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Installation and Configuration Manual 11
Installation Procedure
Installation Procedure Accuracy ConsiderationsElectrical ConnectionsIntroduction
2.5 Reference chamber pressure determination
Once the 3098 specific gravity meter has been placed in its fixture and all relevant pipework and
electrical connections made, the reference chamber pressure needs to be determined.
The gas type and reference chamber pressure define the ‘controlled condition’ at which the unit
allows gas to flow and establishes a direct relationship between density and the specific gravity of the
sample gas.
The choice of reference chamber gas pressure is dependent upon three factors:
• The span of specific gravity to be measured
• The expected change in sample gas supercompressibility, Z
• The accuracy required
The graph below gives an indication of the typical errors associated with using different reference
chamber pressures for natural gas with a reasonably constant specific gravity (in the range of
0.55 – 0.8). This is typical for natural gas metering market, where the gas is available at a line
pressure of 7 Bar abs.
As can be seen, below 7 Bar abs, the total error begins to increase; using a higher reference pressure
will not improve accuracy, but may encourage gas leakage. Therefore, for the conditions specified,
7 Bar is the recommended pressure.
Figure 2-1 Typical total error/°C versus reference chamber pressure
This graph should only be used for natural gas applications, and.gives typical errors seen on the 3098
specific gravity meter if it is not used at the recommended reference chamber pressure.
If the span of specific gravity or change in supercompressibility, Z, is large, and the gas is not a
methane/nitrogen mix, then the best reference chamber pressure can still be determined. The
calculation for doing this is explained in Chapter 4.
Once the desired reference pressure has been found, the 3098 specific gravity meter can now be purge
cycled and then calibrated.
Typical Total Error/°C vs Reference Chamber Pressure
For NATURAL GAS APPLICATIONS ONLY.
-0.035
-0.025
-0.015
-0.005
0.005
0.015
0.025
0.035
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Reference Chamber Pressure (Bar A)
Typ % of FS Specific Gravity /°C
-0.035
-0.025
-0.015
-0.005
0.005
0.015
0.025
0.035
Typ % of FS Specific Gravity /°C