The main possible reasons for the negative leakage value of air tightness leak detector are as follows:
1. The sealing performance of the tested workpiece is better than that of the standard workpiece
The leakage detector needs to display the leakage value (generally kPa). The direct pressure and differential pressure leak detectors need to be compensated and calibrated before displaying the leakage rate. In this process, some leak detection instrument manufacturers call the system zero point calibration, and some instruments become the system zero offset, The corresponding parameter values are required for the test results and the fixed leakage rate (this process is called calibration volume by some leak detector manufacturers, and calibration process by some leak detector manufacturers). Then using this method to correct the leakage detection instrument detection is closer to the state of no leakage, then there will be a negative value. This situation shows that the leak detector is not abnormal.
Therefore, automobile engines, lamps, cameras or parts customers provide standard parts without leakage. The standard workpiece is very important. It should be the one with almost no air leakage. Before the test, the workpiece and pipeline fixture system should be compensated to 0 value (even if there is no leakage in the workpiece and pipeline fixture system, there must be pressure change, because absolutely static air does not exist, and the air will also be affected by the gravity. For details, please refer to the gravity. Therefore, it can be seen that there is always a little pressure attenuation in the test phase of air tightness leak detection equipment).
IfAir tightness testing equipmentIn the process of testing, the tested workpiece is better than the selected standard workpiece (the pressure attenuation is smaller than the selected non leakage standard sample). Since we previously set the leakage value of the non leakage standard workpiece as 0kpa, the leakage value of the tested workpiece is displayed as a negative value.
Suggestion: select the standard parts with the smallest leakage and the best sealing performance as far as possible to do the air tightness test.
2. Ambient temperature change.
Ambient temperature refers to the change of the ambient temperature of the leak detector. Or test the temperature change of the workpiece itself. Or the pipeline temperature from the leak detector to the measured workpiece changes.
The change of temperature affects the leakage value. If the fixture and other conditions of the test workpiece remain unchanged, the temperature rise leads to the expansion of the gas, and the expansion of the gas leads to the decrease of the pressure change, and the test value is smaller than the original value. If the seal of the tested workpiece is better, it may appear.
Suggestion: in the middle of the day, there may be large temperature changes in the outside world, so it's better to heat the engine for 10 minutes before compensation calibration, or do calibration at noon. Try to keep the temperature of leak detector and workpiece consistent with the ambient temperature. The temperature difference in this aspect has been better improved by the built-in temperature compensation system of airtight leak detector of Shenzhen Harris automation technology
3. Change of product volume.
The volume of the tested product may change during the test of the air tightness tester. In the actual test process, we found that when testing the engine oil passage, the customer mistakenly injected oil into the oil passage, resulting in a smaller volume. At that time, there was a large negative value. The sudden change of volume may cause the test result to be smaller and negative. Volume change will lead to negative test results.
Suggestion: pay attention to observe whether the volume of the tested product is equal to that of the master workpiece.
4. The fixture used in the air tight leak detection equipment and the deformation of the pipeline. It may also cause negative leakage value
If the fixture and pipeline are deformed, the whole test volume may change. It may also lead to negative test results. For example, in the test process, there is no leakage in the workpiece and pipeline itself, but at this time, the clamp sealing rubber creep occurs, and the creep causes the internal pressure of the pipeline connected to the workpiece to rise, which may produce a negative value. Therefore, it is also very important to customize the special inspection tooling
Shenzhen Hailisi can provide professional seal leakage detection scheme, customized detection tooling and one-stop detection service