Air Tightness Testing - Common Faults During the Test Process

Ask

Do different air tightness testing machines show differences in test values?

Answer

Air tightness testing is somewhat different from other types of testing. The results of air tightness testing are variable and are related to product deformation, temperature, product volume, fixture sealing, and other factors. Therefore, in air tightness testing, the same product tested with the same parameters may produce different results.

Solution:

1. Make a qualified sample piece. Test it on each machine using the sample, observe the pressure decay (Pa) in the test results, then apply a uniform compensation, defaulting to 0 Pa. The premise of this method is that each individual machine must have good repeatability.

2. By using a calibrated standard leak orifice, the pressure decay is converted into a leak rate. Simply put, each unit of pressure decay value in Pa measured by the tester corresponds to 0 sccm. This also allows the differences between testers to be unified.

Ask

ATEQ leak detector shows a negative value, CTS air tightness tester shows a negative value, COSMO and Innotech leak detectors show a negative value?

Answer

Reasons for negative values:

Because differential pressure testing is comparative, there is a reference port. A negative value means the reference port is leaking more than the test port, which is why a negative value appears. If the negative value is very small, it has little impact.

During testing, temperature variations may occur. (This refers to the temperature difference between the product and the ambient temperature as well as the compressed air temperature.)

Changes in test volume are mainly caused by unstable factory pressure, improper sealing materials, or inappropriate sealing methods.

If the negative value is very large, either the reference side is leaking, or the product deformation is significant, or the instrument is malfunctioning.

Ask

What factors affect air tightness testing and leak detection?

Answer

According to the ideal gas law equation pV = nRT, (P: test pressure, V: tested volume, n: amount of gas substance, T: gas temperature; R: constant)

We can see that the main factors affecting the repeatability of test results include the following aspects:

1. Temperature changes: The temperature of the test workpiece differs from the surrounding air temperature; due to external thermal influences, such as the master part (especially products sensitive to heat transfer), piping, fixtures, etc., heat is transferred into the test workpiece, causing temperature changes that lead to test errors.

2. Changes in volume

(1) Deformation of the test workpiece

(2) Creep occurs in the sealing material

(3) Deformation of the test tube: the test tube is relatively soft and expands under pressure, and the deformation coefficient is not exactly the same each time.

3. Other factors

(1) Damaged or dirty sealing ring

Recommendation: Replace the sealing ring

(2) Scratches or contamination on the sealing face of the workpiece

Recommendation: Wipe the sealing face of the workpiece or replace the test workpiece.

(3) External factors: such as personnel movement, floor vibration, significant changes in the working environment temperature, etc.

Ask

How to troubleshoot continuous failures in air tightness testing?

Answer

Step 1: Check whether the leak detector has any issues. If possible, use a new unit or a previously qualified standard sample for calibration to see whether it works normally.

Step 2: Check the instrument's exterior. First, check whether the air inlet pressure gauge at the rear of the instrument has changed. If it has, adjust the pressure back. Then check the test parameter settings to see whether any value has changed.

Fold the air tightness tester's test tube with a cable tie, then start the equipment to check for leakage after the test is completed. If the equipment shows no leakage, check whether the sealing ring of the test fixture is leaking. Start the equipment without placing a product to see whether the leakage value fluctuates significantly.

Ask

What are the test ranges for gross leaks and fine leaks in air tightness testing?

Answer

There are no specific standards for what constitutes a coarse leak, fine leak, large leak, or micro leak. Different products and different pressures naturally produce different results. Based on our company's experience, at 100 kPa pressure with a 10 s test, a pressure decay within 50 Pa is considered acceptable, 50–100 Pa indicates a micro leak, and above 100 Pa is a large leak.

Ask

Why does the pressure drop at the moment the air tightness tester switches from inflation to pressure stabilization?

Answer

Air leak tester: reasons for a sudden pressure drop at the moment of switching from inflation to stabilization: 1. The inflation time is insufficient, so the displayed value differs from the actual value. 2. The pressure inside the product is not evenly distributed; the pressure is displayed, inflation ends, but in fact inflation is still not sufficient.

Ask

Does inflation time affect air tightness?

Answer

Whether the inflation time has an impact depends on whether the corresponding test pressure is reached during inflation. If the test pressure is reached, there is no impact. However, a sufficiently long inflation time helps stabilize the test data.

Ask

What causes a leak value of 9.999 in air tightness testing?

Answer

It is likely that a COSMO 1866 series leak detector was used. When this data is displayed, it indicates a major leak during testing, and the leak result has exceeded the range.