Temperature transmitters use thermocouples or thermal resistors as temperature sensing elements. The output signal from the sensing element is sent to the transmitter module, where it undergoes processing through circuits such as voltage stabilization and filtering, operational amplification, nonlinear correction, V/I conversion, constant current, and reverse protection. This processing converts the signal into a standard current or voltage output that has a linear relationship with temperature. It is primarily used for measuring and controlling temperature parameters in industrial processes.

   Temperature transmitters typically consist of two parts: the measurement unit and the signal processing and conversion unit. Some transmitters include an additional display unit, and some also feature fieldbus functionality.  

   Inspection and Testing Methods for Temperature Transmitters

During the use of temperature transmitters, regular inspections and performance tests should be conducted. The routine inspection items for temperature transmitters mainly include the contents shown in Table 1. In addition to routine inspections, periodic testing of the metrological performance and insulation performance of temperature transmitters is also required, typically with a testing cycle not exceeding one year.

The metrological performance inspection of temperature transmitters can be conducted in two ways: with the sensor or without the sensor.

1) Inspection with the sensor: Insert the sensing part of the transmitter into a standard temperature source, and calibrate the transmitter's output current by changing the standard temperature input.

2) Inspection without the sensor: Disconnect the sensing element (thermal resistor or thermocouple) of the transmitter, and use a standard resistance source and process inspection equipment to test the signal conversion part of the transmitter.

    In practical use, the probability of failure or deviation in sensing elements such as thermal resistors or thermocouples in temperature transmitters is relatively low, and the introduced error is small. Therefore, inspection without the sensor is usually sufficient for temperature transmitters. In high-demand scenarios or when there are concerns about the performance and accuracy of the sensing element, inspection with the sensor is necessary. The metrological performance inspection of temperature transmitters involves calibrating the measurement error of the transmitter to determine whether it meets accuracy requirements.

1) Calibration point selection: The selection of calibration points should be evenly distributed across the range, generally including no fewer than five points, such as the upper limit, lower limit, and points near 50% of the range.

2) Pre-calibration adjustment: With the sensor disconnected, adjust the corresponding output lower and upper limits by changing the input signal to align them with the theoretical lower and upper limits.

3) Calibration method: For transmitters with sensors, the measurement sequence can start from the lower limit temperature of the measurement range and proceed upward sequentially. At each test point, measurements should be taken only after the temperature in the temperature source has stabilized sufficiently. For transmitters without sensors, input the signal values corresponding to each calibration point smoothly from the lower limit, read and record the output values up to the upper limit, then reverse the direction and smoothly change the input signal back to each calibration point, reading and recording the output values down to the lower limit.

4) Data processing: Principles for processing measurement results and error calculations.