The weather is getting colder, and the coolness is becoming more and more chilly. The leak test is also concerned about the cold air. After a gust of cold air, if left untreated, the product's qualification rate may increase. It is because temperature has a significant impact on the testing of leakage rate. Changes in temperature can cause changes in the leakage characteristics of the leakage hole. In addition, temperature changes during the testing process can also have a significant impact on the leakage value.
1、 The leakage characteristics of leakage holes vary with temperature
From the leakage formula of the hole, it can be seen that the parameters affecting the leakage rate value by temperature include air viscosity, pore size, and thermal expansion and contraction of the hole length. For products made of different materials, the thermal expansion and contraction of pore size and length are not the same, but the viscosity of air is the same at different temperatures.
Comparing the conditions at 20 ℃ and 0 ℃, the air viscosity at 20 ℃ is 1.81 * 10-2 centipoise, while at 0 ℃ it is 1.72 * 10-2 centipoise. By comparison, it can be observed that a change in the viscosity of air alone can cause a deviation of approximately 5%.
2、 Temperature directly affects the leakage rate
For many metal parts, especially castings, their material properties have particularly good thermal conductivity. In winter, there is a significant deviation in outdoor temperature compared to indoor temperature. Especially in the northern heating areas and constant temperature workshops, the outdoor temperature is often below zero, while the indoor temperature is around 20 degrees or even higher. During the testing phase, the air compressed by the pump from outdoors will be heated by the product.
Assuming the internal volume of the product is 2000cc and the outdoor temperature is 0 ℃, the temperature rises by 0.1 ℃ within 30 seconds of testing, and the testing pressure is 0.5bar. According to the ideal gas state equation, PV=nRT
It can be calculated that the pressure increase caused by temperature rise is 0.0005 bar. According to the leakage rate calculation formula, LR=ΔP*V/Patm * t
Causing a reduction in leakage rate to 1.97 sccm. For many products, the target leakage rate value for testing may be 2sccm. After reduction, the leakage rate approaches zero, and the product is basically qualified, with a soaring pass rate. But these qualifications are all illusions, how should we handle them?
1. Regularly calibrate. Calibration is equivalent to peeling and can smooth out the effects of temperature. However, the outdoor temperature varies greatly at different times, and recalibration is required at different times every day. Frequent calibration can seriously affect production.
2. Constant temperature workshop. Precise testing requires a precise environment, and a constant temperature workshop can ensure the consistency of product testing.
3. Gas storage tank. Compressed air extracted directly from outdoors can be heated to a certain extent after passing through a storage tank. The air storage tank can also smoothly pass through the compressed air of the pump, ensuring stable testing.
4. Vacuum testing. Vacuum testing extracts air from the product, and there is no problem of the air inside the product being heated by the product, provided that the product is not hot.
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