DH Instruments COMPASS FOR MOLBOX, User Manual

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COMPASS

for molbox™ User’s Manual
© 1998-2003 DH Instruments, Inc .
Page 26
5.1.7.3 RUN THE TEST
Use [Run] , [Run Test] to start the test sequence.
If you are using manual flow control, COMPASS will prompt you to adjust the gas
flow for the given test point. Flow will be set automatically if your test is using the
<Other Controller> (see Section 9.5.6). Generally, the prover is bypassed while
setting the flow. Once you have allowed sufficient time for flow, pressure and
temperature to stabilize, you can initiate the test point.
COMPASS will display the Test Message: “Press <Start> to begin flow averaging”.
It is important the COMPASS averaging time and the prover stroke be synchronized.
Now you can initiate the stroke of your piston or bell prover. Once the prover
system begins taking data, press the
COMPASS “Start Avg” button. Just prior to
the completion of data taking by the prover, press the “Stop Avg” button in
COMPASS . The goal is for COMPASS to take data for as long a period as
possible, but only during the actual measurement stroke of the prover.
5.1.8 EXAMPLE #8
“How do I set up to calibrate, in units of mass flow, a device that intrinsically measures
volume flow and outputs proportionally to volume flow?”
Some devices (for example bell provers, piston provers, soap film meters, turbine meters)
intrinsically measure
volume flow and have an output that is proportional to volume flow .
However, the objective when the instrument is used is to measure mass flow . Volume flow
is converted to mass flow using the density of the flowing gas which requires knowledge of
the gas pressure and temperature. Some devices include a “flow computer” and/or other
accessories to measure the gas pressure and temperature and calculate mass flow from
volume flow. For example, many piston prover systems include hardware and software to
report mass flow. When this is not the case, the device output is proportional to volume flow,
or it outputs volume flow directly. For example, a turbine meter’s frequency output is proportional
to volume flow and most soap film meters output volume flow. If you would like to calibrate
such a device so that it can be used to measure mass flow,
COMPASS can do so. This requires
real time measurement of gas pressure and temperature at the DUT while calibrating it and
use of the DUT proportional gas density correction. The limitation is that then the mass flow
calibration will only be valid when the DUT is used in its defined “normal operating conditions”
(i.e., reference gas pressure and temperature conditions).
COMPASS DUT gas density corrections only account for the effects of changing density
on DUT readings and not for other, typically smaller, effects on the DUT that may occur when
operating conditions change. Users should attempt to calibrate at conditions that are close
to the DUT normal operating conditions to minimize the effect of possible non-density influences.
5.1.8.1 DUT DEFINITION SETUP
Set up the DUT record in a similar way to the DUT in Example #5 (Section 5.1.5)
with the following changes:
Select [Setup] , [DUT], [Range] tab and select the DUT flow unit. The unit will
be a mass flow unit (e.g., sccm, mg/s, mol/s).
Select [Setup] , [DUT], [Correction] tab and select the <Proportional> Gas Density
Correction Type. You must also enter the DUT’s normal operating conditions