PRINCIPLE OF FLOW RATE LEAK TEST

The flow rate leak test is a test to judge the acceptance by applying constant amount of pressure and measuring the leak amount by flow meter for a product that has large leak rate or product that is used without being completely sealed. Laminar flow meter for volume flow rate and mass flow meter for mass flow rate are used for flow rate leak tests of FUKUDA.

Laminar flow meter

The laminar flow meter measures the flow rate from the difference of pressure between upstream and downstream. The pressure difference can be measured from pressure loss due to viscosity of the flowing fluid or from flowing distance. The characteristic of this method is good reproducibility and response since the differential pressure and flow rate are in proportion.

A laminar element has a structure such that many capillary tubes are bundled along a flow path, and fluid passes through each capillary tube. Since the d of it is small, the Reynolds number is extremely small.

Mass flow meter

A thermal flow rate sensor is used for the flow rate sensor applied to the mass flow meter. On the capillary tubes that work as a sensor, resistive elements with a high resistance temperature coefficient are attached at the upstream and downstream of the tube. When electric currents are applied to the elements, they generate heat. The temperatures of these elements are the same when there is no flow in the pipe. If the fluid begins to flow in the pipe under this condition, the upstream element is deprived of heat by the fluid and the downstream element receives the heat. A temperature difference of ⊿T occurs between the upstream element and downstream element, and the temperature difference ⊿T has a functional relationship with the mass flow rate of the fluid. Therefore, the mass flow rate is measured by the changes of the resistances of electric signals, and then by amplifying and correcting them.

MEASUREMENT METHOD OF FLOW RATE LEAK TEST

In the atmospheric pressure method, the work is positioned at the air pressure source side and the laminar at the atmospheric pressure side. It is used mainly for valve and other areas where the leak location is specified. The test pressure is supplied to the work and the leaked air passes through the laminar and it is exhausted into the atmosphere. The flow rate can be obtained by multiplying the pressure difference at the laminar by the laminar coefficient. The same laminar coefficient can be used when the test pressure is changed.

When there are multiple leak locations or when the leak location is not specified. In the line pressure method, the laminar is positioned between the air pressure source side and the work at the atmospheric pressure side. It is used for assembly parts of engines and other products. The test pressure passes through the laminar and is supplied to the work and is exhausted into the atmosphere. The laminar coefficient in this method depends on the test pressure.