K-Patents PR-33-AC - Manual (Page 33)

6 Configuration and calibration 27
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−
low flow which makes sample to form an unrepresentative film on the prism
−
sample evaporation at high temperature or undissolved solids at low temperature
giving deviations from laboratory determinations
−
an ageing sample which is not representative
−
outside light reaching the prism
Thus calibration using the process liquid should always be made inline.
6.2.3 Direct bias adjustment
The concentration measurement value can also be directly adjusted by changing the
field adjustment parameter f00. The bias adjustment is well-suited to situations where
the difference between the refractometer and the laboratory readings is small, and
where there is no clear temperature or concentration dependence of the correction.
Also, if the field calibration points are from a narrow concentration and temperature
range, the bias adjustment is usually the best option.
The value of the bias parameter f00 will be added to the concentration value:
NEW CONC
=
OLD CONC
+
f00
.
6.3 Default mA output value
In the mA output settings it is possible to set a mA default output value that the instru-
ment returns to in certain malfunction situations. The value can be set to a low or high
mA value, e.g. 3.0 mA or 22 mA. The mA measurement always returns to the mA default
output value when error message is SHORT-CIRCUIT, NO SIGNAL, TEMP MEASUREMENT
FAULT or NO SENSOR. When mA output source is configured to concentration, also NO
OPTICAL IMAGE, NO SAMPLE and PRISM COATED will cause the mA measurement to
return to the default mA output value.
NAMUR is an international association of users of automation in process industries.
The association recommendation NE 43 promotes a standardization of the signal level
for failure information. The goal of NE 43 is to set a basis for proactively using trans-
mitter failure signals in process control strategies. Using these failure signals, instru-
ment faults are separated from process measurements.
NAMUR NE 43 uses the 3.8 to 20.5 mA signal range for measurement information, with
≥21 mA or ≤3.6 mA to indicate diagnostic failures (see figure 6.3). With that informa-
tion, it is easier to detect a failure condition on a refractometer, for example, it clearly
tells you whether you have an empty pipe or a failed instrument.