A thermocouple is a temperature sensor, like a thermal resistor, both are temperature sensors, but the main differences between them are:
1. Signal nature: A thermal resistor itself is a resistor; temperature changes cause positive or negative resistance changes. A thermocouple generates induced voltage changes that vary with temperature.
2. The temperature range detected by the two sensors is different. Thermal resistors generally detect temperatures from 0 to 150 degrees Celsius, with a high measurement range up to about 600 degrees (and can also detect negative temperatures). Thermocouples can detect temperatures from 0 to 1000 degrees Celsius (or even higher). Therefore, the former is for low-temperature detection, and the latter is for high-temperature detection.
3. In terms of material, a thermal resistor is a metal material that is sensitive to temperature changes. A thermocouple is a bimetallic material, i.e., two different metals, which produce a potential difference at the ends of the two different metal wires due to temperature changes.
4. The input modules for thermal resistors and thermocouples in PLCs are also different. This statement is correct, but generally PLCs directly accept 4-20mA signals, and thermal resistors and thermocouples usually have transmitters before connecting to PLCs. If connected to DCS, transmitters are not necessary! Thermal resistors use RTD signals, and thermocouples use TC signals!
5. PLCs also have thermal resistor modules and thermocouple modules that can directly input resistance and thermocouple signals.
6. Thermocouples have types such as J, T, N, K, S, etc. Some are more expensive than thermal resistors, and some are cheaper. However, when including compensation wires, the overall cost of thermocouples is higher. Thermal resistors output resistance signals, while thermocouples output voltage signals.
7. The temperature measurement principle of thermal resistors is based on the property that the resistance of a conductor (or semiconductor) changes with temperature. Commonly used ones include platinum resistors (Pt100, Pt10) and copper resistors Cu50 (range -50 to 150 degrees).
8. The temperature measurement principle of thermocouples is based on the thermoelectric effect. Commonly used ones include Platinum Rhodium—Platinum (index S, range 0~1300°C), Nickel Chromium—Nickel Silicon (index K, range 0~900°C), Nickel Chromium—Constantan (index E, range 0~600°C), and Platinum Rhodium 30—Platinum Rhodium 6 (index B, range 0~1600°C).
Method 1 to distinguish between thermocouples and thermal resistors: Determine by the nameplate. The nameplate will contain specific product and specification information for either a thermocouple or a thermal resistor. By carefully observing and distinguishing, you can tell whether it is a thermocouple or a thermal resistor.
Method 2 to distinguish between thermocouples and thermal resistors: Determine by structure. A thermocouple generally consists of a thermode, insulating tube, protective tube, and terminal box. A thermal resistor has the sensor output load and power supply connected in series.
Method 3 to distinguish between thermocouples and thermal resistors: Determine by the terminal board. A thermocouple has positive and negative poles, while a thermal resistor does not.
You can also look at the number of wires: Thermal resistors generally use three-wire systems, with dual elements having six wires; single thermal resistors rarely use two-wire systems. Thermocouples generally use two-wire systems, with dual elements having four wires.
