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Influence of temperature on airtight detection

The weather is getting cold, and the coolness becomes more and more cold. The leakage test is also worried about the cold air. After a burst of cold air, if there is no treatment, the product qualification rate may increase. The reason is that the temperature will have a great impact on the leakage rate test, and the leakage characteristics of the leakage hole will change due to the temperature change. In addition, the temperature change during the test will also have a great impact on the leakage value.

1、 The leakage characteristics of the leakage hole vary with temperature

It can be seen from the hole leakage formula that the parameters affecting the leakage rate value by temperature include the air viscosity, the size of the hole diameter and the thermal expansion and cold contraction of the hole length. For products made of different materials, the thermal expansion and cold contraction of pore size and pore length are different. The same thing is that the viscosity of air is the same at different temperatures.

Compared with 20 ℃ and 0 ℃, the air viscosity at 20 ℃ is 1.81 * 10-2 centipoise, and at 0 ℃ is 1.72 * 10-2 centipoise. By comparison, it can be found that only the viscosity change of air will cause a deviation of about 5%.

2、 The temperature directly affects the leakage

For many metal parts, especially castings, their material properties and thermal conductivity are particularly good. The outdoor temperature has a large deviation from the indoor temperature in winter. Especially in the northern heating area and constant temperature workshop, the outdoor temperature is often below zero, while the indoor temperature is more than 20 degrees, or even higher. During the test phase, the air compressed by the pump from outside will be heated by the product.

Assume that the internal chamber of the product is 2000cc, the outdoor temperature is 0 ℃, the temperature rises 0.1 ℃ within 30s of the test time, and the test pressure is 0.5bar. According to the ideal gas equation of state, PV=nRT

It can be calculated that the pressure rise caused by temperature rise is 0.0005bar. According to the calculation formula of leakage rate, LR= Δ P*V/Patm * t

The leakage rate is reduced to 1.97sccm. For many products, the target leak rate value may be 2 sccm. After the reduction, the leakage rate is close to zero, and the product is basically qualified, with the qualification rate soaring. But this qualification is false, how to deal with it?

1. Calibrate frequently. Calibration is equivalent to peeling, which can smooth the influence of temperature. However, the actual outdoor temperature varies greatly at different times, and needs to be recalibrated at different times every day, while regular calibration will seriously affect the output.

2. Constant temperature workshop. Precise testing requires a precise environment, and the constant temperature workshop can ensure the consistency of product testing.

3. Air receiver. The compressed air directly extracted from the outdoor can get a certain temperature rise after passing through the air storage tank. The air receiver can also pass the compressed air of the pump smoothly to ensure the stability of the test.

4. Vacuum test. The vacuum test removes air from the product. There is no problem that the air in the product is heated by the product. Of course, the premise is that the product cannot be hot.

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