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A
djustable
Regulator
DryCal
Gas
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MFC
Figure 11 Setup for Calibrating MFCs
7.7 Calibration of Mass Flow Meters (MFMs)
Mass flow meters can be calibrated with the setup of Figure 12, which is similar to that shown for LFE transfer standards. Again, the flow
must not be affected significantly by the Cal=Trak’s cyclic pressure increase. This can be achieved by use of a sonic nozzle as the stable
flow source, or by feeding a fixed restrictor with a precisely regulated pressure of more than 200 kPa. (At 200 kPa [30 PSI], the dynamic
flow decrease caused by the piston’s weight will be about 0.05%.) Alternately, a stable mass flow controller may be used to maintain
specific flows into the MFM under test. In this scenario, the MFC will function as a pressure stabilizer and precision flow regulator with
the Cal=Trak defining the accuracy of the device under test. Contact Sierra Instruments for special Smart-Trak flow controllers designed
for this application.
If the MFM under test must be calibrated at pressure (i.e. because it is always used under pressure) then a fine resolution valve (needle
valve) and a gauge should be installed between the MFM and the Cal=Trak. Adjust the valve until the pressure gauge reads the correct
pressure for the MFM under test.
OR MFC
Figure 12 Setup for Calibrating Mass Flow Meters
7.8 Calibration of Rotameters (Variable Area Flow Meters)
Variable-area meters can become unstable when connected in series with a volume. Cavity resonance may even occur. They are best
calibrated using the setup of Figure 13
. The flow stream is alternately applied to the Cal=Trak and to the device under test. A sonic nozzle
or a large “swamping” pressure restriction is used to render the differences in the two devices’ insertion pressures insignificant with respect
to the required accuracy.
Fixed
Regulator
A
djustable
Regulator
Sonic
Nozzle
OR Porous Plug
DryCal
Gas
Supply
Stable Pressure
> 200 kPa
Flow M eter
Cal=Tra
k
Cal=Tra
k