General guidelines for selecting a control pressure
To select a control pressure using general guidelines:
• To minimize the effect of temperature on the meter, use a higher control pressure.
• To minimize the effect of temperature on compressibility, use a lower control
pressure.
• To minimize the effect of compressibility on calibration, use a lower control
pressure.
• To minimize error in general, use a higher control pressure.
Method for calculating the error associated with a control pressure
Your selection of control pressure determines the range of possible measurement error.
You can calculate the measurement error associated with different control pressures and
use this to help you select a control pressure.
To calculate the measurement error associated with several different control pressures:
1. Use the following table as a calculation aid.
2. Record your process gas and the expected specific gravity range.
3. Record the DTC for the meter. The DTC is provided on the meter's calibration sheet
and on a label inside the transmitter front cover.
4. Check the example control pressures in the table and change them as desired.
5. Using the following equation, calculate the density range for each control pressure.
6. Using the following equations and methods, calculate the measurement error for
each density range.
7. Calculate the total error for each density range.
With this data, you can select a control pressure based on its density range and accuracy.
Be sure to convert the control pressure at 20 °C to the equivalent value at operating
temperature.
Calculation aid for control pressure and associated measurement error
Table 5-1:
Gas _____________________
Specific gravity range _________ to __________
DTC (kg/m³/°C) _____________________
Control pressure at 20 °C lb/in² absolute 18 30 60 100
Bar absolute 1.2 2 4 7
Density range at 20 °C kg/m³
Measurement error (% of
full-scale
specific gravity/°C )
Temperature coefficient er-
ror
Gas compressibility of sam-
ple gas
Velocity of sound in sample
gas
Purging and calibration
32 Micro Motion
®
Gas Specific Gravity Meters (SGM)
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