Honeywell SmartLine Multivariable SMV800 Series User Manual page 45

T/C Fault Detect
:
WARNING
To accurately set the device status and Analog Output, it is highly recommended to
enable T/C or RTD fault detection.
The behavior of the device and process values is explained below when this setting is OFF vs ON to
explain why it is recommended to configure this setting ON always.
If the T/C Fault detect is OFF:
The reported temperature value may or may not be reported as a fault condition depending upon how
the open T/C connection drifts. For active temperature compensation during flow calculations an
undetected open thermocouple may result in a condition where the reported flow value is inaccurate.
For this reason, it highly recommended that open thermocouple detection is turned on so that the
active temperature is used for flow compensation.
Regardless of what device variable is mapped to Analog Output, when the open input condition
occurs, device will report non-critical status, but Flow calculation will use the reported Temperature
value. Note that this case may result in inaccurate Flow value. If the sensor is repaired, the status is
cleared without device reset.
If the T/C Fault detect is ON:
When Temperature is mapped to Analog Output, on detecting open input, device will report critical
status, Temperature value will be set to NaN and Flow value will also be set to NaN. Analog Output
will be in burnout.
When DP, SP is mapped to Analog Output, on detecting open input, device will report non-critical
status, Temperature will be reported value, but Flow value will be set to NaN (when PT failsafe ON),
Analog Output will follow the input DP or SP.
When Flow is mapped to Analog Output, on detecting open input, device will report; critical status
when PT failsafe is ON. Flow will report NaN, Temperature will be reported value, Analog Output
will be at burnout or non-critical status when PT failsafe is OFF. Temperature will be reported value;
Flow calculation will use the Design or Nominal temperature value based on the selected algorithm
and report a valid Flow value. Analog output will follow the calculated flow.
Background
You can turn the transmitter's temperature sensor fault detection function ON or OFF through
configuration.
•
With the detection ON, the transmitter drives the PV3 output to failsafe in the event of an
open RTD or T/C lead condition. The direction of the failsafe indication (upscale or
downscale) is determined by the failsafe jumper on the PWA.
•
When fault detection is set to OFF, these same fault conditions result in the transmitter not
driving the output to failsafe and reporting a non-critical status for an open RTD sensing lead
or any T/C lead. But when an open RTD compensation lead is detected, the transmitter
automatically reconfigures itself to operate without the compensation lead. This means that a
4-wire RTD would be reconfigured as 3-wire RTD, if possible and thus avoiding a critical
status condition in the transmitter when the transmitter is still capable of delivering a
reasonably accurate temperature output.
Revision 6.0
SMV800 Series HART/DE Option User's Manual
Page 33