8
5.3 Diagnostic function
By measuring with two different, thermically coupled sensor elements (NTC,
PT 1000) the unit automatically detects drifts and errors during temperature
measurement�
A mean average value is formed from the measured individual NTC and Pt 1000
values� This mean value is the basis for the provided measured temperature value
and also the basis for the diagnostic messages with the drift monitoring
�
Besides temperature drift other errors can be detected� Via the parameter drEd
you set which of the diagnostic cases is signalled�
The diagnostic message cases are provided by an analogue signal in 2-wire
operation, in addition by a switching signal or IO-Link diagnostic data in 3-wire
operation�
5.3.1 Drift monitoring
For drift monitoring the unit compares the temperature deviation of sensor element
1 (NTC) and sensor element 2 (Pt 1000) from the mean value� The permissible
temperature deviation is defined by the parameters drW = warning threshold and
drA = alarm threshold� Based on the alarm threshold drA the drift failure threshold
drF is automatically calculated (drF = drA + 3 K)� Setting drA = OFF automatically
results in: drF = 8 K�
When values exceed these thresholds, the unit identifies it as a diagnostic case�
t
T [°K]
3
2
1
drW
drA
drF
=
drA
+
3
K
Figure 2: Drift monitoring
Example: NTC sensor element (1) measures 65 °C, Pt 1000 sensor element (2) measures
60 °C� The mean value (3) is 62�5 °C, i�e� both elements deviate by 2�5 K�
With setting drW = 2 °C and drA = 5 °C a warning message would be provided if drEd = OFF�