Aalborg XFM Digital - Manual (Page 26)

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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 III). A reference gas, other
than nitrogen, may be used to better approximate the flow characteristics of cer-
tain 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
70.0˚F (21.1 ˚C), 20 psia (137.9 kPa absolute) inlet pressure and 0 psig outlet
pressure. It is best to calibrate XFM transducers to actual operating conditions.
Specific gas calibrations of non-toxic and non-corrosive gases are available for
specific conditions. Please contact your distributor or Aalborg® for a price quota-
tion.
It is recommended that a flow calibrator be used which has at least four times bet-
ter collective accuracy than that of the Mass Flow Meter to be calibrated.
Equipment required for calibration includes: a flow calibration standard, PC with
available RS-485/RS-232 communication interface, a certified high sensitivity
multi meter (for analog output calibration only), an insulated (plastic) screwdriver,
a flow regulator (for 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). Using Aalborg® supplied calibration and mainte-
nance software to simplify the calibration process is recommended.
Gas and ambient temperature, as well as inlet and outlet pressure conditions,
should be set up in accordance with actual operating conditions.
7.2 Gas Flow Calibration of XFM Mass Flow Meters
XFM Mass Flow Meters may be field recalibrated/checked for the same range
they were originally factory calibrated for. When linearity adjustment is needed or
flow range changes are being made, proceed to step 7.2.3. Flow range changes
may require a different Restrictor Flow Element (RFE). Consult your distributor or
Aalborg® for more information.
7.2.1 Connections and Initial Warm Up
Power up the Mass Flow Meter for at least 15 minutes prior to commencing the
calibration procedure. Establish digital RS-485/RS-232 communication between
PC (communication terminal) and the XFM. Start Aalborg® supplied calibration
and maintenance software on the PC.
All adjustments in this section are made from the outside of the meter
via digital communication interface between a PC (terminal) and XFM.
There is no need to disassemble any part of the instrument or perform
internal PCB component (potentiometers) adjustment.