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Performance Optimisation 3098 Technical Manual
A-2
Note: When sample gas is flowing through the instrument, the reference chamber pressure is indicated on a dial gauge
within the insulated enclosure. The indicated pressure is in bar gauge whilst the pressures quoted in this text are
in bar absolute.
A1.4. Selection of calibration gases
The measurement accuracy of the specific gravity meter can be not better than that defined by the quality of the
calibration gases. Furthermore, the calibration gases should substantially represent the characteristics of the expected
sample gases, especially with respect to their compressibility characteristics.
For example, the use of pure certified methane as one calibration gas and the use of a typical certified gas mix as the
other calibration gas would yield good results. However, since it may be difficult to obtain a certified gas mix, and also
since some gas mixes will stratify in their containers and hence give unreliable quality, it is often better to use two pure
gases such as certified methane and certified nitrogen. In this case it may be necessary to modify the calibration
procedure to make allowance for any non-ideal characteristics of the sample gases.
A2. Recommended Calibration Methods
From the previous descriptions it can be appreciated that there is a choice of calibration procedures. These differ in detail
to suit the operating conditions, the types of gas to be measured, and the availability of calibration gases. However, all
calibrations can be separated into three general tasks as follows:
A2.1. General Calibration Method
Selection of Reference Chamber Gas
This gas should ideally be similar to the sample gas as far as compressibility characteristics are concerned (it is usual for
the sample gas to be used in the reference chamber) in order to minimise the temperature coefficient of the instrument.
Selection of Reference Chamber Pressure
This pressure should be set to a value which minimises the temperature coefficient and also any calibration errors which
result from using non-representative calibration gases.
Calibration and Sample Gases
Having charged the reference chamber to the selected pressure chamber pressure, then suitable adjustment should be
made to the calibration coefficients to ensure minimum error when using the sample gases. These calibration
adjustments can be calculated from a knowledge of the compressibility factors of the calibration and sample gases, or by
establishing the necessary offsets by measurement experience. Paragraph 2.2 details the procedures which can be
adapted to suit any specific calibrations involving gas mixes, and highlights the special problems entailed.
A2.2. Specific Calibration Method
Example for natural gas using methane and nitrogen as calibration gases.
Selection of Reference Chamber Gas
This gas should ideally be similar to the sample gas as far as compressibility characteristics are concerned (it is usual for
the sample gas to be used in the reference chamber) in order to minimise the temperature coefficient of the instrument.
Selection of Reference Chamber Pressure
The reference chamber pressure is selected as follows:
a. To minimise the temperature coefficient, see Section 4.1.4 and Tables 2 and 3.
b. To minimise the compressibility effect between the calibration and sample gases.
Calibration
Apart from the temperature coefficient characteristics, the major potential error sources are:
a. The velocity of sound effect (VOS) of the gas.
b. The compressibility factor (Z) of the gas.