Table 2-1: Terms used in meter setup and measurement (continued)
Term Definition or usage
Wobbe index The ratio of the calorific value of a gas to its specific gravity. Measured in
volumetric units (BTU/SCF, and MJ/SCM).
2.3 Primary process variable: specific gravity, molecular
weight, or relative density
The SGM can operate as a specific gravity meter, a molecular weight meter, or a relative density meter. Your
choice determines the set of process variables that the meter can report, the methods used to measure and
calculate them, and the data that you must supply during setup and configuration.
The primary process variable — specific gravity, molecular weight, or relative density — needs to be specified
as part of the order. However, you can change the primary process variable during calibration.
Related information
Primary process variable and available gas process variables
Primary process variable, gas process variables, and required data
2.3.1 Primary process variable and available gas process variables
The gas process variables that the SGM can report are determined by the primary process variable that you
select during calibration.
Available process variables
Default
measurement unit
Primary process variable
Specific gravity Molecular weight Relative density
Specific gravity Unitless ✓ ✓
Molecular weight g/mol ✓ ✓
Relative density Unitless ✓
Base density g/cm³ ✓ ✓ ✓
Line density g/cm³ ✓ ✓ ✓
Line compressibility Unitless ✓ ✓ ✓
Base compressibility Unitless ✓ ✓ ✓
Calorific value MJ/m³ ✓ ✓
Wobbe index MJ/m³ ✓ ✓
Energy flow MJ/hr ✓ ✓
Concentration (gas purity) Concentration
(% mass)
✓ ✓ ✓
Net mass flow rate g/sec ✓ ✓ ✓
Net volume flow rate l/sec ✓ ✓ ✓
Orientation and planning Configuration and Use Manual
March 2021 MMI-20020954
14 Micro Motion Gas Specific Gravity Meters (SGM)
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