KROHNE OPTIMASS Sensors Concentration Measurement EN - Manual (Page 14)

4
PRACTICAL CONSIDERATIONS
14
OPTIMASS
www.krohne.com 01/2014 - 4000733002 - AD OPTIMASS Concentration R04
Practical considerations
4.1 Accuracy of concentration estimation
The results of the concentration estimation depend on the accuracy of the density
measurements and on the “fit” of the algorithm used to infer the concentration value. Of the two
factors the density measurement itself is obviously the most critical. In order to assess the effect
of inaccuracies in the density measurement, it is necessary to consider the amount that density
will change over a given range of measurment. The concentration accuracy is given by the
equation.
For instance, it is required to measure concentrations in the range 5...10% by mass. Over this
range density varies from 1.018 to 1.038 g/cm3, a change of 0.020 g/cm3. If the density accuracy
of the flow meter is 0.002g/cm3, then, assuming that the variation of density with concentration
is linear, the approximate accuracy of the measurement will be:
Obviously this simple calculation takes no account of temperature, or of the fit of the algorithm,
but it very quickly gives an indication of what is achievable with any given density meter.
There are other factors that also limit the ability of the meter to read an accurate density figure.
Air / gas inclusions Air / gas inclusions Air / gas inclusions Air / gas inclusions
If the process fluid has significant gas content this will cause the
Coriolis meter to under-read density. This is not a fault of the meter but a problem with the
application. If it is not possible to prevent air getting into the fluid various measures can be
taken to try to minimise the effect. The meter should be installed vertically to prevent air
collecting within it (this may not help with bent tube instruments). Try pressurising the line
so that the any air bubbles are compressed.
To ensure a good density measurement:
• Install the meter as directed in the Handbook
• make sure that the process product is free from entrained bubbles
• Carry out on site calibration of the density measurement, prefereably at the normal operating
temperature for the process product being measured. For more information on calibration,
please refer to the relevant converter Handbook.
C
Accy
concentration accuracy
Δ
C
change in concentration
Δ
ρ change in density
ρ
Accy
density accuracy
C
Accy
concentration accuracy
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