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32 Micro Motion 3098 Gas Specific Gravity Meter
Accuracy Considerations
With this decided, the 3098 specific gravity meter can be calibrated by following the calibration
procedure described in Section 2.6.
Note: In the case where only one calibration gas is available, the time period of the density meter at
zero density/specific gravity (for example,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 Section 2.6. You can enter this information directly into the Calibration Certificate example in
Section 4.4. For an online version of this certificate, download the Calibration Certificate Excel file at
www.micromotion.com (located on the 3098 products page) or access the calcert.xls file on the floppy
disk shipped with the product.
For more specific details on calibration see Appendix A.
4.3 Operation at low reference pressure levels
One of the design features of the 3098 specific gravity meter 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 specific gravity (SG) measurement.
For reference pressures greater than 3 bar absolute (3 bar A), this situation will not occur in the unit
flow range of (0.2–60 cc/s). However, it may occur if the reference pressure is less than 3 bar A and
the flow rate > 50 cc/s.
It is recommended that in order to achieve the optimum accuracy when performing specific gravity
(SG) measurement, the corrections for VOS and compressibility are taken into consideration. This can
be done by following the procedure described in Appendix A.