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3098 Technical Manual Accuracy Considerations
4-1
Chapter 4
Accuracy Considerations
This chapter
This chapter provides a method for estimating the accuracy of 3098 measurements under various conditions.
4.1 Accuracy Considerations
The ‘controlled condition’ which establishes a direct relationship between density and the specific gravity of the sample
gas is mainly determined by the pressure and the type of gas used in the reference chamber. The choice of reference
chamber gas pressure is dependent upon:
1. The span of specific gravity to be measured.
2. The expected change in sample gas supercompressibility, Z.
3. The accuracy required.
The exact choice in reference gas pressure is made after considering all the error sources for that application. To simplify
the selection, Table 1 is provided which can be reproduced by the user. In general, unless a pump is used to boost the
pipeline pressure, the reference gas pressure at 20°C must be at least 10% less than the minimum line pressure, to
ensure gas flow over the operating temperature range.
Example 1
When a gas has a relatively low and reasonably constant specific gravity, and is available at a line pressure greater than 7
Barg (100psig) such as natural gas measurement in the range 0.55-0.8, a very high accuracy is possible using a reference
pressure of 7 Barg. (See Example 1 table below for a worked example).
Example 2
If large range specific gravity measurements are to be made, or where changes in the supercompressibility factor of the
sample gases become significant, (such as in flare gases or air/CO
2
mixes), a much lower reference gas pressure is
required. (See Example 2 table below for N
2
/CO
2
mix).