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GENERAL PROCESS - REF. #GP008 | View PDF
THE CHALLENGE
A plumbing fixture maker needed to detect very low leak rates in their products, in the range of 1.0 sccm or less. Such levels are easily masked or confused by “noise” from seal and pipe leaks, elastic seal compressions, hose lengths and diameters, etc. A previous method employed to find leaks at this level involved a continuously fed pressure test; however, it created inaccuracies due to minute pressure fluctuations.
 THE BROOKS SOLUTION
A Brooks Mass Flow Meter (MFM) was considered for the application. However, there were still a few problems associated with this solution:
- Unless the gas flow rate is significantly above the full scale range of the MFM, a Mass Flow Meter sized to accurately measure a 1 standard cc per minute leak will not fill the large volume of the plumbing fixture quickly enough to be practical.
- The sensor cooling effect of the gas when operating an MFM above the full-scale range causes it to be inaccurate until thermal equilibrium of the sensor has recovered. The recovery time of a saturated thermal sensor can be 40 seconds or more.
- Filling the fixture at low flow rates causes the seals to creep into their final positions, causing pressure fluctuations.
- Expansion of the supply tubing is slow, and tends to continue after filling.
Subsequently, a large tube MFM bypass was employed that contains valves to take the meter out of the loop while the fixture is filling. Once the fixture is filled and the pressure is equalized, the bypass is closed and the MFM path is opened, to quickly measure flow between the pressure vessel and the plumbing fixture. This eliminates measurement inaccuracies for the following reasons:
- A bypass allows the fixture and pressure vessel to be filled very quickly through a large bore tube.
- The flow isn’t passing through the MFM during the fill cycle; therefore, sensor inaccuracy due to cooling is eliminated.
- The seals move quickly into their final position, eliminating “noise” due to creeping.
- Supply tubing fills and expands to its final position very quickly, eliminating “noise”.
 This method significantly improves the test results because it rapidly achieves and maintains a quiescent pressure condition during the test phase. Brooks provided rescaled thermal mass flow meters for measuring the very small leak rate. The MFMs were found to be extremely accurate and repeatable in this application. Brooks’ rescaled meters always show the same readings for the same test pieces, indicating their very high accuracy, repeatability, and reliability for this application.
Owing the extreme stability and sensitivity of Brooks Instrument thermal mass flow meters, any of the several different thermal meters that Brooks produces will perform well in this application. Two specific devices merit special consideration:
- The Model 5860S is a premier digital thermal device with exceptional performance, accuracy, and reliability.
- The Model Mf60i provides water- and dust-proof protection courtesy of its NEMA-4X packaging.
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