Scaling Temperature Transmitters for Accurate Measurements

Temperature transmitters are employed to transmit a signal from a temperature sensor, such as a RTD or thermocouple, to a control or measurement device. These temperature transmitters amplify and condition the signal generated by the sensor before transmitting it to the recording device. They can also reduce noise from EMI and RFI that may interfere with signals generated by temperature sensors and enhance the measurement accuracy. Although PLC and DCS systems record measurements over the entire sensor range, it is possible to calibrate a temperature transmitter to any particular range within its capabilities. Accuracy is improved by limiting the measurements to a narrow range.

Thermocouple Transmitter Calibration

Two copper wire leads are used to connect a thermocouple transmitter to an unregulated power supply. The leads are used to power the transmitter and to transport output current to a recording device. After receiving a signal from the thermocouple, the transmitter processes it and transports output current that is directly relative to the millivolt input from the thermocouple. The signal begins at 4 mA for temperatures at the low-end range and increases to 20 mA for temperatures at the high end. The transmitter can either be mounted inside or on the surface of a protection head. The thermocouple extension wires are replaced by two copper wires used to transmit the 4 to 20 mA signal and supply DC voltage to the transmitter.

Thermocouple or RTD Temperature Transmitters

Thermocouple or RTD Temperature Transmitters

Equipment needed for calibration of the thermocouple transmitter includes:

  • Precision mV source with mV resolution of 0.001 and mV accuracy of ±0.002 or
  • Precision DVM with mV accuracy of ±0.002 mV and an adjustable mV source with mV resolution of 0.001
  • Precision DMM with mA resolution of 0.001 and mA accuracy of ±0.002
  • Reference thermocouple
  • A stable ice bath

An ice bath is prepared by filling a glass beaker with crushed ice made from distilled water and adding sufficient distilled water to create slush, but care must be taken to ensure that enough water is not added for the ice to float. The reference thermocouple should then be inserted. Alternatively, a thermocouple calibrator can be employed instead of the ice bath, voltage source and DVM.

For calibration using the ice bath, the transmitter needs to be connected to the DMM monitor and dc power supply. Then, the reference probe should be connected to the mV source or DVM using a copper wire. In order to calibrate using a thermocouple calibrator, the transmitter has to be connected to the DMM monitor and dc power supply, and the input thermocouple wires from the calibrator should be connected to the transmitter. It must be ensured that the thermocouple wire is the same calibration as that of the transmitter and that the wiring polarities are correct.

Users need to find the Z (zero) and S (span) potentiometers on the transmitter. The manufacturer’s specifications can be referenced to achieve the mV input values for the Z (zero) and S (span) adjustments in proportion to the required temperature range. The appropriate Z (zero) and S (span) values can be selected by using a calibrator. The DC mV source should be set to the Z (zero) mV value corresponding to the low end of the temperature range, and the Z potentiometer needs to be adjusted to read 4.000 mA on the DMM monitor. Then, the DC mV source should be set to the S (span) mV value corresponding to the high end of the temperature range, and the S potentiometer can be adjusted to read 20.000 mA on the DMM monitor. The potentiometer adjustments can be repeated until the values displayed are precisely 4.000 mA and 20.000 mA.

RTD Transmitter Calibration

Generally powered by an unregulated power supply, the RTD transmitter is compatible with 2 or 3 wire RTDs. Once the transmitter receives the input, it sends output current that is directly proportional to the RTD sensor. The transmitter can be mounted either inside or on the surface of a protection head. Two copper wires are employed to supply dc voltage to the transmitter and transmit the temperature signal.

Equipment needed for calibration of the RTD transmitter includes:

  • Precision DMM with mA accuracy of ±0.002 and mA resolution of 0.001
  • Precision RTD simulator
  • Precision Decade Resistance Box with ohm accuracy of ±0.02 and ohm resolution of 0.01

The transmitter has to be connected to the DMM monitor and DC power supply. This transmitter can then be attached to either a Decade Resistance Box or an RTD calibrator using copper wire. Users need to find the Z (zero) and S (span) potentiometers on the transmitter. The manufacturer’s specifications have to be referenced in order to acquire the ohmic values for the Z (zero) and S (span) adjustments that correspond to the preferred temperature range.

If an RTD simulator is used, then the appropriate Z (zero) and S (span) values have to be selected. Next, the Decade Resistance Box should be set to the Z (zero) ohmic value corresponding to the low end of the temperature range, and the Z potentiometer has to be adjusted to read 4.000 mA on the DMM monitor. Then, the Decade Resistance Box has to be set to the S (span) ohmic value that corresponds to the high end of the temperature range, and the S potentiometer has to be adjusted to read 20.000 mA on the DMM monitor. The potentiometer adjustments need to be repeated until the displayed values are exactly 4.000 mA and 20.000 mA.

Conclusion

Temperature transmitters provide a great deal of flexibility in scaling the analog output signal in relation to the input. In order to achieve increased accuracy, temperature transmitters separate the signal, filter noise and amplify the signal. Thermocouple and RTD transmitters offer a full scale accuracy of ±0.1%. In addition, temperature transmitters provide stability by separating signals from electromagnetic and radio frequency interference. Due to interaction between the S and Z potentiometers, repeat calibration is necessary.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

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