Aalborg GFM - Manual (Page 18)

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See also section 8. TROUBLESHOOTING. If after attempting to remedy the
above conditions, a malfunction still persists, return the meter for factory service,
see section 1.1.
At this point the calibration is complete. However, it is advisable that several addi-
tional points between 0 and 100%, such as 25%, 50%, and 75% flow be checked.
If discrepancies are found, proceed to step 7.3 for Linearity Adjustment.
7.3 Linearity Adjustment
7.3.1 Connections and Initial Warm Up
At the 9-pin “D” connector of the GFM transducer, connect the multimeter to out-
put pins [2] and [3] for 0-5 VDC (or pins [8] and [9] for 4-20 mA)-(see Figure 2.a).
If calibration to a new flow range or different gas is being performed, it may be
necessary to remove any jumpers at J1.A, J1.B, J1.C, and J1.D before the begin-
ning the linearizing procedure.
Power up the Mass Flow Meter for at least 30 minutes prior to commencing the
calibration procedure.
7.3.2 ZERO Adjustment
Shut off the flow of gas into the Mass Flow Meter. To ensure that no seepage or
leak occurs into the meter, it is good practice to temporarily disconnect the gas
source. Using the multimeter and the insulated screwdriver, adjust the ZERO
potentiometer [R34] through the access window for 0 VDC (or 4 mA respectively)
at zero flow.
7.3.3 25% Flow Adjustment (using R33 potentiometer)
Reconnect the gas source. Using the flow regulator, adjust the flow rate to 25% of
full scale flow. Check the flow rate indicated against the flow calibrator.
Adjust the setting for potentiometer [R33] by using the insulated screwdriver
through the access window, until the output of the flow meter reads 1.25VDC
±63mV (or 8mA ±0.25mA).
Using the flow regulator, adjust the flow rate until the output of the flow meter
reads 0.5 VDC (or 5.6mA). Check the flow rate against the flow calibrator. If the
flow rate indicated by the calibrator is within 10% ± 1.5% of F.S. then skip para-
graphs 7.3.4, 7.3.5 and proceed directly to paragraph 7.3.6, if not, perform 10%
flow adjustment according to paragraph 7.3.4.
CAUTION: The minimum voltage on 0-5 Vdc output can be in the range of 7
to 25 mV. Trying to reduce voltage below this level may increase negative
zero shift. This shift may be invisible on devices without LCD display. Stop
R34 zero potentiometer adjustment if voltage on 0-5 Vdc output is in the
range from 7 to 25 mV and does not decrease any lower.
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