4. OPERATION
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© 2007 DH Instruments, a Fluke Company
the GFS Tools data files. These updates are known as
samples
. The rate of
sampling is set by recording a sample at a fixed amount of mass change
throughout the sampling period, so that only meaningful mass changes are used
for flow calculation. Since a fixed amount of mass change is used to define the
sampling interval, flow updates occur much faster at higher flows than at low flow
rates. This is a logical characteristic of a system that is based on measurement
of changes in mass.
4.1.4.3
AVERAGES AND BEST FITS
Each reference flow rate that is displayed during and accumulation represents
the system’s measurement of the average mass flow rate since the start of the
accumulation. The total accumulated mass and total elapsed time is used to
calculate mass flow. In an attempt to record new mass and time values as
frequently as reliable values are available, some instability of the displayed mass
flow and/or error of the device under test can be observed. This instability is
partially due to actual instability in the flow that is controlled and partially due to
limits in the system’s ability to read and synchronize mass, time and DUT output
readings. This instability falls well within the limits of acceptable performance of
the system, but it is clear that to achieve the best results that GFS2102 is
capable of delivering, some averaging of the real-time mass flow indications
should be done. When averaging is used, the resulting dynamic output is more
stable and overall repeatability is improved between accumulations.
GFS Tools uses a rolling average mass value setup with a user-definable
number of consecutive samples to be averaged. Although not recommended, if
the user would like to use a real-time mass value instead, the rolling averaging
time can be set to 1 sample (see Section 5.5.6.1).
GFS Tools also uses a rolling average flow setup with a user-definable number
of consecutive samples to be averaged. If the user would like to use a real-time
calculated flow instead, the rolling averaging time can be set to 1 sample (see
Section 5.5.6.1).
A best fit of the mass vs. time curve is available for flow and flow error calculation
in GFS Tools as well. This method uses a fit of all of the calculated flow values
during an accumulation instead of just a rolling average of just a few, and
appears to give the best repeatability possible, but the method is not as reliable
when AutoZero updates occur during an accumulation because of discontinuity
of mass measurements. The best fit flow may be a useful output for users when
measuring high flow with short test times not requiring an AutoZero (see Section
5.1.5.6, 5.7.8).
4.1.5
SETTINGS FOR FLOW SAMPLING
GFS2102 is a dynamic gravimetric flow standard, which means that changes in mass are
measured and flow calculated “on the fly” as the gas is flowing. But the key benefit of the
dynamic property of the system is not in the fact that real-time flow can be displayed. The
ultimate goal of low-uncertainty flow measurement requires that a significant amount of mass
and time are used in each accumulation, as with static gravimetric measurements. More
important benefits of the dynamic system are that problems associated with starting and
stopping the flow are eliminated since steady flow can be established before the
measurement starts and the fact that the system can run multiple automated test points
unattended. One could say that the only calculated flow rate or DUT flow error that is
important for a GFS2102 accumulation is the one that is determined at the end of the
accumulation when uncertainty is minimized. But it is instructive for the user to view the real-
time changes in calculated flow or flow error as the accumulation progresses. It gives the
user a guide as to how stable the calculated flow value is and when enough time and mass
has accumulated to produce the very repeatable results.