What Causes Unstable Helium Leak Test Results?
Helium leak test results can become unstable because of high helium background, fixture or seal leakage, insufficient evacuation, changing test pressure, temperature variation, calibration issues or inconsistent test conditions. An unstable reading does not automatically mean the leak detector itself is faulty.
At Access Technology, our team supports manufacturers by reviewing the leak detector together with the test fixture, vacuum system, helium supply, controls and actual test sequence before recommending the next technical action.
What Does an Unstable Helium Leak Test Result Look Like?
An unstable helium leak test result usually appears as a reading that drifts, changes between repeated tests or does not return to a consistent baseline.
Common signs include:
- Leak rate changes between repeated tests
- Reading drifts instead of stabilising
- Helium background remains high
- Pass/fail result changes for the same part
- Detector takes longer to settle
- Zero value changes between cycles
- Results differ after helium spraying
- Results change when the fixture or seal position changes
These symptoms can come from the test system, environment, component or test procedure, so the detector should not be assessed in isolation.
What Are the Main Causes of Unstable Helium Leak Test Results?
The most common causes are changing helium background, fixture leakage, insufficient pump-down, inconsistent test pressure, temperature changes, trapped helium, poor vacuum conductance, calibration variation and inconsistent test sequencing.
1. High Helium Background
High or changing helium background is one of the most important causes of unstable leak-rate readings.
Possible sources include:
- Excessive helium spraying
- Helium accumulating around the test area
- Previous test cycles
- Poor ventilation
- Helium migrating through the vacuum system
A high or changing helium background can affect the measured leak-rate signal because helium reaching the detector may come from the surrounding test environment as well as from the component under test.
For a broader explanation of the test principle, see how helium leak testing works.
2. Fixture or Seal Leakage
The measured helium may come from the test fixture rather than the component being tested.
Possible leakage points include:
- O-rings
- Gaskets
- Clamping surfaces
- Fixture connections
- Hoses and fittings
- Damaged seals
- Vacuum connections
Inconsistent clamping can also change the sealing condition between cycles. If the fixture does not return to the same mechanical condition each time, the measured leak rate may change even when the component is unchanged.
3. Insufficient Pump-Down Before Measurement
If measurement starts before the test system reaches a repeatable vacuum condition, the leak-rate reading may continue to change during the measurement window.
Residual gases may still be leaving the system, background may still be decreasing and vacuum pressure may still be changing.
We therefore review whether the system has reached a repeatable vacuum condition before the leak-rate value is used for comparison or pass/fail judgement.
4. Test Pressure Is Not Consistent
Changes in test pressure or pressure differential can change the measured leak rate.
Possible sources of variation include:
- Helium supply pressure
- Component test pressure
- Regulator behaviour
- Pressure timing
- Pressure decay during the cycle
- Different pressure conditions between tests
The appropriate test pressure depends on the component, test method and defined acceptance criteria. For repeatable results, the relevant pressure conditions should be controlled consistently from one test cycle to the next.
5. Temperature Changes During Testing
Temperature changes can affect precision helium leak testing through changes in seals, fixture dimensions, gas conditions and calibrated leak behaviour.
The effect may be more noticeable when the test requirement is very sensitive or when components move between significantly different temperatures before testing.
We therefore consider whether temperature conditions are sufficiently consistent for the required leak-rate limit and test method.
6. Helium Is Trapped in O-Rings or Test Components
Helium can remain temporarily trapped in elastomer seals, porous materials, fixture surfaces or internal cavities after a previous test.
Possible sources include:
- O-rings
- Elastomer seals
- Porous materials
- Internal cavities
- Fixture materials
- Previous helium exposure
If the fixture or part has not returned to a similar starting condition before the next cycle, repeated tests may produce different readings.
7. Poor Vacuum Conductance or Restricted Connections
Narrow hoses, long connections, restrictive fittings, small valves or restricted fixture passages can slow the movement of helium between the test component and the detector.
This can increase response time and make the measured signal slower to stabilise.
Where vacuum-side restrictions are suspected, we also review the connection arrangement and related vacuum conditions. Our article on how vacuum piping affects pump performance explains the broader effect of piping and restrictions on vacuum-system performance.
8. Calibration or Zeroing Is Not Repeatable
Calibration and zeroing need consistent starting conditions if test results are expected to be comparable.
Variation may occur when:
- Zeroing is performed at different background levels
- Calibration starts before the system stabilises
- The calibrated leak condition changes
- Temperature changes between calibration and testing
- The test setup is altered
- Different procedures are followed between operators or shifts
Calibration procedures should follow the leak detector manufacturer’s instructions and the requirements of the actual test process.
9. Test Sequence or Automation Timing Changes
In an automated leak test system, repeatability depends not only on the detector but also on whether valves, sensors, pressure steps and measurement timing occur consistently from one cycle to the next.
Important sequence variables can include:
- Evacuation time
- Valve opening and closing
- Helium introduction
- Pressurisation time
- Dwell time
- Measurement window
- Venting
- Fixture release
- PLC sequencing
Small timing changes can mean that one measurement is taken after the system has stabilised while another is taken too early.
Through our automation and material handling solutions, we can also assess how leak test equipment interacts with valves, sensors, PLC controls and production sequencing.
Is the Helium Leak Detector Always the Problem?
No. An unstable leak-rate reading can come from the test fixture, seals, vacuum condition, helium background, test pressure, calibration, temperature or automation sequence.
| Area | What We Check | Why It Matters |
|---|---|---|
| Leak detector | Background, calibration, signal stability | Confirms whether the instrument is operating consistently |
| Test fixture | Seals, clamps, connections | Fixture leakage can appear as component leakage |
| Vacuum system | Pump-down, valves, conductance | Unstable vacuum can affect measurement conditions |
| Helium supply | Pressure, concentration, delivery | Changing tracer conditions can change the result |
| Test sequence | Timing, PLC, measurement window | Different timing can produce different readings |
The detector should therefore be assessed as part of the complete leak test system rather than as an isolated instrument.
For manufacturers that require equipment and integration support, we provide helium leak testing equipment in Malaysia together with vacuum-system, fixture, control and technical integration support according to the application.
Why Do Results Change Between Repeated Tests on the Same Part?
Repeated tests can give different results when the test setup has not returned to the same starting condition before each cycle.
Possible reasons include:
- Trapped helium
- Fixture or seal movement
- Changing background
- Different evacuation time
- Temperature or test-sequence variation
What Information Helps Us Diagnose Unstable Leak Test Results?
Accurate test information helps us identify whether the instability is related to the detector, fixture, vacuum system, helium supply or test sequence.
Useful information includes:
- Leak detector make and model
- Test method: vacuum or sniffer
- Required leak-rate limit and observed readings
- Test pressure
- Helium pressure or concentration
- Fixture design
- Seal type
- Component volume
- Vacuum level, evacuation time and measurement window
- Test cycle sequence
- Background reading
- Calibration method
- Recent maintenance
- Photos or drawings of the fixture and connections
- PLC or alarm information for automated systems
If the test method itself is still being evaluated, our guide on vacuum vs sniffer helium leak testing explains how the two approaches differ in sensitivity, test pressure, leak location and automation requirements.
For new systems, our guide on information needed for helium leak testing system selection also explains the application details manufacturers should prepare before equipment selection.
Helium Leak Test Troubleshooting Guide
The following table provides a practical starting point for identifying where unstable results may be coming from.
| Symptom | Possible Area to Check | Why It Matters |
|---|---|---|
| Leak rate drifts upward | Helium background, trapped helium | Residual helium may still be entering the detector |
| Leak rate changes between cycles | Fixture, seals, test sequence | Starting conditions may not be repeatable |
| Background remains high | Test area, previous helium exposure | High ambient helium can mask or distort small leak signals |
| Reading changes after reclamping | Fixture or seal | Connection condition may be changing |
| Detector takes a long time to stabilise | Pump-down, vacuum conductance | Test system may not have reached stable vacuum conditions |
| Pass/fail changes on the same part | Calibration, pressure, timing, fixture | One or more test conditions may be inconsistent |
| Result changes with temperature | Fixture, seals, calibrated leak condition | Temperature can influence precision measurements |
This table is a starting point only. The actual cause should be confirmed from the complete test setup and operating sequence.
How Can We Improve Helium Leak Test Repeatability?
Improving repeatability means making the test starting conditions and measurement sequence as consistent as practical.
Useful actions include:
- Control helium use and background
- Allow sufficient pump-down
- Keep test pressure consistent
- Maintain fixtures and seals
- Standardise calibration and test sequence
- Control temperature where required
These controls can help improve repeatability, but final test capability depends on the leak-rate requirement, equipment, component and actual application.
Need Help Stabilising Helium Leak Test Results?
If your helium leak test readings are inconsistent, our team can review the detector, fixture, vacuum system, helium background, controls and test sequence to identify the areas that should be checked. At Access Technology, we provide helium leak testing equipment, integration and technical support according to the actual test requirements and agreed project scope.
FAQ
The leak rate can change because of helium background, fixture leakage, trapped helium, changing vacuum conditions, test pressure variation, temperature or inconsistent test timing. The detector is only one possible source of instability.
Yes. High or changing helium background can add to the detected helium signal and make the measured leak rate drift or become difficult to stabilise.
Yes. Leakage from O-rings, gaskets, fittings, clamps or fixture connections can appear as component leakage. Fixture sealing should therefore be checked when readings change between tests.
The test setup may not have returned to the same starting condition because of trapped helium, fixture movement, background changes, different evacuation time or test-sequence variation.
No. Unstable readings can originate from the fixture, vacuum system, helium supply, seals, test pressure, calibration or automation sequence. The complete test setup should be reviewed before assuming the detector is faulty.
Conclusion
In summary, unstable helium leak test results can be caused by helium background, fixture leakage, seals, insufficient pump-down, changing test pressure, temperature, calibration or inconsistent test sequencing. The detector itself is only one part of the complete test system.
At Access Technology, our team supports manufacturers by reviewing the leak detector together with the fixture, vacuum system, controls and actual test conditions before recommending the appropriate technical action.