Leak & Flow Test Temperature Compensation
When using a mass flow type leak detector to measure air leaks, the resulting measurement is composed of: The decrease in the number of air molecules (the leak) in the test volume, and; an apparent, or virtual leak, determined by the instantaneous rate of change of temperature of the test air (dT/dt). For our leak & flow test temperature compensation method, the temperature change is caused by the following factors:
- Adiabatic heating or cooling (see note 1)
- Part Temperature
- PreTest Type Temperature Compensation Technique
Adiabatic Heating or Cooling (see note 1)
When a part at atmospheric pressure and ambient temperature is pressurized, the incoming air compresses the residual air in the part causing that air to heat. The resulting air temperature, which is now warmer than the part, cools to the temperature of the part. This is usually the dominant thermal effect in the leak test. When a part is at atmosphere and then evacuated, the process is reversed. For upstream testing (see note 2), the speed at which the air temperature stabilizes determines the length of time for
the leak test. Incorporating volume fillers into the fixture can reduce this time. Volume fillers not only reduce the quantity of air being compressed but also reduce the response time of the system by decreasing the distance air molecules travel before reaching the fixture or test part walls.
When the test chamber is closed, an amount of ambient temperature air will be trapped in the fixture prior to pressurizing the test part. If the temperature of the ambient air is changing faster than the temperature of the fixture, the temperature of the entrapped air can be higher or lower than the fixture. The changing temperature of the entrapped air affects the leak measurement in the test volume. This effect is most pronounced in low-pressure systems (<~ 5 psig test pressure). A volume filler block decreases the time constant of the system to reduce this effect.
The rate at which a part reaches ambient temperature is proportional to the difference between the part temperature and ambient temperature. When a cool part is pressurized, the test air will cool down toward the temperature of the part. As the part temperature rises toward ambient, the test air temperature will rise with the part temperature. This temperature rise of the air will increase test air pressure and create a virtual leak that will mask the actual leak. The process is reversed for a part that is warmer than ambient.
For downstream testing with a cool part in a sealed test chamber, the effect is positive since this temperature effect will cause a pressure rise in the belljar and create a virtual leak.
PreTest Type Temperature Compensation Technique
A leak test measurement, made prior to the introduction of test pressure, is proportional to the virtual leak caused by temperature effects. When the leak is measured at the end of the test, the actual leak can be determined. Since temperature effects are proportional to absolute pressure, the pre test reading must be scaled by the ratio of the absolute pressure of the test divided by the absolute pressure during the pre test. There is an additional factor that scales the cooling dT/dt rate at the end of pre test to dT/dt at the end of
the test (note 3).
Measured Leak = Actual Leak ± Apparent Leak
Actual Leak = Measured Leak – (TCF x Pressure Ratio x Pre test Measured Leak)
- This effect occurs when the pressure in a thermally isolated volume is changed.
- This description refers to two methods of leak testing: Upstream in which leak detection occurs within the test part or in the pressurization path; and Downstream in which leak detection is measured outside the part under test, typically in a chamber that captures leakage from the part.
- TCF is the ratio of dT/dt at the end of the pre test time and at the end of the test time. A part that cools slowly like a cast iron block will have a factor close to 0.950, whereas a part like an aluminum oil cooler will have a factor close to 0.700.
- Does not require additional temperature sensors. Pre test involves measuring the flow effects of any temperature changes rather than inferring the effects from a temperature measurement.
- Is independent of the temperature profile across a part.
- Compensates for certain effects such as trapping.
- Works well in downstream testing. Both pre test and leak test are performed at the same ambient pressure and the pressure ratio is 1.0.
- May be influenced by previous tests. When a test vents at the end of the test cycle, a thermal transient remains in the fixture and in the test circuit. Since pre test is the first phase of the next test, these residual thermals in subsequent tests cause errors.
- Requires an additional pre test time period which will typically increase cycle time by 20% to 30%.
- The test reading must be scaled by the pressure ratio of the test. When testing upstream at higher pressures, small errors during pre test cause large errors in final reading. For this reason, pre test temperature compensation for upstream detection of small leaks at high pressure is difficult.
- Fixture stability - since the pre test period is the first phase of the test, the fixture needs to be stable sooner than if reading at the end of the test.
- The TCF factor needs to be adjusted when cycle times are changed.
Each application must be carefully evaluated but the following guidelines are generally valid:
Downstream testing with pretest
- Low Leak (< 0.5 ccm) applications: compensation limited to ± 5 deg F
- Large Leak (> 0.5 ccm) applications: compensation limited to ± 15 deg F
Upstream testing at low pressure (< 3 bar)
- Low Leak (< 5 ccm) applications: compensation limited to ± 5 deg F with pretest.
- Large Leak (> 5 ccm) applications: compensation limited to ± 15 deg F with either pretest or probe.
Upstream testing at high pressure (> 3 bar)
- Low Leak (< 5 ccm) applications: compensation limited to ± 3 deg F with probe
- High Leak (> 5 ccm) applications: compensation limited to ± 10deg F with either pre test or Probe
Consult Our Experts
Please contact InterTech for further information or help with your test application. We are happy to answer any questions you may have. Give us a call at 847-679-3377 or fill out the form and we'll be in touch as soon as possible.
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