molbox™ RFM™ OPERATION AND MAINTENANCE MANUAL
© 1998-2007 DH Instruments, a Fluke Company
Page 152
Table 25.
Troubleshooting Checklist (Continued)
SYMPTOM
PROBABLE CAUSE
SOLUTION
UNABLE TO MAKE MOLBLOC
-
S FLOW
MEASUREMENTS OVER FULL EXPECTED FLOW
RANGE
.
molbloc-S back pressure too high
The downstream pressure on molbloc-S
must always be significantly lower than the
upstream pressure to maintain critical flow.
When upstream pressure is reduced, the
back pressure ratio may become too high to
make valid flow measurements. (3.1.2)
Unable to make molbloc-S flow
measurements over full expected flow
ranges.
Flow units use reference
temperature other than 0° C.
The molbloc-S flow range table displays
flow ranges expressed in slm @ 0° C. If
you use volumetrically based flow units with
another reference temperature, the
minimum usable flow value may be
increased by up to 9% for a given molbloc-S
upstream pressure. (1.2.4.2.1, 3.4.3.3)
Unable to reach molbloc-S maximum flow Pressure regulator supplying
molbloc-S set too low.
If you are controlling flow with a control
valve downstream of a fixed pressure
regulator, you must have sufficient supply
pressure to reach the maximum desired
molbloc-S flow. Be sure not to exceed
molbox RFM maximum operating pressure.
(2.3.4, 3.6.3)
Flow indication appears to be incorrect
and flow unit is a volumetrically based
mass flow unit (sccm, scfh, etc.).
Reference temperature or
compressibility consideration in
volumetrically based units is
inconsistent.
Set units correctly for consistency between
values being compared. Be sure to select a
unit starting in “u” for volumetrically based
mass flow units if reference temperature is
not 0
°
C. (3.4.3)
Flow indication appears to be incorrect
and there is a K following the flow unit on
the display.
A K factor is being applied
incorrectly or inadvertently.
Turn off or correct K factor. (3.4.1)
Flow indication appears to be incorrect
and you are using AVERAGING function.
Averaging period is wrong and
does not correspond to reading
period of the device you are
comparing with.
Adjust averaging period and/or be sure to
synchronize averaging period of the molbox
RFM and the device you are comparing to.
(3.4.6.2)
Flow indication is negative.
Flow in your system is reverse
from what you expect; molbloc
upstream and downstream
pressure lines are switched.
Check system and correct flow; check
pressure lines and switch if necessary.
Pressure transducer (RPT) indications are
out of range.
Incorrect RPT calibration
coefficients; RPT(s) has (have)
been overpressured.
Correct calibration coefficients; have
transducers replaced if damaged. (5.2)
molbloc-S Tare does not execute as
expected
Expectation of tare function
operation is incorrect for
molbloc-S.
Normal molbox RFM operation. The
molbloc-S tare function has a different
purpose and method of operation from
molbloc-L tare and does not require saving
a fixed tare value. 3.4.4.1.2
Cannot
tare
molbox due to excessive
tare
value.
One or several RPTs are
defective; RPT calibration is bad;
there is a leak inside molbox RFM
causing a differential pressure.
Check performance of RPTs; check
calibration of RPTs; leak check molbox
RFM. (5.2 and 3.2.4.3.1)
Leak check does not indicate pass/fail in
molbloc-S operation.
Improper expectation for
molbloc-S leak check.
Normal molbox RFM operation. molbloc-S
system leak check function in molbox RFM
does not have pass/fail criteria assigned as
molbloc-L leak check does. (3.4.4.3.2)
Measured pressure display has too
much/not enough resolution.
Resolution setting needs to be
changed.
Use
[RES]
to change resolution setting.
(3.4.9)
Resolution is greater than the resolution
set by
[RES]
and/or resolution is changing
in the lower part of the molbloc range.
molbox RFM has the microrange
option and the option is ON which
causes resolution to increase by a
factor of 10 when the differential
pressure across the molbloc is
< 12.5 kPa (1.8 psi).
Operation is normal if microrange is on.
Change resolution setting and/or turn
microrange off if desired. (3.4.9 and 3.4.7)
