Leak Test of Sensors with the Test Medium compressed Air

Transcription

1 DOI /sensor013/B7.3 Leak Test of Sensors with the Test Medium comressed Air Dr. Joachim Lasien CETA Testsysteme GmbH, Marie-Curie-Str , 4071 Hilden, GERMANY, Abstract As sensors are used in the diverse industrial alications this comonent has to fulfill a variety of requirements. This includes leak tightness against dirt and moisture, whose requirements are secified in the IP rotection classes. Penetration of liquids can cause serious malfunction of the sensor and the associated electronics. Thus, the sensors have to be leak tight. The leak testing of the sensors as an end-of-line test integrated in the roduction line is of articular imortance. The basics of leak testing and imortant ractical asects of the leak test rocess are treated. Key words: leak tightness, IP rotection classes, leak test, end-of-line test, hood test. Test Medium comressed Air The definition of the IP rotection classes describes how a laboratory test has to be erformed. But of course, such a test cannot be integrated as 100%-inline rocess in the roduction line. Based on the results of the laboratory exeriments the requirements for a corresonding leak test in the roduction line have to be worked out. Ideally the test ressure and the ermissible leak rate can be defined. Deending on the leak rate a suitable test medium and test rocess have to be chosen to fulfill the technical requirement. Table 1 shows some of the most common used test media and their secific features. Widesread is the use of the test medium comressed air for the 100 % in-line leak testing in industrial roduction lines. The test medium comressed air can be used down to an air leak rate of 10-3 mbar*l/s (deending on the test art). If a sensor has to be waterroof an air leak rate of 10 - mbar*l/s (= 0,6 cm³/min) is commonly assumed. Tyical examles of automotive sensors, which are tested with comressed air are oil ressure sensors, ABS sensors, level sensors. Tab. 1. Overview about leak test media (1 mbar*l/s = 60 cm³/min). Test Medium Air Leak Rate Method Remarks Water > 10 - mbar*l/s Detection of bubbles Qualitative method Localisation of leaks Comressed Air > 10-3 mbar*l/s Measurement of ressure decay res. ressure rise (relative ressure sensor, differential ressure sensor) Quantitative method Easy to handle Deending on the volume Temerature sensitivity Integral method Localisation of leaks => Use of leak sray Hydrogen (Forming Gas) > 10-6 mbar*l/s Measurement of H -concentration Quantitative method No volume deendency No temerature sensitivity Sniffing: Localisation of leaks Helium > 10-9 mbar*l/s Measurement of He-concentration Quantitative method No volume deendency No temerature sensitivity Evacuation for small leak rates Sniffing: Localisation of leaks AMA Conferences SENSOR 013, OPTO 013, IRS

2 DOI /sensor013/B7.3 Test Method Pressure Decay Test for direct fillable Test Parts Test arts with small volumes, which can be filled directly, have often to be tested in a very short total test time by the use of comressed air. The overall test consists of the consecutive hases: filling, stabilizing, testing and duming. In the filling hase, the test art has to be filled to reach the test ressure. The stabilizing hase is necessary so that air disturbances (caused by the filling rocess and generated by the switching of the internal valves of the test device) can subside. In case of a ositive relative test ressure the air is comressed adiabatically during the filling rocess, which heats u the air. So, the temerature of the comressed air has to adjust to the temerature of the test art. In the testing hase, the rate of ressure loss is measured and comared with the ermitted tolerances. The testing hase must be long enough to generate a significant measurement value. In the testing hase mostly a very sensitive differential ressure sensor is activated to measure the ressure decay with high resolution. A stable measurement hase is characterized by the effect, that the leakageinduced ressure loss is roortional to time. Finally follows the duming hase (see Fig. 1). Fig. 1. Princile of the ressure decay test of direct fillable test arts. Leak Rate Formula The exected ressure decay in time in the test hase can be calculated by the use of the socalled leak rate formula. This is an aroximation for the leakage induced ressure decay in a stable test regime (eq. (1)): Pa QL [cm³ /min] t s V [cm³] eff Pa 60 s/min (1) where /t = Pressure decay in time V eff = Effective test volume This is the sum of the volumes of the test art, the adation, the measuring line and the measuring circuit inside the test device Q L = Leak rate (1 mbar *l/s = 60 cm³/min). The air escaes into the ambient atmoshere with an atmosheric ressure of arox Pa (absolute ressure) AMA Conferences SENSOR 013, OPTO 013, IRS

3 DOI /sensor013/B7.3 Test Method "Closed Comonents" for encasulated Test Parts Encasulated test arts, like most of the sensor tyes, cannot be filled inside with ressurized air. A tyical examle to demonstrate this test method is an encasulated hotoelectric sensor (see Fig. ). These tyes of test arts are laced in a hood which encloses the test art as closely as ossible. The hood is ressurized. The temoral ressure decay of the ressure in the test art is measured. This tye of test is called "closed comonent test" or "hood test". Fig.. Photoelectric sensor as an examle for a encasulated test art. Hereby occurs the following roblem: If the test art has a major leak (so-called gross leak), it is already filled with comressed air during the filling hase of the hood. In this case, only the tightness of the hood surrounding the test art would be measured. Therefore it has to be controlled in the first ste that the test art has no gross leak (gross leak test). After this the fine leak test is erformed by means of a ressure decay measurement (see above). The gross leak test is done in the following way (see Fig. 3): A reservoir volume, which is integrated in the test device, is filled to a ressure 1 and searated from the ressure regulator. Then the check valve of the internal reservoir volume is oened and the air is flooded through the measuring line into the hood. Hereby the ressure decreases to a lower ressure. The ratio / 1 is a measure for the volume which is filled. The ressure values are measured with a relative ressure sensor. Fig. 3. Princile of the leak test of encasulated test arts. The total test consists of two stes: After the gross leak test follows the fine leak test. If the test art has no gross leak, the following alies (eq. ()): V R () 1 VR VM VL VH VPe In case of a gross leak the internal volume of the test art is filled additionally and the following alies (eq. (3)) V R (3) 1 VR VM VL VH VPe VPi AMA Conferences SENSOR 013, OPTO 013, IRS

4 DOI /sensor013/B7.3 where V R = Reservoir volume V M = Volume of the measuring circuit V L = Volume of the measuring line V H = Volume of the emty hood V Pe = External volume of the test art V Pi = Internal volume of the test art (filled in case of a gross leak) These relationshis are a result of the ideal gas equation, assuming constant temerature (isothermal change of state). In the initial state, the volumes are at atmosheric ressure level, so that in these formulas, the ressures are to be used as ositive and negative relative ressures. Examle The outer volume of a sensor is 5 cm 3. The sensor is encasulated and has an internal volume of cm 3, which is filled in case of a gross leak. The hood, surrounding the sensor, has a volume of 10 cm 3. The test device has an internal reservoir volume of 45 cm 3, the internal measuring circuit has a volume of 5 cm 3. The measuring line has a length of 0.5 m and an inner diameter of 4 mm. This corresonds to a line volume of 6.3 cm 3. In this case, the ressure ratio is in case of a sensor without gross leak. If the sensor has a major leak, there is a ressure ratio of In case of a non gross leak sensor the fine leak follows. To erform the fine leak test at a test ressure of = 500 mbar, a ositive ressure of 1 = 681 mbar has to be set as the start value. Technical Features of the Leak Tester Fundamental hysical effects (temerature adjustment of the comressed air, disturbances in the comressed air) can hardly be influenced, but the test device can be otimized regarding the comonents which have a sensitive influence uon the test rocess. The distinction between a test art with gross leak and a test art without gross leak is more difficult if the fillable internal volume of the test art is quite small comared to the sum of the other volumes (see eq. (1) and ()). By the use of a relative ressure sensor for the measurement of the ressures 1 and a difference of at least about 0.0 between the gross leak ressure ratio and the non-gross leak ressure ratio is needed for a stable leak test rocess integrated in the roduction line. This can be imroved by the use of more sensitive ressure sensors (see below). The resolution and the total test time in the leak test of small-volume test arts can be otimized by the following rocedures: Use of a measuring line with a small inner diameter, rovided this does not hinder the filling rocess too much. If the measuring line has an inner diameter of 3 mm, the line volume can be reduced by 44% as an equal length measuring line with 4 mm inner diameter. Use of secial internal electrical valves that have an extremely low switching kick. This reduces the time for the relaxation of the air disturbances caused by the switching rocess. Reduction of the internal volumes of the test device (esecially measuring circuit volume). Use of a hood that encloses the test art as closely as ossible. Hereby the test art tolerances have to be taken into account. In ractice it is ossible, that the hood has just a surrounding distance of only 0. mm to the outer contour of the test art. Use of a much more sensitive ressure sensor (differential ressure sensor instead of relative ressure) to detect even finer differences in the ressure ratio / 1. Practical Alications High Seed Pressure Decay Test and Detection of Assembly Defects With such an otimized instrument like the leak tester, tye CETATEST 515 in the variant "closed comonents, high resolution" (see Fig. 4), the assembly of microarts can be tested, too. The volume which can be filled with air can be taken as a measure to detect assembly defects like a missing o-ring. In a test art, which has a fillable volume of 0 cm 3 and which can be filled directly, a volume difference of only 0.03 cm 3 can be detected by using a ositive relative ressure of 900 mbar This corresonds to the volume of an o-ring of 1 mm diameter and 1 mm cross section. Fig. 4. Leak tester tye CETATEST 515 for the leak test of encasulated test arts with small volume. AMA Conferences SENSOR 013, OPTO 013, IRS

5 DOI /sensor013/B7.3 The leak test of encasulated small volume test arts (external volume arox. 0,5 cm³) have been realized in the roduction line with a total test times in the range of 1 to s. This can result in several million cycles in one year in a roduction line working to full caacity. The internal comonents in the test device must be designed accordingly for a long service life. In the leak test of low-volume secial valves by the ressure decay leak test using the CETATEST 515 (in the variant "ressure decay high seed ) a total test time of just 750 ms has been realized. In this very short time a comlete leak test cycle consisting of filling, stabilizing, testing and duming is erformed. AMA Conferences SENSOR 013, OPTO 013, IRS