When conducting air tightness testing, we will encounter a situation where the leakage value is negative even though the single positive pressure method is used for testing. What is the reason for this?

Before understanding this problem, we first need to know the purpose of air tightness testingTest principle, the product cavity is filled with gas of a certain pressure, and the gas supply is cut off after reaching the set pressure value. After the gas pressure is stabilized for a period of time, the pressure sensor monitors the attenuation of the pressure. If the attenuation exceeds the judgment threshold, the product is judged to be leaking.

 Figure 1-1Air tightness detection principle diagram

 Figure 1-2 During the gas inspection process, the pressure changes as the test stage progresses.

andleakage valueIt refers to the value obtained by subtracting the pressure after testing from the pressure after stabilization during the test.

Therefore, a negative value indicates that the ratio of the stabilized pressure to the test pressure is rising.Why does this happen?

Why is the leakage value negative?

According to the ideal gas equation of state（Equation of state of ideal gas）From the PV=nRT formula, it can be concluded that when a gas is in equilibrium, the pressure is related to the volume and temperature. From theoretical knowledge and past practice, we have summarized the following reasons.

1. Seal (or sealant) too high.If the position limit is not set properly, the sealing ring will not be pressed into place once during the test. The sealing ring will be squeezed all the time during pressure stabilization or testing, and the volume of the sealed cavity will become smaller. According toPV=nRT leads to P=nRT/V, V becomes smaller, that is, the pressure P value will increase, so a negative value appears.

 picture2-1 The sealing ring itself has a certain height

2. Mold limit is not done properly.During the product testing process, the inflation pressure causes the product to deform (mainly for bags or tough shells), or the silicone inside the seal to deform, causing the product to be tested toThe internal volume,becomes smaller during the testing phase and appears,in negative values.

 Figure 2-2 shows the product to be tested with a limiting column.

3. temperature change.The ambient temperature of the detector, the temperature of the mold itself, and other environmental temperatures increase, based onP=nRT/V As T becomes larger, the P pressure will also increase, resulting in a negative value.

4. Product deformed during testing.For example, on the back case of a watch, the cavity becomes larger when pressurized. During pressure stabilization or testing, the cavity becomes smaller due to its restoration to its original shape, and the pressure rises. If the rising pressure is subtracted from the standard end pressure, a negative value will appear.

5. Cylinder pressure is too high, causing the product to be pressed downwards during the test, squeezing the internal space of the product.V becomes smaller, causing P to become larger, resulting in a negative leakage value.

6. In the process of using the differential pressure detection method,Standard port air leakage, while the product under test on the other side has no leakage, which will also cause the leakage value to appear negative.

Figure 3-1 Principle diagram of differential pressure air tightness detection

Corrective measures

1. Put it on the air intake pipe of the tooling“Insulation tube”；

2. Don't let the cold wind/The air conditioner blows towards the detector; try to keep the external pressure consistent with the internal pressure in the test fixture;

3. If it is a sealing ring/The glue is too high and the mold support is not done well. You can add a suitable one to the test fixture during the assembly process.“Limiting column”for support;

4. Product deformation must be resolved using differential pressure detection, which can offset the impact of deformation;

5. These situations can be avoided by replacing the cylinder with a smaller bore;

6. If the standard port leaks, the leakage value can be restored to normal by replacing the standard parts.

The above are the common reasons for negative values ​​in our tests, for your reference only.


Air tightness knowledge

PV=nRT，ideal gas equation of state（Equation of state of ideal gas） ，Also known as the ideal gas law and the universal gas law, it is an equation of state that describes the relationship between pressure, volume, and temperature of an ideal gas in equilibrium.it was founded on boyle-Proposed by French scientist Benoit Pierre Emile Clapeyron in 1834 on the basis of Mallot's law, Charles' law, Guy-Lussac's law and other laws.

In the formula:p is the pressure (Pa), V is the gas volume (m³), T is the temperature (K), n is the amount of gas substance (mol), and R is the molar gas constant (also called the universal gas constant) (J/(mol.K)). 

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