Leak & Flow Test Detection Methods
- Pressure Decay Testing
- Differential Pressure Decay Testing
- Mass Flow Testing (InterTech Patented Technology)
- Helium Leak Testing
Pressure Decay Testing
In pressure decay testing, the part is pressurized, sealed, removed from the pressure source, and monitored for pressure changes over time. The pressure in the part is measured twice - at the beginning and end of known time interval. The leak rate can then be calculated from the change in pressure and the time over which is occurred. Although the relatively low costs for instruments and fixtures might initially seem to make pressure decay testing the least expensive of the dry air methods, there are hidden costs. Because pressure measurements are made at two different times, this method is especially vulnerable to outside interference such as changes in ambient temperature, drafts, test part deformity and seal creep. It is also inherently slower than a one-step process. These factors often mean added testing costs to achieve the required accuracies.
Differential Pressure Decay Testing
Differential pressure decay is a variant of pressure decay method that yields superior accuracy. In this method, a reference volume is pressurized along with the test part. Both are then isolated from the pressure source but connected by a differential pressure transducer. As air leaks from the pressure in the part drops, while the pressure in the reference volume remains constant. The transducer measures the pressure differential between the non-leaking referencing volume and the the part. Two pressure measurements are required over a time interval, and the leak rate is calculated from the change in pressure, and the time over which it occurred. Like the pressure decay method, this is an indirect measurement of leakage rate in which one must calculate the leak rate from the time and pressure data.
Differential pressure decay testing is also vulnerable to the same kinds of outside interference as the pressure decay method, such as drafts in the plant, ambient temperature changes and seal creep. The actual volume of test parts and test circuits must also be known in order to calculate leak rates. For this reason, differential pressure decay testing is usually not good fit for any application that requires detecting leak rates less than 1 sccm (standard cubic-centimeter per minute). Neither is is generally suitable for detecting very large leaks with flow rages greater to 100 sccm because the pressure drops too rapidly to allow accurate measurements.
As with simple pressure decay testing, instrument and fixture costs for this differential pressure decay leak testing method is relatively low, but this does not translate into lower testing costs in most applications because of the time interval required between two measurements. In typically assembly operations, pressure decay testing can add between 20 percent and 40 percent to overall assembly time per part.
Mass Flow Testing (InterTech Patented Technology)
InterTech patented mass flow leak testing is a one-step process that is often superior to various pressure decay methods in that it is less prone to adiabatic effects. It also yields a direct measurement of leak rate, instead of a leak rate calculated from pressure changes and time intervals. As in differential pressure decay, a reference volume is pressurized along with the test part. Both are then isolated from the pressure source but connected to mass flow transducer. As air leaks from the part, the pressure in the part drops, and air flows through the transducer from the reference volume into the test part. The leakage flow is measured instantaneously in sccm. A constant-pressure regulated aur supply line can also be used. For increased measurement accuracy, a bias leak can be added to the system to verify that the mass flow testing instrument is working properly.
A mass flow transducer employs temperature-sensitive resistors that measure the temperature of the air from a leak as it flows across them. These resistors are in two of the four arms of an electrical bridge circuit, and a small heating element lies between the two resistors. The leakage air flow crosses first one resistor, then the heater, then across the other resistor. Both resistors are exposed to the heater so, if there is no leak, the resistors will be at the same temperature. If there is a leak, the temperature of the first resistor will drop and that of the second resistor will rise, unbalancing the bridge and generating output voltage proportional to the mass flow. Once the instrument is calibrated correctly, it yields a direct measurement of the leakage rate in one step.
Mass flow sensors need to be specifically tailored to the leak test requirements of a given application. They also require automatic temperature compensation to ensure accuracy. When these requirements are met, mass flow testing can detect leaks as small as 0.008 sccm. The larger the part volume, the better mass flow test methods are when compared to pressure decay leak testing methods. The faster recovery times in mass flow testing and the ability to handle pressure cycling often make it the method of choice in a broad range of leak testing applications.
InterTech Patented Downstream Mass Flow Leak Test Technolgy
Downstream mass flow leak testing is used for difficult applications where there is a requirement for testing at pressures in excess of 150 psig up to 10,000 psig, short test times, measurement of small leaks, very accurate and repeatable results, and/or precise temperature compensation.
Helium Leak Testing
Helium mass spectrometers are the most costly instruments used for leak testing. Additional costs are involved for the helium gas, and these instruments typically require more maintenance than other air leak testing systems. Nonetheless, they are often the instruments of choice for demanding applications that require accurate measurement of leaks less than 0.00001 sccs (standard cubic-centimeters per second). For example, helium mass spectrometers are instrument of choice for any application that involves lethal gases.
In the helium testing method, the part to be tested is enclosed within, but isolated from, a vacuum test chamber that is connected with the mass spectrometer. The part is then pressurized helium and the test chamber is evacuated. If a leak exists anywhere on the part, helium will escape into the mass spectrometer, where its presence triggers a signal from the spectrometer electronics. There are several variations on this method, but this approach is the most frequently used in manufacturing applications.
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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