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molbox™ RFM™ OPERATION AND MAINTENANCE MANUAL
To run the leak test, select <1run>.
molbox RFM actuates internal valves to connect the upstream and downstream
RPTs together, so they are both measuring an equal system pressure. (see
Figure 10).
Close an isolation valve downstream or plug the exhaust of the flow system that
is being tested (downstream of the molbloc and the DUT if the molbloc/molbox is
being used to test another device).
Open an isolation valve upstream of the molbloc to allow the working pressure to
pressurize the system.
© 1998-2007 DH Instruments, a Fluke Company
Figure 10. molbox RFM
Internal Pneumatic Schematic
– SYSTEM LEAK CHECK – CHECKING OFFSET AND STABILITY
molbloc-S operation
FOR SYSTEMS WITH MASS FLOW CONTROLLERS (MFCS)
Keep in mind that the valves in most MFCs are not intended to provide a
complete gas shutoff and so they may not be suitable to close off the test
system. If an MFC is downstream of the molbloc and its downstream port
is open to atmosphere, it is best to close the system by connecting a cap to
the MFC outlet fitting or by connecting a shutoff valve downstream. If the
MFC valve is closed (most MFCs have normally closed valves) when the
operating pressure is applied from the upstream side, most of the gas will
be stopped by the MFC valve and will not immediately fill the volume between
the MFC valve and the downstream cap or valve. If this happens, the gas
may leak by the MFC valve to fill this volume during the test and cause a
pressure decay and an apparent system leak. The solution is to send a
setpoint signal to the MFC to open the MFC valve while pressurizing the
system. Then close the MFC valve (remove the setpoint signal) after the
system is pressurized to avoid heating of the test gas by the energized valve.
Page 52
1. High Isolation:
Open
2. Low Isolation:
Open
3. Bypass:
Open
4. Mirorange Bypass: Open
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