Dwyer GFM - Manual (Page 17)

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7.2.3 SPAN Adjustment
Reconnect the gas source. Using the flow regulator, adjust the flow rate to 100% of
full scale flow. Check the flow rate indicated against the flow calibrator. If the devia-
tion is less than ±10% of full scale reading, correct the SPAN potentiometer [R33]
setting by using the insulated screwdriver through the access window, to eliminate
any deviation. If the deviation is larger than ±10% of full scale reading, a defective
condition may be present.
LIKELY REASONS FOR A MALFUNCTIONING SIGNAL MAY BE:
✓ Occluded or contaminated sensor tube.
✓ Leaking condition in the GFM transducer or the gas line and fittings.
✓ For gases other than nitrogen, recheck appropriate “K” factor from the Gas Factor Table.
✓ Temperature and /or pressure correction errors.
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
additional 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
All adjustments in this section are made from the outside of the meter, there is no
need to disassemble any part of the instrument.
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 neces-
sary to remove any jumpers at J1, J2, and J3 before beginning 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.