Determine the reference chamber pressure
The reference chamber pressure must be appropriate to your application. You must know the desired
reference chamber pressure before you begin the purge and calibration process. The reference chamber
pressure also affects the inlet sample supply pressure and the outlet vent pressure that you must maintain in
the system.
Guidelines for reference chamber pressure
Use the following guidelines for reference chamber pressure:
• Between 17 psia (1.17 bara) and 101 psia (7 bara), at 68 °F (20 °C)
• Less than the inlet sample pressure by 15% to 25%
• Greater than the outlet vent pressure
These pressure settings work for most applications. Use them as starting point, or use them to calculate
reference chamber pressure using data that is specific to your process.
Related information
Calculate measurement errors using reference chamber pressure
5.2.5 Multiple calibrations
The SGM can store calibrations for up to four different process gases or ranges. Each calibration is generated
by an independent calibration procedure and contains an independent set of calibration coefficients. This
feature allows you to switch between process gases or ranges without recalibrating the device.
If you plan to use more than one calibration:
• Perform all calibrations using the same measurement option: specific gravity, molecular weight, or
relative density.
• Set calibrations as either two-point calibrations or three-point calibrations.
• Complete each calibration before beginning the next calibration.
• Choose to add calibrations at a later time. You do not need to perform all calibrations at the same time.
Important
It is possible to use a different reference chamber pressure for each calibration. If you do, change the
reference chamber pressure in the meter whenever you change the active calibration. If you do not change
the reference chamber pressure to match the active calibration, measurement accuracy will be affected.
Calibration ranges
• If you have a specific gravity meter, the meter will automatically calculate the specific gravity output
variable for all four ranges during operation.
• If you have a molecular weight meter, the meter will automatically calculate the molecular weight output
variable for all four ranges during operation.
• If you have a relative density meter, only one calibration is applied at a time. A control selects the active
calibration.
• For specific gravity and molecular weight meters — energy, energy flow, compressibility, and Wobbe index
are calculated only from the active calibration. Gas purity is automatically calculated for the associated
Purging and calibration Configuration and Use Manual
March 2021 MMI-20020954
30 Micro Motion Gas Specific Gravity Meters (SGM)
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