6. TROUBLESHOOTING
Page 151
© 1998-2007 DH Instruments, a Fluke Company
Table 25.
Troubleshooting Checklist (Continued)
SYMPTOM
PROBABLE CAUSE
SOLUTION
Flow indication appears to be incorrect.
Reference pressure transducer(s)
(RPT) are inoperable or out of
calibration.
Use
[P&T]
to view pressure indications.
Recalibrate RPTs if necessary. (3.4.5
and 5.2)
Flow indication appears to be incorrect.
The gas selected is not the gas
flowing through the molbloc.
Select the correct gas. (3.4.2)
Flow indication appears to be incorrect
while using molbloc-S to measure air flow
Incorrect humidity ratio is being
used.
Reselect Air as the test gas and make sure
the humidity ratio W is entered properly. W
should be zero for dry air and should be
calculated from ambient conditions when
measuring ambient air. 3.4.2.2.
Flow indication appears to be incorrect.
The gas selected is the gas
flowing through the molbloc but the
molbloc has not been calibrated
for that gas.
Verify that the molbloc has calibration
coefficients for the flowing gas, have the
molbloc calibrated for the flowing gas if
necessary or accept lower accuracy flow
measurements. (1.2.4)
Flow indication appears to be incorrect.
Gas flowing through molbloc is not
pure.
Check purity rating of gas used; purge
molbox/molbloc if flowing gas was just
changed. If gas is a known mix, use the
flow
adder
and
multiplier
to correct for the
mix. (3.4.4.2, 3.5.6)
Flow indication appears to be incorrect.
The molbloc is not being used at
the correct pressure for its
pressure dependent calibration
type.
Check molbloc calibration certificate for
pressure dependent calibration type and
check operating pressure. (1.2.4 and 3.4.5)
Flow indication appears to be incorrect.
molbox RFM pressure and/or
Reynolds numbers are outside of
limits.
Observe pressure and Reynolds numbers
and adjust pressure/flow to be inside of
limits. (1.2.4 and 3.4.5)
Flow indication appears to be incorrect.
Flow through molbloc is in wrong
direction.
Check that flow through molbloc is in
direction of arrow on molbloc. Switch if
necessary.
Flow indication appears to be incorrect.
Leak in pressure lines from
molbloc to molbox RFM.
Check lines for leaks and correct if
necessary. (3.4.4.3)
Flow indication appears to be incorrect.
A flow
adder
/
multiplier
has been
incorrectly or inadvertently applied.
Check and correct if necessary. (3.5.6)
Flow indication appears to be incorrect.
molbloc calibration has changed
due to contamination.
Check molbloc for contamination. Contact
your
DHI
Authorized Service Provider if
liquid or particulate contamination is
detected.
Flow indication appears to be incorrect in
the low end of the range.
molbox RMF is equipped with
microrange but the option is not
on.
Turn on microrange option. (3.4.7)
Flow indication appears to be incorrect by
a constant offset amount.
molbox RFM needs to be correctly
“zeroed” (
tared
) at the current
operating pressure.
Tare
molbox RFM. Be sure to
tare
“upstream” if the upstream pressure is
constant and downstream if the downstream
pressure is constant. (3.4.4.1)
Flow indication appears to be incorrect
and molbloc is upstream and reading high.
Leak downstream of molbloc.
Check for and correct leak if present.
Consider using molbox RFM SYSTEM
LEAK CHECK function. (3.4.4.3.2, Leak
Check System)
Flow indication appears to be incorrect
and flow rate is less than 50 sccm.
Flow in system is not in steady
state; too much volume between
molbloc and test; unstable
pressure on molbloc.
Be sure flow is stable before reading,
reduce dead volumes, install precision
regulator.
Unable to make molbloc-S flow
measurements over full expected flow
range.
MFC or flow control valve is
connected dowsntream of molbloc-
S.
Valve or regulator used to control flow
through molbloc-S must always be
upstream of the molbloc-S. When operating
an MFC with molbloc-S, the MFC must
always be upstream (3.1.2)
