KROHNE CORIMASS MFC 85 EN - Manual (Page 16)

16
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 which also limit the ability of the meter to read an accurate density
figure.
•
Air / Gas Inclusions. If the process fluid has a 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
to try to collapse any air bubbles.
•
Temperature Shocks. Corimass meters have temperature compensation for their density
measurements, and can cope with slowly changing temperatures. If however the meter is
subjected to a rapid change in the temperature, then temporary errors in the density
reading may result, whilst the whole instrument achieves a new thermal equilibrium. Such
errors can persist for several minutes after the initial shock.
•
Response Time. Corimass meters display a ”steady state” density value. They cannot
track fast changes in density. If the fluid passing through the meter has a sudden change in
density, then the meter, owing to internal filtering will take about 30 seconds before the
density reading settles to the new value.
To ensure a good density measurement :
a) Ensure meter is properly installed and clamped.
b) The product must be free from air bubbles (see above).
c) Site calibrate the density measurement, preferably at the process’ normal operating
temperature, as described in the standard operating manual.
5.2 Brix Measurements
Many industries use the Brix scale to monitor/control the sucrose content in their product(s).
Typically, they will use a refractrometer to do this. A small sample of the product is taken
from the line and placed on the refractrometer. The refractrometer then measures the
refractive index of the sample.
The refractive index of sucrose solutions varies with both concentration and temperature.
Modern refractrometers also measure temperature, and can then display a inferred Brix value,
based on the assumption that the sample it measured was just a solution of sucrose in water.
This whole process is an off-line procedure and can be very inconvenient. In-line
refractrometers are available, but these are notoriously unreliable and need frequent cleaning
and maintenance.
In practice very few companies work with pure sucrose solutions, but they still use a
refractrometer to measure Brix! In the case of soft drinks manufacturers their product will
contain many other components in addition to sucrose (maybe glucose, fructose, fruit acids,
proteins, suspended particulates etc.). Many of these other components may affect the
product’s overall refractive index. Consequently, the refractrometer reading becomes only an
approximation to the actual sugar content.