www.miinet.com
Moore Industries-International, Inc.
- 13 -
User’s Manual
238-760-00K
May 2019
Functional Safety Dual Input Smart HART® Temperature Transmitter
STZ
SECTION 1
2ohms
40ohms
35ohms
WARNING
RTD WIRE
RESISTANCE
RTD
Wi
re
Resistanc
e
T/C Circuit Resistanc
e
CAPTURED
VALUE
ALARM
2ohms
35ohms
Different Values
Same Values
RTD WIRE
RESISTANCE
CAPTURED
VALUE
WARNING
& ALARM
200ohms
4,000ohms
3,500ohms
3,500ohms
WARNING
T/C CIRCUIT
RESISTANCE
CAPTURED
VALUE
ALARM
200ohms
Different Values
Same Values
T/C CIRCUIT
RESISTANCE
CAPTURED
VALUE
WARNING
& ALARM
TIME
Initial
Minimal
Difference
Typical Drift Over Time
T
USER
DEFINED
DRIFT
LIMITS
Drift Alert
Activated
Sensor Drift Detection
Drifting in sensors occurs from a number of different causes including environmental
contamination, vibration, and extreme temperature fluctuations. This can lead to false sensor
readings. The STZ
can be configured to detect these false readings by using a Range Alarm to
monitor the absolute differential between two sensors (e.g., RTD & T/C or two RTDs or
T/Cs) at the same point. This method can provide a range alarm warning or alarm to indicate
that there is an issue with one of the sensors. See Chart below for clarification.
In this example a Range Alarm is set with the Absolute Differential as the source. An out of
range warning or alarm is configured with the low and high limits set to 0C° to 5C° respectively,
creating an alarm when Sensor 1 and Sensor 2 readings drift apart by more than 5C°.
Note:
When setting the drift limit please allow for sensor tolerances and differences in sensor
response times to avoid false drift alerts.
Corrosion Detection
Corrosion of the sensor connections is primarily caused by oxidation or electrolysis which can
lead to inaccurate readings with RTD measurements. The STZ is able to detect corrosion on 3W
or 4W RTDs by measuring the change in compensating lead wire resistance between terminals 3
and 4 providing an alert when preset value limits are exceeded.
Additionally the STZ monitors the status of T/C circuits. High resistance values in T/C circuits
could indicate a potential impending T/C burnout.