8.3.2 Configure gas compressibility measurement using a field
communicator
Gas compressibility measurement is required for the following process variables: line density, calorific value,
Wobbe index, and energy flow. Gas compressibility measurement can also be used to calculate
compressibility as an independent process variable.
Prerequisites
You must know whether you will measure line compressibility and base compressibility or you will use fixed
values.
If you will use fixed values, you must know the line compressibility and base compressibility values for your
application.
If you will measure compressibility, you must be able to supply gas composition data to the meter, for the
following constituents:
• Carbon dioxide (CO2)
• Nitrogen (N2)
• Hydrogen (H2) (required only if you plan to measure compressibility according to SGERG 88)
Gas composition must be measured in % by volume.
If you plan to poll an external device for % CO2, % N2, or % H2, ensure that the meter has the required polling
slots available. The meter provides four polling slots, and they may be already in use. You may need to use a
fixed value or digital communications for some external values. To check the current polling configuration,
choose Configure → Manual Setup → Inputs/Outputs → External Device Polling.
Procedure
1. Configure the meter to use fixed values or to measure compressibility.
a) Choose Service Tools → Maintenance → Modbus Data → Write Modbus Data.
b) To use fixed values, write 0 to Coil 442.
c) To measure compressibility, write 1 to Coil 442.
2. If you are using fixed values:
a) Write the line compressibility value to Registers 4183-4184, in 32-bit IEEE floating-point format.
The default value is 0. The range is unrestricted.
b) Write the base compressibility value to Registers 4141-4142, in 32-bit IEEE floating-point
format.
The default value is 1.0. The range is 0.7 to 1.1.
No further actions are required.
3. If you are measuring compressibility, complete the configuration as described in the following steps.
4. Choose Configure → Manual Setup → Measurements → Optional Setup → Gas Measurement →
Setup Compressibility.
Configuration and Use Manual Configure process measurement using a field communicator
MMI-20020954 March 2021
Configuration and Use Manual 71
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