Aalborg DFM Digital - Manual (Page 36)

Gas flow calibration parameters are stored in the Gas Dependent portion of the
EEPROM memory separately for each of 10 calibration tables. See APPENDIX I for
complete list of gas dependent variables.
Note: Make sure the correct gas number and name is selected as
current on the main DFM screen. All adjustments made to the gas
linearization table will be applied to the currently selected gas.
The DFM gas flow calibration involves building of the table of the actual flow val-
ues (indexes 114, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134) and corre-
sponding sensor readings (indexes 113, 115, 117, 118, 119, 121, 123, 125, 127,
129, 131, 133). Actual flow values are entered in normalized fraction format:
100.000 % F.S. corresponds to 1.000000 flow value and 0.000 % F.S. corresponds
to 0.000000 flow value. The valid range for flow values is from 0.000000 to
1.000000 (note: DFM will accept up to 6 digits after decimal point). Sensor read-
ings are entered in counts of 12 bits ADC output and should always be in the range
of 0 to 4095. There are 11 elements in the table so the data should be obtained at
increment 10.0 % of full scale (0.0, 10.0, 20.0, 30.0, 40.0, 50.0, 60.0, 70.0, 80.0,
90.0 and 100.0 % F.S.).
Note: Do not alter memory index 113 (must be 120 counts) and 114
(must be 0.0). These numbers represent zero flow calibration point
and should not be changed.
If a new gas table is going to be created, it is recommended to start calibration
from 100% full scale. If only linearity adjustment is required, calibration can be
started in any intermediate portion of the gas table. Using the flow regulator,
adjust the flow rate to 100% of full scale flow. Check the flow rate indicated against
the flow calibrator. Observe flow reading on the DFM. If the difference between cal-
ibrator and DFM flow reading is more than 0.5% F.S., make a correction in the sen-
sor reading in the corresponding position of the linearization table (Index 133). If
the DFM flow reading is more than the calibrator reading, the number of counts in
the index 133 has to be decreased. If the DFM flow reading is less than the cali-
brator reading, the number of counts in the index 133 has to be increased. Once
Index 133 is adjusted with a new value, check the DFM flow rate against the cali-
brator and if required perform additional adjustments for Index 133. If a simple
communication terminal is used for communication with the DFM, then "MW"
(Memory Write) command from the software interface commands set may be
used to adjust sensor value in the linearization table (see section 8.3 for complete
software interface commands list). Memory Read "MR" command can be used to
read the current value of the index. Assuming the DFM is configured with RS-485
interface and has address "11", the following example will first read the existing
value of Index 133 and then write a new adjusted value:
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