the integrity of many components such as valve plungers and seals or, stated oppositely, the leakage rate of such parts and of the composite device. 
     When such devices are tested for their component and seal integrity (leak rate), it is necessary to employ leakage testers which utilize pressurizing mechanisms such as pumps, pressure sensing devices and data processors which compute the leak rate of the tested component. Typically, a part or component will be rejected if the leak rate is above a certain, predetermined threshold and will be accepted if the leak rate is below another certain, predetermined threshold. 
     This activity raises the question of how the leak sensing equipment itself is calibrated or tested since an improperly operating leak tester generally does not include self-diagnostic components or programs and it may indicate improper pass-fail test results initially or for a period of time before it is checked, found to be out of calibration and re-calibrated. Furthermore, since different equipment from different manufacturers which was designed for different leak tests may not be accurately and identically calibrated, it becomes advisable to standardize such testing and calibration to ensure satisfactory, consistent and repeatable test results and product integrity regardless of the equipment utilized to perform a leak test. 
     The present invention is directed to a method addressing the foregoing difficulties. 
     SUMMARY 
     The present invention provides a method of determining the accuracy and repeatability and thus the acceptability of leak testing instrumentation comprising the following steps: providing a two chamber vessel having an access port and a flow controlling reference orifice associated with each chamber and a third reference orifice communicating between the two chambers, providing a leak testing device and connecting such leak testing device first to one of such ports, pressurizing the associated chamber and, with the associated orifice open, observing and recording the pressure measured by the leak testing device under test as a function of time, that is, the leak rate. The second test repeats this activity with the other chamber and the other orifice. A third test is undertaken with the third orifice open. One of the chambers is smaller and incorporates a smaller orifice and the other chamber is larger and incorporates a larger orifice thus achieving leak testing under different conditions, i.e., pressures and flow rates. The tests may be repeated many times to check the repeatability of a leak testing device. The testing may also be conducted at temperatures slightly above ambient. 
     Thus it is an aspect of the present invention to provide a two chamber vessel having a respective port and a flow controlling reference orifice associated with each chamber 
     It is a further aspect of the present invention to provide a two chamber vessel having a port and a flow controlling reference orifice associated with each chamber and a third reference orifice selectively communicating between the two chambers. 
     It is a still further aspect of the present invention to provide a method for determining the performance of a leak testing device. 
     It is a still further aspect of the present invention to provide a method for determining the performance of a leak testing device and determining its acceptability for use. 
     It is a still further aspect of the present invention to provide a method for determining the performance of a leak testing device and accepting or rejecting it. 
     It is a still further aspect of the present invention to provide a method of testing a leak detection device at multiple pressures and flow rates. 
     It is a still further aspect of the present invention to provide a method of testing the repeatability of a leak detection device. 
     It is a still further aspect of the present invention to provide a method of testing a leak detection device at temperatures slightly above ambient. 
     Further aspects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a schematic view of a test arrangement utilizing a two chamber vessel according to the present invention; and 
         FIG. 2  is a flow chart setting forth the steps of a method of determining the performance and acceptability of leak detection devices and instruments according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     With reference to  FIG. 1 , a general arrangement of a test apparatus is schematically illustrated and generally designated by the reference number  10 . The test apparatus  10  includes a two chamber leak test container or vessel  20  which is preferably a cylinder having an outside wall  22 , a first circular end wall  24 , a bulkhead  26  and a second circular end wall  28 . The outside wall  22 , the first end wall  24  and the bulkhead  26  define a first, smaller chamber  30  which preferably has a volume of 200 cubic centimeters (12.20 cubic inches or 0.0528 gallons). A first fitting  32  provides access to the first chamber  30  through the first end wall  24 . The outside wall  22 , the bulkhead  26  and the second end wall  28  define a second chamber  50  which preferably has a volume of 1 gallon (3.79 liters or 231 cubic inches). A second fitting  52  provides access to the second chamber  50  through the second end wall  28 . 
     Disposed adjacent the bulkhead  26  and selectively providing fluid communication between the first chamber  30  and second chamber  50  is a bulkhead or third orifice  72 . The bulkhead or third orifice  72  is preferably a fixed, pre-determined and pre-selected size which provides a flow (leak) rate of 160 standard cubic centimeters of air per minute at a pressure of 15 p.s.i. (103.4 kPa). The bulkhead or third orifice  72  is disposed in series with a two position (on-off) valve  74  such that the valve  74  may be opened to provide fluid communication between the first chamber  30  and the second chamber  50  through the bulkhead or third orifice  72  or closed to terminate and prevent such fluid communication. Preferably, the two position valve  74  is operated by an electric motor or solenoid  76  but the valve  74  may also be pneumatically or hydraulically operated. 
     The first fitting  32  is in fluid communication with one (a first) port of a first T fitting  34  through suitable tubing or a fluid line  36 . It will be appreciated that the fittings  32  and  34  as well as all the other fittings, the tubing or fluid line  36  as well as all the other tubing or fluid lines, such as the tubing or fluid line  56 , and the valves, such as the two position valve  72 , are relatively large compared to the control orifices so that they have negligible or no effect on the testing performed with the apparatus  10 . Another (second) port of the first T fitting  34  communicates through such tubing  36  to a sensing port  38  of a leak testing device or instrument  40  which is undergoing a first portion (high pressure) testing with the present test apparatus  10 . Yet another (third) port of the first T fitting  34  communicates with a first orifice  42 . The first orifice  42  is sized to provide a defined and known leak rate of 1.2 standard cubic centimeters of air per minute at an initial pressure of 15 p.s.i. (103.4 kPa). 
     In fluid communication with the yet another (third) port of the first T fitting  34  through the first orifice  42  and a first three way valve  44  is a source of pressurized air  48 . The first three way valve  44  is preferably operated by an electric motor or solenoid  46  but may also be operated manually, pneumatically or hydraulically. The source of pressurized air provides dry air at 15 p.s.i. (103.4 kPa) gauge. The first three way valve  44  may be positioned or adjusted, as illustrated in  FIG. 1 , to provide pressurized air to the first orifice  42 , the first T fitting  34 , the leak testing device  40  and the first chamber  30 . The first three way valve  44  may also be positioned or adjusted to terminate the flow of pressurized air from the source  48  and allow the first chamber  30  and the leak tester  40  to vent through the first orifice  42 . Preferably, as noted above, the first three way valve  44  is relatively large in size, i.e., having large internal passageways, so that it has negligible or no effect on the flow rate established and controlled by the first orifice  42 . 
     The second fitting  52  which is disposed on the second end wall  28  of the vessel  20  is in fluid communication with one (a first) port of a second T fitting  54  through suitable tubing or a fluid line  56 . Another (second) port of the second T fitting  54  communicates through such tubing  56  to a sensing port  38 A of a leak tester  40 A which is undergoing a second portion (low pressure) testing with the present test apparatus  10 . Typically, the leak testing device or instrument  40 A will be the same leak testing device  40  previously tested with the first, smaller chamber  30  and the first orifice  42  although it should be apparent that it may be another leak testing device  40 A undergoing low pressure testing. Yet another (third) port of the second T fitting  54  communicates with a second orifice  62 . The second orifice  62  is sized to provide a defined and known leak rate of 20 standard cubic centimeters of air per minute at an initial pressure of 15 p.s.i. (103.4 kPa). Additionally, a second three way valve  64  is in fluid communication between the yet another (third) port of the second T fitting  54  and the source of compressed air  48 . The second three way valve  64  is preferably operated by an electric motor or solenoid  66  but may also be operated manually, pneumatically or hydraulically. The second three way valve  64  operates in the same manner and provides the same functionality with the second orifice  62  as the first three way valve  44  does with the first orifice  42 . 
     Referring now to  FIGS. 1 and 2 , a flow chart presenting the test procedure to determine the suitability (acceptability) of a piece of leak testing device or instrument  40  is illustrated and generally designated by the reference number  100 . Preferably, the test procedure  100  takes place at 65° F. to 75° F. (18° C. to 24° C.) ambient temperature and 40% to 70% relative humidity. The test procedure  100  begins with a start or initialization step  102  and moves to a step  104  which determines and/or sets the optimum leak test cycle time. Since these leak tests will be repeated many times, it is important and beneficial that a nominal total time be determined for one cycle of the leak test. That is, if the to-be-performed leak test nominally requires 30 seconds, setting the equipment cycle time at 5 minutes not only simply wastes equipment and personnel time but also delays the results of the testing. Contrariwise, if a particular leak test requires one minute to complete, setting the cycle time to any value less than that is obviously a grave error. In a following process step  106 , the desired leak tests with the three orifices  42 ,  62  and  72  providing three distinct leak rates, are performed. 
     In a decision point  108 , the three readings (volume per unit of time) on the leak testing device or instrument  40  under test are compared to the three known leak values (1.2 sccm through the first orifice  42 , 20 sccm through the second orifice  62  and 160 sccm through the bulkhead orifice  72 ). If the three values determined by the leak testing device  40  under test are within 5%, that is, within plus or minus 5%, of these values, the leak testing device  40  is determined capable or acceptable and the decision point  108  is exited at YES. If any of the three values determined by the leak testing device  40  under test are not within 5% of the predetermined values, the decision point  108  is exited at NO and the leak tester  40  is rejected in a process step  110  as being unacceptable and incapable of proper leak testing. 
     Next a process step  112  is executed wherein the data relating to the first orifice  42  and the second orifice  62  is evaluated with the bulkhead or third orifice  72  engaged or closed. Then a decision point  114  is entered and it is determined whether the leak testing device or instrument  40  under test has provided results that are within 5%, that is, within plus or minus 5%, of the leak rate of the first orifice  42  and of the second orifice  62 . If either of the results is not within this tolerance, the decision point  114  is exited at NO and the leak testing device or instrument  40  is rejected in a process step  116  as being unacceptable and incapable of proper leak testing. If the results are within the 5% tolerance, the decision point  114  is exited at YES and the leak testing device  40  is determined capable or acceptable. 
     The process  100  now splits into two parallel paths which are both performed and may be performed in either order before joining or returning to a single path. To the left is a process step  120 A which is referred to as a Type 1 high pressure study utilizing only the first chamber  30  and the first orifice  42 . This study is repeated fifty (50) times and when completed the values of Q1c and C gk1  are calculated. Q1c is simply the average of the fifty test runs and C gk1  is the gage capability index which is a measure of the accuracy and repeatability of a system, in this case the leak testing device  40 . C gk1  can be defined as 
               C   gk     =         0.1   ·   RF     -              x   _     g     -     x   m                3   ⁢     s   g                   where                 x   _     g     =       1   n     ·       ∑     i   =   1     n     ⁢           ⁢     x   i                   and               s   g     =         1     n   -   1       ⁢       ∑     i   =   1     n     ⁢       (       x   i     -       x   _     g       )     2                 
The Xg (bar) term is the average flow (Q1avg) or Q1c. Xm is the specified value for the orifice  42  for the chamber  30  in the test. RF is the process tolerance or the Q1c. Xi is the flow of the individual measurements (i is 1 through 50 measurements).
 
     A decision point  122 A is then entered which determines whether C gk1  is greater than 1.6. If it is not, the decision point  122 A is exited at NO because the leak testing device  40  under test is incapable of performing accurate high pressure leak tests repeatedly and is therefore unacceptable as indicated at step  124 A. If the value C gk1  is greater than 1.6, the decision point  122 A is exited at YES because the leak testing device  40  under test is capable of performing accurate high pressure leak tests repeatedly and is therefore acceptable. 
     Returning to the split in the process  100 , to the right is a process step  120 B which is referred to as a Type 1 low pressure study utilizing only the second chamber  50  and the second orifice  62 . This study is repeated fifty (50) times and when completed the values of Q2c and C gk2  are calculated. Q2c (Q2avg) is simply the average of the fifty test runs and C gk2  is the gage capability index which, as noted above, is a measure of the accuracy and repeatability of a system, in this case the leak testing device or instrument  40 . A decision point  122 B is then entered which determines whether C gk2  is greater than 1.6. If it is not, the decision point  122 B is exited at NO because the leak testing device  40  under test is incapable of performing accurate low pressure leak tests repeatedly and is therefore unacceptable as indicated at step  124 B. If the value C gk2  is greater than 1.6, the decision point  122 B is exited at YES because the leak testing device  40  under test is capable of performing accurate low pressure leak tests repeatedly and is therefore acceptable. 
     The process then moves to a common process step  130  which commences evaluation of the leak testing device or instrument  40  at varying room (ambient) temperatures. First, a process step  132  is encountered which heats the leak test vessel  20  to at least 10° F. (5.5° C.) above room (ambient) temperature. When the leak test vessel  20  is so heated, a process step  134  is entered in which three high pressure tests with the first (high pressure) chamber  30 , utilizing the first orifice  42 , with a one minute dwell or delay between each test, are performed. Finally, a second, similar process step  136  is entered in which three low pressure tests with the second (low pressure) chamber  50 , utilizing the second orifice  62 , with a one minute dwell or delay between each test, are performed. 
     Data is collected from each of the process steps  134  and  136  and, returning to the process step  134 , such data from the three high pressure tests is utilized to calculate Q tHp in a process step  142  and L capH in a process step  144 . Q tHp is obtained in the Type 1 high pressures tests of step  122 A and L capH is calculated from the equation
 
 L   cap =(cycle time/ C   gk )×[1+| Q   t   −Q   avg |/( Q   avg )]
 
where Q avg is the average of the fifty runs performed in steps  122 A and  1228 , Q t is the high or low pressure value referenced directly above and the cycle time is in seconds. Similarly, data from the process step  136  is utilized in a process step  146  to calculate Q tLp and, in a process step  148 , to calculate L capL. These four values (Q tHp, L capH, Q tLp and L capL) are utilized in a process step  150  to compare the performance of various instruments  40  which have undergone testing according to the just described sequence with the above-described equipment for selection purposes. That is, certain leak testing devices  40  or leak testing devices  40  from certain manufacturers may always prove to be superior and thus acceptable, others may prove to be acceptable under certain conditions and still others may never provide proper results and thus be unacceptable for any purpose or use.
 
     The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.