• If the SGM is operating as a molecular weight meter, you must enter the molecular
weight of the gas.
During the calibration, you must be able to flow each calibration gas through the meter, in
the order of their specific gravity: lowest to highest.
5.2.4 Pressure
You must be able to control the sample pressure and the vent pressure. In a typical
installation, a pressure regulator is installed to control sample pressure. The meter
includes a pressure indicator that allows you to verify sample pressure.
The pressure in the system must meet the following requirements:
• The sample pressure must be less than the maximum pressure of the meter
(12 bar a).
• The sample pressure must be greater than the control pressure. In a typical
application, it must be 15% to 25% greater than the control pressure.
• The control pressure must be within the following range: 1.2 to 7.0 bar a at 20 °C (17
to 101 psi a at 68 °F).
• The control pressure must be greater than the vent pressure.
At the end of the purge procedure, the reference chamber is filled with the reference gas
to the appropriate control pressure. At this point, the reference chamber is sealed.
Determining the control pressure
The control pressure must be appropriate to your application. You must know the desired
control pressure before you begin the purge and calibration process. The control pressure
also affects the supply pressure and the vent pressure that you must maintain in the
system.
Control pressure refers to the pressure inside the reference chamber. The control pressure
to use depends on three factors:
• The expected range of the process variable, from lowest specific gravity to highest
specific gravity
• The expected change in gas supercompressibility (Z)
• The required measurement accuracy
You can select a control pressure using general guidelines or you can calculate a control
pressure using data that is specific to your meter and your process gas.
Requirements for control pressure
The control pressure must meet the following requirements:
• Between 1.2 and 7 bar a (17 and 101 psi a), at 20 °C
• Less than the sample pressure, by 15% to 25%
• Greater than the vent pressure
Purging and calibration
Configuration and Use Manual 31
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