GFS2102™ OPERATION AND MAINTENANCE MANUAL
© 2007 DH Instruments, a Fluke Company
Page 46
4.1.3
AUTOZERO
In order for the GFS2102 mass measurements to be made with the precision and
repeatability that is required for the system’s target flow uncertainties, the drift of the balance
that occurs over time must be removed from the GFS mass measurements. The AMH-
GFS2102 platform allows a reference mass to be loaded on the balance at intervals
throughout a flow measurement, without disturbing the gas flow, in order to check the
balance for drift. The time interval at which this is done can be selected in GFS Tools.
Using AMH and the calibrated GFS reference mass, the balance zero and span can be
calibrated at any time, and this function can be automatically programmed to occur before
each flow measurement if desired. It is known that the primary type of drift that occurs in the
balance over time is a “zero shift” and not a span error, so each time a new AutoZero is
performed, subsequent mass readings use a corrected zero so that balance drift does not
have a significant effect on the system’s ability to measure the critical mass change over long
test times.
For low flows, which result in long test times, several AutoZero functions will occur during one
measurement. For higher flows, often no AutoZero occurs. When a fixed time interval is
selected for the AutoZero function to take place, the system will determine if and when it
should be done during a test. (See Sections 5.5.2.7, 5.7.4).
4.1.4
GFS2102 DATA COLLECTION METHODS AND
CONVENTIONS
4.1.4.1
ACCUMULATIONS
During GFS2102 measurements, mass and time data is continuously collected
from the time that the test is started until either the user or pre-set test limits end
the measurement. Raw mass readings are taken from the balance and are
corrected for drift by way of the AutoZero function and for air buoyancy. Since
the gas is flowing out of the reference gas cylinder, the mass readings are
successively smaller as the mass is depleted from the cylinder. Although the
mass is in fact being depleted, we are actually interested in quantifying the
amount of gas that is flowing through the GFS2102 hardware and the device
under test, not the mass that is still contained in the cylinder, so the change in
cylinder mass is reported by GFS Tools software as a positive value known as
the
accumulated mass
. A single GFS flow measurement that is completed after
a sufficient amount of mass and time change occurs is known as an
accumulation
.
In the scope of a typical multi-point flow device calibration, an accumulation
would be the culmination of many samples into one final reading or data point at
a given flow rate. It is possible to setup GFS Tools to take multiple
accumulations at each flow point. For details on setting up GFS Tools tests, see
Section 5.5.2.
4.1.4.2
SAMPLES
Throughout an accumulation, mass measurements are read from the balance at
a rate of several readings per second. This is necessary to achieve good results
at higher flows where the rate of mass change is relatively large. At low flows,
the rate of mass change is so slow that short term noise, or perturbations, of
mass output from the balance can cause more movement in mass values over a
small time period than the actual flow does. If all of these mass measurements
are translated into flow values, the effect would be a flow indication that is rapid
and unstable looking. All of the high speed balance and time output is not used,
less frequent updates of mass and time are selected for display and logging in