Leak testing is often viewed as a simple pass/fail test: pressure is applied, the result is measured, and the test specimen is then evaluated to determine whether it meets the requirements. In practice, however, reliable and repeatable results require careful consideration of numerous influencing factors—including temperature, stabilization time, test volume, and parameter settings.
Setting up a leak test process begins with understanding the test conditions, creating a stable environment, and demonstrating repeatable measurement behavior. Only then is the process finally calibrated using a calibrated standard leak.
The following six steps provide a practical approach to developing a robust leak test process using the Pressure Differential Method.
Step 1: Define your test specifications
Before configuring a leak test system, you must first clearly define the test requirements.
The most important parameters include:
The differential pressure method is one of the most commonly used techniques in industrial leak testing. In this method, the system measures the change in pressure over a defined period of time. This change in pressure is then converted into a leak rate using a calculation that takes the effective test volume into account.
The key difference is that the device directly measures the change in pressure—not the leak rate itself. The leak rate is calculated from the measured pressure curve, the measurement time, and the configured volume value.
Understanding this relationship is crucial, as an incorrectly configured volume value significantly affects the displayed leak rate, even though the actual measured pressure curve remains unchanged.
Step 2: Create a temperature-stable environment
Temperature is one of the most influential factors in leak testing.
During filling, the test air is compressed and heated. During evacuation, it can cool down. If the test specimen has not sufficiently acclimated to ambient conditions before the measurement, the resulting pressure changes may be mistakenly interpreted as a leak.
Even a temperature change of just 1 Kelvin can cause a pressure change of several hundred Pascals at a nearly constant volume. At approximately 1 bar absolute pressure and room temperature, this amounts to roughly 340 Pascals; at higher absolute pressures, the effect is correspondingly greater. Compared to the pressure changes of only a few Pascals frequently observed during leak testing, this influence is significant.
Reducing temperature effects
To improve stability, a heat exchanger and a buffer volume can be used. This brings the test air closer to room temperature before it reaches the test specimen.
Best practices include:
During manual leak tests, temperature variations can also be caused by the operator. Simply holding a test specimen for several minutes can heat it up enough to alter the measurement result. Consistent handling and waiting times are therefore essential.
Step 3: Determine the correct test cycle duration
Once the test environment is sufficiently stable, the appropriate cycle sequence is determined.
Before optimizing the cycle times, the entire test setup should first be verified. To do this, the test specimen is tested without the standard leak source activated, using deliberately generous fill, stabilization, and measurement times. This allows you to determine whether the test specimen, tubing, connections, and adapters are sufficiently leak-tight or whether a reproducible baseline leak is already present.
A typical differential pressure leak test consists of four phases:
1. Filling
During the filling phase, the system establishes the configured test pressure within the test specimen. The time required depends, among other factors, on the test pressure, test volume, supply pressure, tubing, and dimensional stability of the test specimen. It should be kept as short as possible but with sufficient margin to ensure that the test pressure is reliably achieved within the specified tolerance even under normal process fluctuations.
2. Stabilization
The stabilization phase is one of the most important parts of the testing process. Filling or evacuating the chamber causes thermal effects. In addition, test specimens that are not dimensionally stable may expand under pressure or settle mechanically. Before the measurement begins, these changes must have subsided to the extent that a sufficiently stable and repeatable pressure curve is obtained during the measurement phase.
If the stabilization time is too short:
A longer stabilization time generally yields a value that is closer to the thermally stabilized result. In production, however, this requirement must be balanced against the available cycle time. The goal is not the longest possible stabilization time, but rather the shortest time required to demonstrate a reliable and repeatable process. Repeatability alone does not prove that the thermally stabilized true value is being measured.
3. Measurement
During the measurement phase, the system evaluates the pressure change over a defined period of time. A shorter measurement time can reduce the cycle time but increases the relative influence of small fluctuations. The process must therefore be validated to determine a measurement time that provides a sufficiently large and repeatable signal.
As a practical guideline, a pressure change signal of approximately 20 to 40 Pascals caused by the boundary leak during the measurement phase can be helpful in many applications. This range is not a universally applicable acceptance criterion and must always be evaluated in relation to the test task, sensor range, test volume, and required measurement capability. For example, if a measured value fluctuates by 1 Pascal, this has a significantly greater impact on a total signal of 10 Pascals than on one of 30 Pascals.
4. Venting/Depressurizing
At the end of a positive pressure test, the test pressure must be safely relieved. In a negative pressure test, the test specimen is controlled to return to ambient pressure. The time required depends, among other factors, on the pressure level, test volume, and flow cross-sections. Before removing or opening the test specimen, there must be no overpressure or underpressure remaining that could endanger the operator.
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Step 4: Verify repeatability before calibration
Calibration should only be performed after the test process itself has stabilized. Before adjusting the displayed leak rate value, it must be verified that the system delivers repeatable results under comparable conditions.
An endurance test—in which multiple measurements are taken on the same test specimen—helps identify the following:
A common pattern is that the measurement results change gradually during the first few cycles and then reach a plateau. This may indicate that the pause time is too short. The test specimen does not fully return to its original temperature conditions between tests, but instead approaches thermal equilibrium over several cycles. After a longer interruption, the next measurement may therefore start at a significantly different level. Ideally, the results should fluctuate only slightly around a stable average value starting with the very first measurement.
Different materials and designs behave differently in this regard. A metallic, dimensionally stable test specimen often stabilizes more quickly, while plastic components or deformable test specimens may require significantly longer waiting times.
An endurance test is an effective practical preliminary assessment of repeatability; however, it does not replace any required measurement system analysis or process capability study. As a general guideline, a variation of significantly less than 10 percent of the relevant measurement signal may indicate good repeatability; however, the specific evaluation criteria required in each case are binding.
Keep in mind: The goal is not always the maximum possible accuracy, but rather an accuracy appropriate for the test task, which is ensured by repeatable and traceably validated results—as accurate as necessary, not as accurate as possible.
Step 5: Calibrate the test process using a calibrated standard leak
Once the process is stable and repeatable, it can be calibrated using a calibrated standard leak.
A calibrated standard leak provides a known leak rate at a defined test pressure and thus serves as a reference for the entire test process. During calibration, the parameterized volume value is adjusted so that the displayed leak rate matches the value of the standard leak.
Changing the configured volume value does not alter the physically measured pressure curve. It affects only the conversion of the pressure change into the displayed leak rate.
The volume value effective for the test process is critical. This includes not only the internal volume of the test specimen but also relevant portions of the tubing, adapters, and test equipment. The value can initially be estimated geometrically or by means of a volume determination and then calibrated using the calibrated standard leak.
Calibration using a calibrated standard leak also improves comparability between different test benches and production sites. However, different effective volumes, tubing configurations, and environmental conditions must still be considered and validated separately for each test process.
Step 6: Validate the process with and without the standard leak connected
The final step is to validate the entire process.
A robust leak test process should be evaluated under at least two conditions:
Without the standard leak connected
This verifies whether the test setup is sufficiently leak-tight and whether hoses, connections, seals, adapters, and the test specimen itself do not cause any impermissible or unstable background leakage.
With the standard leak activated
By activating the calibrated standard leak, the test verifies whether the system reliably and reproducibly detects and evaluates the defined limit leak.
The focus of validation is on repeatability and the reliable distinction between good and defective parts. A correctly calibrated single measurement is worthless if the process does not consistently produce the same result under comparable conditions.
The ultimate goal: A reliable process, not just a passed test
A successful leak testing process is based on a series of controlled steps:
This structured approach enables manufacturers to reduce misclassifications, improve process capability, and develop leak testing systems that function reliably even under real production conditions.
Safe and reliable leak testing with innomatec
With over 40 years of experience in industrial leak testing, innomatec supports leading manufacturers worldwide in the design, optimization, and implementation of reliable leak testing solutions. Our expertise helps companies achieve precise and repeatable results and improve process reliability across a wide range of applications.
Are you ready to optimize your leak testing process? Contact innomatec today to discuss your application and find the right solution for your testing needs.