2.3.2 Primary process variable, gas process variables, and required
data
The gas process variables are calculated from a combination of measured variables, calculated variables,
process data from external devices, and user-specified values. For each process variable that you want the
meter to report, you must be able to supply all required external data and configuration values. Specific
requirements are determined by the primary process variable.
Note
The meter does not measure certain process variables directly. External devices are required for the following
process variables:
• Line pressure
• Gas composition (% CO, % CO2, % H2, % N2)
• Flow rate (mass or volume)
Note
• If you use temperature data from the meter, the data will represent the gas inside the measurement
chamber.
• If you use temperature data from an external device, the data will represent the gas at the location of the
temperature probe.
Table 2-2: Gas measurement when the primary process variable is specific gravity
Process variable to be reported Process data provided
by the SGM
Required process data
from external devices
Required user-
specified values
Specific gravity Specific gravity
Molecular weight Specific gravity Molecular weight of air
Base density
Molecular weight
Base compressibility
Base pressure
Base temperature
Line density
Temperature data from
meter (RTD)
(1)
Base density
Line compressibility
Base compressibility
External temperature
(2)
Line pressure
Base pressure
Base temperature
Line compressibility
NX 19
Sample temperature
(RTD)
(1)
Specific gravity
External temperature
(2)
Line pressure
% CO2
% N2
Molecular weight of air
NX 19 Mod
Sample temperature
data from meter
(RTD)
(1)
Specific gravity
External temperature
(2)
Line pressure
% CO2
% N2
Configuration and Use Manual Orientation and planning
MMI-20020954 March 2021
Configuration and Use Manual 15
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