When the same product is tested twice, the results get lower each time—it’s not a problem with the airtightness equipment; it’s a flaw in the method.

In laboratories and on production lines, there is a phenomenon that has been a recurring headache for engineers:The Same Product, under the same environmental conditions, the same pressure, and the same test duration,After conducting several consecutive tests, the leakage value was highest on the first test, lower on the second, and continued to decrease on the third.The

The data looks something like this:

Test Rounds	Leakage value
1st Time	100 Pa
The 2nd	91 Pa
The 3rd	85 Pa
The 4th	80 Pa

When seeing these figures, most people's first reaction is: "Is it...Airtightness TesterPoor repeatability? Could the sensor be drifting?"

Don't rush to replace the equipment just yet. The real problem often isn't with the instrument, but rather withEach test did not start from exactly the same initial state.

I. Under Pressure, the Product Has Changed

To understand this phenomenon, we need to go back to the most basic principles of physics.

The essence of leak testing isObserve changes in pressure inside the sealing system over a period of time. Under approximately steady-state conditions, the relationship between pressure P, volume V, amount of gas n, and temperature T follows the ideal gas law:

PV = nRT

This means thatAny change in volume, temperature, or gas quantity will cause the pressure conditions to change accordingly.

The problem is:After the initial inflation and pressurization, the test lines and products are not “completely motionless.”. Elastic expansion, wall deformation, and deformation of the seal ring due to pressure may occur,This results in a slight change in the internal effective volumeThe

Let me give a simple example. Suppose a certain product’sThe initial internal volume is 100 mL, after the first pressurizationDue to slight expansion of the material, the effective volume has temporarily become 100.5 mL.. This change has virtually no impact on everyday use, but for airtightness testing requiring Pa-level precision,A volume change of 0.5 mL is sufficient to affect the results of the pressure measurement.

More importantly, after the first test is complete, the product does not instantly return to its completely initial state. The material is elastic and exhibits time-dependent behavior—It takes time for the stress to be released and for it to spring back into shape.The

This is the root of the problem:

The initial state at the time of the first test is no longer the same as the initial state at the time of the second test.

II. Rubber and Metal: Their Behaviors Are Completely Different

Different materials vary greatly in their sensitivity to this phenomenon.

Rigid metal parts typically have thick walls and high rigidity; when subjected to pressure, their volume changes very little, and they return to their original shape relatively quickly, resulting in relatively good stability during continuous testing.

Case Study on Repeatability in Airtightness Testing of Metal Sensors: An Error of ±1 Pa Over 11 Consecutive Tests

Plastic seals undergo more pronounced elastic deformation when pressurized and require a certain amount of time to recover; therefore, sufficient rest periods must be allowed between consecutive tests.

Rubber products are the type that requires the most attention. In addition to elastic deformation, rubber also exhibits significantViscoelasticity—Their deformation recovery process is time-dependent; they do not return to their original shape immediately after pressure is relieved. Sealing rings, hoses, diaphragms, and flexible shells all fall into this category.

That is precisely why,In continuous testing of rubber and plastic products, the pattern of “a high initial reading, followed by a gradual decrease, and then stabilization” is the most typical.. This isn't due to equipment instability; rather, it is a true reflection of the material's physical properties.

III. Since the pressure wasn't fully released, the second test was already "incorrect."

This is the crux of the matter.

Suppose that after the first test is completed, the product is not sufficiently depressurized before proceeding directly to the second test. In this case, the following conditions may exist simultaneously inside the product:Residual positive pressure, incomplete recovery of wall deformation, residual internal stresses in the material, and a gas distribution pattern different from that of the first cycleThe

Under these circumstances, the pressure change detected by the instrument during the second test is not “A Simple Leak" is not caused by that, but rather by:

Instrument reading = Change caused by actual leakage + Change caused by the product's own condition

By the second and third times, after the product had been pressurized multiple times, the material deformation gradually tended toward “Saturated", and as subsequent changes in state become increasingly small, the additional signal superimposed on the leakage value also becomes increasingly weak—so the data appears as "The more we test, the lower the numbers get".

Conducting repetitive assessments in this manner is equivalent toCollect data under different experimental conditions and use it to demonstrate the instrument's stability.. The method itself is flawed.

IV. High sensitivity acts as a magnifying glass for problems, not their source.

There is a common misconception here that needs to be corrected.

Many engineers believe that,The more sensitive an instrument is, the more likely it is to malfunction. In fact, the opposite is true.

The instrument's high sensitivity means that it can not only detect actual leaks but also capture subtle changes within the test system itself—Tightness of workholding fixtures, piping conditions, temperature fluctuations, product deformation, residual pressure. These are all real physical variables that have been accurately recorded by high-precision instruments.

This isn't a problem with the instrument; on the contrary, it shows that the system is sufficiently sensitive. What we really need to do is get these non-leak-related factors under control, rather than replacing the unit with a “Slow to react", Low-precision devices that cannot detect these variables are merely fooling themselves.

Engineering with Dedication and Integrity JC-C10005 Differential Pressure Airtightness TesterFor example, the logic is as follows:Inflate → Equalize pressure → Detect pressure changes → Calculate leakage → Determine whether it passes inspection. Throughout the entire process, the pressure difference between the test end and the reference chamber serves as the key criterion for evaluation. This mechanism places high demands on the consistency of the test environment—and that is precisely why,It is the standardization of the method that determines the validity of the data.

V. How to Conduct Scientific Continuous Testing

The following is a standard process that has been proven effective in practice.

The first step is to completely relieve the pressure.

After each test, ensure that the internal pressure of the product returns to the specified initial state,We can't carry over any residual pressure into the next round.The

The second step is to give the product enough time to recover.

This step is particularly important for rubber, plastics, thin-walled parts, and flexible structures. There is no one-size-fits-all answer to how long this will take,This must be determined through prototype validation.(math.) genusThere are significant differences in recovery characteristics among different materials, structures, and pressure conditions.The

The third step is to maintain a consistent temperature

The product may experience a temperature rise after the first test; if another test is conducted immediately afterward,Temperature differences can directly affect pressure readings.. In precision testing scenarios, this factor cannot be ignored.

Step 4 is to ensure that the tooling is in a consistent state.

For each test,The clamping force, sealing position, and connection status must all be consistent.. The variations you measure must be attributable to the product, not to fluctuations in the production equipment.

Step 5 is when you conduct a follow-up assessment

Provided that the initial conditions are consistent in the first four steps,Conduct at least 10 consecutive tests on the same product, and record the mean, range, and standard deviation.—Only data like this can truly reflect the equipment's repeatability.

If you want to quickly determine whether “Is it an instrument issue or a method issue?", you can make this comparison:

Test Methods	Operation Description	Data Characteristics	Determination
Method A (Direct Continuous)	After each session, do not relieve pressure or wait; start the next one immediately.	100→91→85→80 Pa—continuous decline	Don't doubt the equipment just yet.
Method B (Standard Process)	Each time, fully depressurize and wait for the product to return to its initial state before testing.	82→81→82→82 Pa—stable fluctuations	Equipment repeatability is normal

Comparing the data from the two methods often directly reveals the source of the problem.

VI. A Simple Controlled Experiment

If you want to quickly determine whether “Is it an instrument issue or a method issue?", you can make this comparison:

Test Methods	Operation Description	Data Characteristics	Determination
Method A (Direct Continuous)	After each session, do not relieve pressure or wait; start the next one immediately.	100→91→85→80 Pa—continuous decline	Don't doubt the equipment just yet.
Method B (Standard Process)	Each time, fully depressurize and wait for the product to return to its initial state before testing.	82→81→82→82 Pa—stable fluctuations	Equipment repeatability is normal

Comparing the data from the two methods often directly reveals the source of the problem.

VII. Procurementairtightness meter...this is the point that's most easily overlooked

When selecting a model, most people ask only one question:What is the accuracy?

However, accuracy is only a static parameter. What truly determines whether a piece of equipment can be used on your production line isData stability during continuous testing under standardized test conditionsThe

The best way to verify a product before purchasing it is to ask the supplier to conduct a complete repeatability test on-site:The same product, the same test fixture, the same pressure, the same test duration, the same environment, and an identical pressure relief and recovery procedure, with more than 10 consecutive tests...look at the distribution of the actual data—rather than just focusing on a single, best-case result.

A machine that is truly suitable for mass production linesAir tightness testing equipment...must be able to withstand this test.

Conclusion

When you see a steady decline in test data, the first question you should ask isn't “Is the instrument accurate?", rather:

Does every test start from exactly the same initial state?

For products such as rubber, plastic, and thin-walled structures, the elastic deformation, volumetric change, and material stress recovery that occur after the first pressurization are all genuine physical processes. If the next test is started immediately without sufficient depressurization and recovery, systematic data drift is inevitable.

First, standardize the testing methods; then evaluate the instrument's performance. This is the most basic—and most easily circumvented—principle in the verification of repeatability for airtightness testing.

Shenzhen Jingchenggongke Technology Co., Ltd.(JCGK), as a leading company in China’s airtightness testing industry, has been deeply involved in the sector for over a decade and has seen countless cases where improper testing methods led to false positives of “Instrument Malfunction"I've also encountered quite a few cases where customers purchased equipment from low-cost suppliers and then found themselves with no one to turn to for help—"Those competitors who only sell equipment and don’t provide technical support often disappear without a trace once problems arise.The

In contrast, Jingcheng Engineering (JCGK) offers more than just a singleAirtightness Tester, but ratherProduct Analysis + Test Method Analysis + Tooling Analysis + Instrument Validationa comprehensive solution, as well as after-sales support throughout the entire lifecycle of the equipment.

If you are facing testing challenges that your current supplier cannot resolve, please feel free to contact Jingcheng Engineering at any time for a free technical consultation.. This is because the foundation of data quality lies in the consistency of the entire testing system—not just the specifications of the instrument itself.