Temperature Transmitter Preventive Maintenance

In this article, you will learn the temperature transmitter preventive maintenance procedure and the RTD & thermocouple sensors validation.

Contents

Tools

We need the following tools to carry out the maintenance of temperature transmitters.

Digital Multimeter
Decade Box for RTD
Scandura or Beamex for thermocouple as a primary sensing element
HART communicator
Tool bag with all necessary tools

The above tools must be calibrated with master instruments and must have their validity to use for our calibration & maintenance activities.

We need calibration stickers. We have to stick the updated labels on the transmitters and other instruments after completing the PM job.

We have to carry all the required personal protective equipment (PPE) for our PM job.

We need the datasheet for the instruments which are to be planned for maintenance. So we need datasheets for the temperature element and transmitter.

Temperature Transmitter Preventive Maintenance Procedure

Take appropriate work permits and take necessary approvals.
Bypass the interlock if the temperature transmitter is having any interlocks (Always take a bypass approval from operations and fill the Interlock Bypass Form).
If a temperature transmitter is used for process control using a control valve, then ask the operation engineer to put the control valve in manual mode.
Verify the temperature transmitter tag number. You should always ask the value shown by the temperature transmitter on DCS/SCADA/PLC and confirm the value in the field. This is a double confirmation for identifying the proper tag.
Note down the present value shown by the temperature transmitter before the PM job.
Open the back cover of the temperature transmitter and you will find the cable connection terminals.
Note down all the cable connections. So that after job completion, all cable connections are properly restored back.
Check the supply voltage.
Remove the sensor cable connections.
Connect Decade Resistance Box in case of RTD and Scandura or Beamex in case thermocouple with the temperature transmitter. (in place of sensor cables)
Connect the HART communicator to the temperature transmitter
As per the range of the temperature transmitter, configure output in Scandura or Beamex for feeding it to the temperature transmitter. (set resistances in decade resistance box for RTD)
Now apply input values and note down the output of the transmitter (confirm in field display or in HART communicator as well as on DCS/SCADA/PLC).

% of Range	Input from TCI	Value on field display or HART 375/475	Value on DCS/SCADA/PLC
0	0
25	50
50	100
75	150
100	200
75	150
50	100
25	50
0	0

( For example, The above table is filled for temperatures from 0 to 200 degrees Centigrade)

RTD and Thermocouple Testing

We use thermowells for the installation of thermocouples and RTDs in the industries. So we can remove the installed RTDs and thermocouples safely for testing purposes.

Now take RTD or thermocouple to the instrument workshop or calibration lab.

First, you have to note down the temperature sensor range from the datasheet.

Insert the temperature elements in the temperature bath for validation. Also, insert the reference standard temperature sensor in another slot of the temperature bath.

Connect the RTD/thermocouple to temperature baths for measurement purposes, it is available for a few vendor products. (we can also connect them to a multifunction calibrator in order to read the temperature values.)

Set the required temperature on the temperature bath and check the sensor’s output reading. Repeat the same step for different points for the temperature range given in the datasheet.

Use the below-mentioned table to note down the values

% of Range	Input from temperature bath	Output from RTD or thermocouple
0	0
25	50
50	100
75	150
100	200
75	150
50	100
25	50
0	0

If both the transmitter and sensor generate the output with an error less than the maximum permissible error, then both are ok. If the transmitter is having error value of more than the maximum permissible error, then calibrate the transmitter. And after calibration, recheck all the values.
If the temperature sensor is having error value of more than the maximum permissible error, then replace the sensor because it is not possible to calibrate the temperature measuring primary element (RTD and thermocouple). If the sensor does not pass the calibration test then discard it.
Now take the temperature sensor back to the installation location (field) and install it in the original location. Restore the connections as they were before the start of the PM job.
Verify the condition of the cable lugs. If found in bad condition, then do the cable relugging.
Close the back cover of the temperature transmitter and apply monsoon tape or silastic whichever is applicable.
After connections, crosscheck the present values with the old values taken before the PM job and confirm with the operation engineer that they are valid (Because many times due to ongoing process values may differ). If found not ok, then recheck the connections. (optional step)
Check the tightness of the thermowell bolts. If loose, then do proper tightening.
Apply a calibration sticker on the temperature transmitter with the appropriate due date.
Clean the temperature transmitter.
Restore the interlocks if any are bypassed and also ask the operation engineer to put the loop again in auto mode if taken in manual mode.
Close the work permit.
Fill up the calibration record.