The accuracy of a temperature transmitter is THE key aspects to look into, as with many instrumentation transmitter, a temperature transmitter needs to be adjusted or calibrated. So the current output of 4-20mA represents the temperature range of the process variable being measured, for example, a process variable temperature range of zero degree Celsius to 100 degree Celsius will produce a current range of 4 to 20 mA.

In every calibration scenario, a sensor is replaced by a device that can simulate the output of the sensor over the entire process variable temperature range.  For a thermocouple, the sensor is replaced by a device that produces a range of millivolts. For an RTD sensors, the sensor is replaced by a device that produces a range of resistances. Devices produces resistances such as decade box,  today ,we’ve got signal reference calibrators to generate various reference signals for RTD and thermocouples which makes the calibration for temperature transmitters a lot more easier,  For an old style temperature transmitters, they got two potentiometers on the unit, Zero and Span, when devices simulate temperature sensor signals feed the transmitter with a Zero signal, the output of the transmitter should be 4mA, the real output could be off which is not 4mA, the whole purpose of calibrating is to make the output 4mA, so what you do is fine tune the Zero potentiometer on the temperature sensors until you get the output reading at 4mA, For span calibration , the process is similar, feed the temperature transmitter with a span signal, and then fine tune the span potentiometer until you get the reading from the output at 20mA. For modem head mount or Din rail mount temperature transmitter, there is no Zero or Span potentiometer on the unit itself, instead, factory will have software, all you have to do is connect the temperature transmitter to the PC, and then fine tune the output on the software to 4mA and 20mA which corresponds to the minimum and maximum inputs. We at Maxwell, provide software for recalibration purpose for our head mount temperature transmitters.