metso automation kajaaniMCAi, Installation, Operating & Service Manual

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kajaaniMCA i – Installation, Operating & Service
- 12.10 -
W4610201 V2.5 EN
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
15 25 35 45 55
Tem p. (°C)
MCAi - Lab (%)
Fig. 12.16. Effect of temperature on the consistency error.
12.C.2. Correction curve for temperature compen-
sation
Water temperature affects the propagation velocity of
microwaves. The sensor therefore measures process
temperature with a Pt-100 temperature sensor, and
performs linear temperature compensation. However,
at temperatures below 30°C (86°F) the effect of temper-
ature is not quite linear. If the process temperature is
below 30°C (86°F) – either permanently or from time
to time – a correction curve is needed for the tempera-
ture compensation.
The correction curve is determined by using the
results of laboratory follow-up. The laboratory follow-
up data must include consistency measured by the
laboratory and by the MCAi, and the process tempera-
ture measured by the MCAi sensor. You can either read
the MCAi results from the Main Menu, or take follow-
up samples by using the S
TART SAMPLE
function (see
section 12.B.2). The latter alternative will directly give
the average values during sampling.
Using the laboratory results, draw a graph with
temperature on the X-axis and the (MCA
i
– Lab. Cs) on
the Y-axis. Enter the correction curve as point pairs on
the resulting graph (temperature / MCAi – Lab. Cs).
The MCAi will create the correction curve by drawing
a line between the entered points and then extending the
line outside the last points at both ends.
NOTE: Do not make any changes to the consistency
calibration or chemicals compensation during the labo-
ratory follow-up period!
At least two points are needed for the curve. To
avoid compensation errors due to inaccurate laboratory
results, make sure that the points are not too close to
each other. The recommended minimum difference
between the points is about 5°C (9°F). When determin-
ing the curve for a larger temperature range, it is
advisable to take a sufficient number of points, at
regular intervals. The following examples illustrate the
principle of entering the correction curve for tempera-
ture compensation.
• Example 1.
The normal process temperature is 40...50°C
(104...122°F) but drops temporarily to 20°C (68°F)
when the process is started up. The graph (MCAi – Lab
Cs vs. Temperature) shown in Fig. 12.16 was drawn by
using laboratory samples taken while the process was
being started.
This graph shows that when the temperature gets
below 30°C (86°F) it has an effect on the MCAi
measurement. Enter the graph as the correction curve as
follows:
1. Select D
IAGNOSTICS
/T
EMP
.C
OMP
(Fig. 12.16).
2. Choose max. 6 point pairs (temperature, MCAi –
Lab) from the curve “MCAi – Lab vs. temperature”.
Choose point pairs at even temperature intervals, so
that when the MCAi connects the points with lines
the resulting graph is representative of the original.
Note that the error must be calculated by deducting the
laboratory consistency from the MCA
i reading, NOT vice
versa!
3. Enter the point pairs (Temp1, MCA – Lab1) on the
display and press function key SEND. The HART
communicator will send the data to the MCAi.
NOTE: Make sure that you use the same “MCAi – Lab”
value for the last two points! Otherwise the correction
curve will continue using the slope between the last two
points.