Temperature Transmitter

Overview

A temperature transmitter is a stand-alone instrument that converts a small sensor signal (mV from a thermocouple or Ω from an RTD) into a standard current-loop output (4–20 mA) suitable for PLC or analog data acquisition. Most industrial sensors are deployed 10–500 m from the control room, and sending their raw signal over long distances introduces noise, voltage drop, and cross-talk. The transmitter sits at or near the sensor, converts the delicate signal to a robust current loop immune to wiring resistance and EMI, and delivers a 4–20 mA output that any instrument can read without calibration. Transmitters handle RTD four-wire connections, thermocouple cold-junction compensation, sensor break detection, and temperature linearization.

The instrument comprises a Input Module accepting thermocouple or RTD connections, a Signal Conditioner isolating the signal from noise, a Linearization Processor converting raw sensor reading to temperature, and a Output Driver pushing the 4–20 mA loop. The Transmitter Housing is a compact DIN-rail device mounted in the control cabinet.

How it works

For RTD (resistance thermometer detector) inputs, the Excitation Source drives a precision 1 mA current through the Pt100 sensor. The resulting voltage drop (typically 0–100 mV over −50 °C to +100 °C) is filtered by Input Filter to reject power-line hum, amplified by Input Amplifier, and isolated by Isolation Amplifier to break ground loops.

For thermocouple inputs, the Reference Junction thermistor automatically compensates for cold-junction (reference) temperature, which is essential because thermocouple voltage depends on both hot-junction temperature and the reference-point temperature. NIST polynomial tables for K-type, J-type, and other standards are burned into Calibration Memory during factory calibration.

The Microcontroller samples the ADC continuously and applies the Callendar–Van Dusen linearization equation for RTDs or the NIST inverse polynomial for thermocouples to convert resistance or voltage to temperature in °C. Two-point field calibration allows adjustment if the sensor has drifted: the operator immerses the sensor in ice and boiling water, buttons the transmitter to accept calibration points, and the MCU stores the offset and slope.

The Digital-to-Analog Converter and Current Sink then drive the 4–20 mA loop at the calculated temperature. An open-sensor or over-temperature fault is flagged by pushing the loop to 3.6 mA (below 4 mA), which any receiver recognizes as an alarm condition. The Isolation Amplifier provides 1500 V galvanic isolation between sensor input and loop output, preventing ground loops and protecting both sensor and control electronics if wiring touches live conductors.