M O L B L O C / M O L B O X L O W M A S S F L O W C A L I B R A T I O N S Y S T E M A F M E T C A L
C O N F I G U R A T I O N S
I N S T A L L A T I O N A N D O P E R A T I O N M A N U A L S U P P L E M E N T
©2000-2008
DH Instruments, a Fluke Company
Page 18
Leak Check
There are two leak check options available through molbox1: a molbox leak check and a system leak check. The molbox leak
check is a diagnostic tool that is not used during normal operation. The system leak check is a tool that can be used regularly to
make sure that the molbloc and other molstic components are installed properly and are free of significant leaks. Only the
system leak check is available through COMPASS software.
The molbox1 system leak check tests for leaks by monitoring pressure decay in a closed, pressurized system. The test pressure
will be the outlet pressure of the molstic regulator (Figure 1, B), 76 psia (525 kPaa). Since it is only reasonable to leak test
system components in this way if they are normally operated at elevated pressures, the components downstream of the metering
valve are not included in the closed system during a typical system leak check. The system should be plugged just downstream
of the molstic metering valve using the VCR plug included in the AFMETCAL Configuration Kit. The plug is connected in
place of Fitting 1 from Figures 1 and 2.
Procedures can be developed to leak check DUTs and their connections by closing off the system downstream of the DUT
before running the system leak check. However, DUTs that are normally operated at lower pressures will usually fail the
pressurized leak check even they do not leak at their normal operating pressures. DUTs may also be damaged by the elevated
pressure if they are designed for low pressure use. In general, the components included in the closed system during a system
leak check should not include the DUT unless it is a DUT that is normally used with a back pressure near the system’s
maximum, 50 psig.
Following are steps for performing a typical system leak check with the AFMETCAL hardware setup:
Install the molbloc to be used.
Connect the VCR plug to the outlet of the VCR close coupling downstream of the metering valve using a Viton O-ring in
the VCR fitting.
Open the molstic metering valve.
If you are operating the molbox1 manually, select
[TARE], <LEAK CHECK>, <SYSTEM>
from the molbox front
panel so that the upstream and downstream pressure values are displayed.
If you are using COMPASS software, there are two ways to run a system leak check. When in
[RUN DUT]
or
[RUN
molbox]
, select the
[Leak Test]
icon, then click
<OK>
in the first message box. Wait for the “Make any leak test
preparations…” message. If you are running a test, a system leak check can be scripted into the beginning of the test.
The same “Make any leak test preparations…” message will be displayed after test initialization.
Open the molstic shut off valve.
The system pressure is displayed on either the molbox1 display or the COMPASS screen, depending on where you are
running the leak test. When the system pressure reaches the normal working pressure, close the molstic shut off valve.