• If the measured temperature is outside the operating temperature
range for the process connection (-32°C ... +170°C), the diagnostic
output provides an alarm message, the analogue output changes to
the fault value which is defined with the parameter FOU2.
The fault number Fnr = 53 or Fnr = 54 is set.
Longer operation of the sensor outside the operating temperature
range can lead to permanent measuring faults and drifts!
Drift / fault monitoring
To monitor the drift the sensor compares the temperatures of two dif-
ferent sensor elements that are thermically coupled in the sensor tip.
Normally these temperatures should be identical.
Due to the usual manufacturing tolerances a temperature difference of max.
0.1K can also occur with new sensor elements. This does not affect the drift
monitoring function.
If there is a drift in one or both sensor elements, the sensor detects
them due to the difference between the two measured temperatures.
The sensor compares the difference with the set warning / alarm
thresholds (
drW
,
drA
). If the thresholds are exceeded the diagnostic
output provides a corresponding warning / alarm message. The ana-
logue output still provides a temperature-proportional current signal.
In case of high temperature changes (e.g. filling of a hot medium into
a cold vessel) there may be a short-term difference between the mea-
sured values of both measuring elements. To prevent a drift warning
or alarm in these cases, a delay time can be set by means of the sen-
sor parameter
ddr
.
Sensor backup / redundancy switching
If one of the two temperature measuring channels fails (e.g. short cir-
cuit of one of the two sensor elements) the temperature can still be
measured with the working channel (= sensor backup). However, drift
monitoring is not possible any more.
The characteristics of the sensor if one measuring element fails is defi-
ned by means of the sensor parameter
drEd
.
29
ENGLISH
