The temperature transmitter is an electronic instrument widely used in the field of industrial process control. Its core function is to convert the weak signals detected by temperature sensors (such as thermocouples, thermal resistors RTDS or thermistors) into standard industrial process signals (the most common being 4-20mA DC current signals or digital signals), and then transmit this signal to display instruments, controllers, data acquisition systems or actuators located in the control room or at a distance.
The working principle of a temperature transmitter can be summarized into the following key steps:
Temperature perception and raw signal generation:
A temperature sensor (usually a thermocouple or thermal resistor such as Pt100) comes into direct contact with the medium being measured to sense its temperature changes.
Thermocouple (T/C) : Based on the Seebeck effect, when there is a temperature difference between two different metals at the measuring end (hot end) and the reference end (cold end), a thermoelectric potential (millivolt-level voltage signal, mV) proportional to the temperature difference will be generated in the circuit.
Thermal resistance (RTD) : such as Pt100, based on the physical property that the resistance value of the metal conductor increases with rising temperature (positive temperature coefficient). Temperature changes cause its resistance value to change (for example, it is 100Ω at 0℃).
Thermistors: Based on the characteristic that the resistance value of semiconductor materials changes significantly with temperature, they are classified into negative temperature coefficient (NTC) and positive temperature coefficient (PTC) types.
Signal conditioning (key step) :
Amplification: The original signal generated by the sensor (MV-level voltage or resistance changes) is extremely weak. The electronic circuit inside the transmitter first linearly amplifies it to a standard level suitable for subsequent processing.
Cold end compensation (for thermocouples) : The thermoelectric potential generated by a thermocouple is a function of the temperature difference between the hot end and the cold end (reference end, usually located at the internal terminal of the transmitter). To obtain an accurate measured temperature (relative to 0℃), the transmitter needs to measure the actual temperature at its terminal (cold end temperature), calculate the thermoelectric potential that needs to be compensated based on this temperature, and superimpose (or equivalent process) it onto the original signal, thereby eliminating the error caused by the change in cold end temperature.
Linearization: The thermoelectric potential/resistance-temperature relationship between thermocouples and thermal resistors is not a perfect straight line but has a certain degree of nonlinearity. The transmitter usually stores the linearization curve corresponding to the sensor type inside (or calculates it using a formula). The amplified/compensated signal is linearized to directly and linearly represent the measured temperature value.
Low-pass filtering: It removes high-frequency noise that may be present in the signal (such as electromagnetic interference, vibration interference, etc.) to enhance the stability and accuracy of the signal.
Signal conversion
Convert the analog signal (voltage) that has been conditioned (amplified, compensated, linearized, filtered) and precisely represents the measured temperature into an industrial standard output signal.
The most commonly used output signal is the 4-20mA current signal: the converted current signal flows through the loop. Zero temperature or the lower limit of the range usually corresponds to 4mA, and the full-scale temperature corresponds to 20mA. Why 4-20mA?
4mA zero offset: It can conveniently distinguish truly effective low signals (4mA) from sensor disconnection line faults (0mA).
Strong anti-interference: Compared with voltage signals, current signals are not sensitive to changes in wire resistance and voltage drops during long-distance transmission and are less likely to be interfered by electromagnetic noise.
Two-wire power supply: Many transmitters adopt a two-wire design, that is, they provide power and transmit current signals simultaneously through two wires. The minimum value of 4mA ensures the transmitter's own minimum operating current requirement (commonly referred to as the "active zero point").
Signal transmission
The converted standard signal (such as 4-20mA) is transmitted to the remote end through wires. Due to its standardized characteristics, control rooms or PLCS and other equipment can directly receive and process this signal for:
Display the temperature value (on the panel table, DCS/SCADA operator station).
Input to the controller (such as a PID controller) for logical operations and regulation.
Stored in the historical database or used for alarm judgment.
Drive the actuator (if temperature-based control is required).
