16.2
Setting the MFC/MFM Zero
MFC zero must be set correctly and consistently to avoid any flow inaccuracies that might
be present in a mass flow controller or meter.
16.2.1 Background
It is normal to experience zero drift in the flow meter output during the life of an mass flow
controller (MFC). MFC users should understand what MFC zero offset is and how to adjust
for its drift. When an MFC is first installed, it is normal to re-zero the device to ensure the
best results. "MFC Zero" is the signal produced by the mass flow sensor when gas is
not flowing. This signal is measured at the output terminal of the MFC with respect to the
signal common terminal. The MFC zero must be set such that the output reads 0.0% when
the MFC is not flowing any gas. Zero offset is a major contributor to flow inaccuracies
par
ticularly at lower control setpoints, where the zero offset error becomes a large
percentage of the total flow signal. Because of this problem, it is imperative that the MFC
zero be set correctly and consistently. There are two common errors that can result in
errant zeroing.
16.2.2 Gas Inlet Pressure
The inlet pressure is an often overlooked source of potential zero error on MFCS. Many
high density, low viscosity gasses exhibit slight changes in zero output as a function of
MFC inlet pressure. MFCs flowing gasses such as tungsten hexafluoride and many fluoro
carbons are especially sensitive to this problem. A simple way of minimizing this problem
is to keep the inlet pressure as low as possible.
Note: This does not apply to pressure buffered "reverse flow" controllers as the pressure
sensitive portion of the MFC is on the downstream side of the MFC.
16.2.3 MFC control valve leakage
Because the MFC is not intended to be used as a positive shut-off device, the MFC valve is
not guaranteed to be leak tight when the valve is closed. What this means is that setting
the MFC zero while there is a pressure differential across the MFC can be a problem
because the MFC will be measuring the valve leakage. Setting the zero under these
conditions will r
esult in a calibration er
r
or
. In or
der to minimize fl
ow er
rors, it is suggested
that the user follow the zeroing procedure below.
16.2.4 Factory Suggested Zeroing Procedure
1. Mount the MFC in the intended physical orientation. The gas delivery system should be
at the intended operating temperature.
2. Insure the MFC flow path is filled with process gas. If the MFC was recently purged or
newly connected to the system, run process gas for 5 to 10 minutes. Alternatively,
evacuate the MFC using a vacuum pump then r
un pr
ocess gas long enough to full the
MFC and gas line.
18
©2005 Celerity
, Inc.
#199-001-0006 REV
.F 4/05
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
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