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Accuracy Considerations 3098 Technical Manual
4-4
4.2 Calibration (for non-natural gas applications)
The instrument is supplied with its reference chamber empty and thus in an un-calibrated condition. After installation on
site it is necessary to decide what reference chamber pressure to use, and then to charge and calibrate the instrument as
described in the Installation Procedure (Chapter 2).
Some examples of how to calculate these reference chamber pressures are given in Examples 1 and 2, which show the
best pressures for a natural gas and a N
2
/CO
2
mix application.
Once this has been done, the gases to be used for calibration need to be defined. The calibration gases to be used must
be of known specific gravities and substantially represent the properties of the line gas to be measured (i.e compressibility,
viscosity etc.) For example, if measuring a natural gas which is substantially methane and carbon dioxide, then these two
gases in their pure forms or at defined specific gravities should be used in the calibration.
With this decided, the 3098 can be calibrated by following the calibration procedure described in Chapter 2, the Installation
Procedure.
Note
In the case where only one calibration gas is available, the time period of the density meter at zero density/specific gravity
(i.e., vacuum conditions), which is included on the meter temperature coefficient calibration certificate, can be used as
time period τ
y
. Under this condition, calibration is less accurate due to the non-homogenic condition of a vacuum and its
effect on supercompressibility compensation. An example of the meter temperature coefficient calibration certificate is
given below.
Once the calibration has been performed, the coefficients can be calculated using equations (1) and (2) in the Installation
procedure or by using the calibration certificate shown below. (An Excel program, calcert.xls, to perform these
calculations is contained on the 3½” disk which came with this manual).
For more specific details on calibration see Appendix A.
4.2.1 Operation at Low Reference Pressure levels
One of the design features of the 3098 is that two orifice plates are used to control and regulate the flow of sample gas
through the unit, one of which is placed at the output port and is used to reduce the stresses placed on the unit’s
diaphragm. It is important to note that in order to increase the sample gas flow rate, the pressure at the input port must be
increased. As this pressure is increased, the pressure across the output orifice increases. If this pressure exceeds that of
the gas inside the reference chamber, the diaphragm will not regulate the input gas pressure and hence not allow an SG
measurement.
For reference pressures greater than 3 bar absolute (3 bar A), this situation will not occur in the unit flow range of (0.2 -
60cm
3
/s). However, it may occur if the reference pressure is less than 3 bar A and the flow rate > 50cm
3
/s.
It is recommended that in order to achieve the optimum accuracy when performing SG measurement, the corrections for
VOS and compressibility are taken into consideration. This can be done by following the procedure described in Appendix A.