Flow Control for Air Duct Leakage Test

• A pressure controller sets the pressure applied to a device under test (DUT) and the gas flow is measured by a thermal mass flow meter to conduct an air duct leakage test
• A DUT may be everything, ranging from a small hole to a volume of 10 liters
• Eliminate pressure drop over the mass flow meter by keeping the pressure controller as close to the DUT as possible

An air duct leakage test can be applied to closed systems as well as open systems, temperatures and high pressures, to make the reaction steps as effective as possible and it is related to quality control: more and more ISO certifications require the documentation of quantitative leakage data.

Leak Test for Air Ducts using Flow Controller

The basic principle is simple: consider a device under test (DUT), which may be everything, ranging from a small hole to a volume of 10 liters. Apply a pressure by using a pressure controller, and measure the gas leakage using a gas mass flow meter. The use of the specific pressure controller and gas flow meter depends on the setting and measuring range.

Considerations for Air Duct Leak Testing

• Test to check or guarantee leak tightness
• Temperature stabilization

Conducting Air Duct Leakage Test

A practical realization of the measuring principle is as follows:
Place the DUT below a certain gas pressure as specified by the customer. The ‘leakage rate’ under that specific condition has to be known. Suppose this initial pressure is 5 bars, then we need to know what incoming flow is necessary to balance the outgoing leakage, in order to keep the pressure stable.

Pressure Controller and Flow Meter for Air Duct Leak Testing

A pressure controller sets the pressure and the gas flow is measured by a thermal mass flow meter. Even a liquid-related application can be tested in this way by performing gas flow measurements. In the case of testing air ducts, we use pressurized air delivered by the customer’s compressor. Normal air can be used as well, when water and oil have been removed prior to testing.

However, there are some catches. In order to eliminate the pressure drop over the mass flow meter, the pressure controller needs to be as close to the DUT as possible. This means the pressure controller and its valve need to be separated physically.

It is very common to test multiple DUTs in succession. To reduce the testing time and to improve the stability of the test, two shut-off valves are placed between the setting and measuring equipment and the DUT. Especially for cases with a very small leakage rate, a safe bypass has to be added to the setup in order to fill the DUT quickly. Using such a safe bypass avoids approving a defect product. The solutions to all these ‘catches’ are shown in the flow scheme.

Temperature stabilization prior to conducting a measurement is essential. For example, due to gas expansion a temperature variation of 4°F already results in a volume change of 0.7%. Any leaks smaller that this volume change cannot be measured when the temperature fluctuates.