metso automation kajaaniMCAi Installation, Operating & Service Manual Download Page 54

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kajaaniMCA

i – Installation, Operating & Service

- 12.7 -

W4610201 V2.5 EN

Fig. 12.9. Consistency error caused by changes in chemical
content.

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0.1

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Mlev (u)

MCAi - Lab (%)

The following functions can be used to eliminate the
disturbing effect of various process variables.

Chemicals compensation may be needed if the

chemicals content of the process varies considerably.

If the measurement appears to be affected by process

temperature, the temperature compensation can be cor-
rected, section by section, by applying a linear curve.
Correction is needed if the process temperature goes
below 30°C (84°F).

12.C.1. Chemical compensation
• Principle

Large variations in the chemicals content of the pulp
affect the measurement of microwave propagation time.
As a result, the MCAi will show a too high reading
when the chemicals content increases. Chemicals also
affect the attenuation of the microwave signal, causing
a lower signal level result. Thus a correlation can be
found between the consistency error (MCAi – Lab) and
the measured signal level, and this correlation can be
applied to eliminate the measurement error caused by
changes in the chemical content.

If large variations occur in the chemicals content of

the process, the consistency, signal level and tempera-
ture measured by the MCA

should be included in the

laboratory follow-up, as well as the conductivity and
consistency measured in laboratory. Based on the col-
lected data, the signal level measurement can be cor-
rected by chemicals compensation if needed.

Fig. 12.9 shows an example graph based on labora-

tory follow-up results; consistency error (MCA

– Lab.)

is on the Y-axis, signal level on the X-axis. Mlev is the
abbreviation used for the MCAi signal level.

As the graph shows, the consistency error (MCAi –

Lab) correlates with the signal level. This indicates that
the error is caused by chemicals and thus it can be
eliminated by chemicals compensation. To make sure,
we can make a graph with conductivity (reflecting the

12.C. Special Functions

chemicals content) is on the X-axis. The graph should
then be a rising straight line; the error (MCAi – Lab.)
grows when conductivity increases.

• Setting chemicals compensation

NOTE: Select the calibration points so that the differ-
ence between their chemicals contents and signal levels
is as great as possible.

To define the calibration points take two calibration

samples, or use samples entered earlier (new sampling
is then not needed). The latter method is more reliable
if there is data from a long-time laboratory follow-up,
as a larger number of sample points are then available.

In the example shown in Fig. 12.10, the calibration

points are selected from the laboratory follow-up data
as follows: Choose a calibration point with a high signal
level. At this point the chemicals content has been low
and the MCAi has not shown a too high reading. Choose
a second calibration point with a low signal level. At
this point the chemicals content has increased and the
MCAi has shown a high reading.

Then enter the values of laboratory and MCAi

measurements to the sensor: lab. Cs and conductivity,
and consistency, temperature and signal level measured
by the MCAi. Make sure to give the consistencies
measured by the MCAi and laboratory so that the error
(MCAi – Lab.) is of the right magnitude; the absolute
Cs levels have no significance. In other words, it does
not matter whether the Cs values for sample 2 are MCAi
= 3.2 and Lab = 3.0 or MCAi = 5.2, Lab = 5.0. Before
determining the compensation graph through samples 1
& 2, the MCAi will perform temperature compensation
based on the temperature and conductivity.

When the chemicals compensation is in use, the

MCAi will place the measured signal level result (Mlev)
on the compensation line, determine the error (MCAi –
Lab.) and correct the measured consistency by the
calculated amount.

Fig. 12.10. Choosing the calibration points.

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M le v (u)

MCAi - Lab (%)

Sample 2

Sample 1