In addition to the delay, the pressure differential across the flowmeter creates an uncertainty in the actual test part pressure. This problem is made worse by the fact that the uncertainty is greater for large leaks. For these reasons, it is essential to use a flow meter designed to operate with a very low pressure differential.
The recommended mass flow meter is the Hastings HFM 300. It is chosen for its ability to accurately measure flow to within 0.75% of Full Scale while operating with extremely low pressure differential. Whereas typical mass flow meters require 4 inH2O pressure differential to operate, the HFM 300 only requires 0.25 inH2O. For your leak test, 16 times less pressure differential means 16 times faster! This is demonstrated in the actual test case below.
Another important consideration when selecting the proper instrument is avoiding what is known as “fold-over”. This problem arises in some typical flow meters when they actually misreport a very high (over-range) flow as a normal flow and allow a bad part to pass. The HFM 300’s sensor design correctly interprets over-range flows; its output signal will not come back on scale. The HFM 300 will not report false acceptable readings.
Flow meter Full Scale range – 10 sccm
Volume of test part – 1 liter plus volume of tubing downstream of the flow meter
Leak rate – 10 sccm
Pressure differential of HFM 300 – 0.25 inH2O
Pressure differential of typical mass flow meter – 4 inH2O