Dwyer GFM - Manual (Page 15)

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Remove the block and connected screens. Clean each of the removed parts as
necessary. If alcohol is used for cleaning, allow time for drying.
Inspect the flow path inside the transducer for any visible signs of contaminants.
If necessary, flush the flow path through with alcohol. Thoroughly dry the flow
paths by flowing clean dry gas through.
Re-install the inlet parts. Be sure that no dust has collected on the O-ring seal.
It is advisable that at least one calibration point be checked after re-installing
the inlet fitting - see section 7.
7. CALIBRATION PROCEDURES
NOTE: REMOVAL OF THE FACTORY INSTALLED CALIBRATION SEALS AND/OR
ANY ADJUSTMENTS MADE TO THE METER, AS DESCRIBED IN THIS SECTION,
WILL VOID ANY CALIBRATION WARRANTY APPLICABLE.
7.1 Flow Calibration
Dwyer Instruments' Flow Calibration Laboratory offers professional calibration
support for Mass Flow Meters, using precision calibrators under strictly controlled
conditions. NIST traceable calibrations are available. Calibrations can also be per-
formed at customers' site using available standards.
Factory calibrations are performed using NIST traceable precision volumetric
calibrators incorporating liquid sealed frictionless actuators.
Generally, calibrations are performed using dry nitrogen gas. The calibration can
then be corrected to the appropriate gas desired based on relative correction [K]
factors shown in the gas factor table - see Appendix 2. A reference gas, other than
nitrogen, may be used to better approximate the flow characteristics of certain
gases. This practice is recommended when a reference gas is found with ther-
modynamic properties similar to the actual gas under consideration. The appro-
priate relative correction factor should be recalculated - see section 9.
It is standard practice to calibrate Mass Flow Meters with dry nitrogen gas at 70 F
F (21.1 F C), 20 psig (1.4 bars) inlet pressure and 0 psig (0 bar) outlet pressure. It
is best to calibrate the GFM transducers to actual operating conditions. Specific
gas calibrations of non-toxic and non-corrosive gases are available at specific
conditions. Please contact your distributor or Dwyer for a price quotation. It is rec-
ommended that a flow calibrator of at least four times better collective accuracy
than that of the Mass Flow Meter to be calibrated be used. Equipment required for
calibration includes a flow calibration standard and a certified high sensitivity mul-
timeter (which together have a collective accuracy of ±0.25% or better), an insu-
lated (plastic) screwdriver, a flow regulator (example: metering needle valve)
installed upstream from the Mass Flow Meter and a pressure regulated source of
dry filtered nitrogen gas (or other suitable reference gas).
The gas and ambient temperature, as well as inlet and outlet pressure conditions
should be set up in accordance with actual operating conditions.
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