1.Overview
① Definition
RTD,Resistance Temperature Detector
A temperature sensor based on the variation of metal resistance with temperature.
② Principle
The resistance value of metals varies with temperature. The temperature can be calculated by measuring the resistance value.
2.Materials
Metals such as platinum (Pt), copper (Cu) or nickel (Ni) are usually used
Among them, platinum resistance (PT100) is the most common.
3.Characteristics
① High precision: Within the range of -200℃ to 850℃, the measurement accuracy of the thermal resistance is relatively high.
② Good stability: After long-term use, its characteristics change little.
③ Good linearity: The temperature-resistance characteristic curve is close to linearity, which is convenient for calibration and calculation.
④ External power supply required: The resistance of a thermal resistor needs to be measured through an external current source
4.Signal generation
According to the temperature change, the resistance value is converted into a voltage or current signal by using the measurement circuit
①Two-wire measurement: Simple but easily affected by lead resistance.
②Three-wire measurement: It can better compensate for the influence of lead resistance.
③Four-wire measurement: The most accurate, fully compensating for lead resistance.
5.Signal processing
A constant current source is needed to drive the RTD, measure the voltage across its terminals, calculate the resistance value based on the voltage and the known current, and then calculate the temperature.
6.Application circuit
Constant current source drive, voltage measurement, analog-to-digital conversion, temperature calculation
1.Overview
① Definition:
Thermocouple, a type of temperature sensor based on the thermoelectric effect (Seebeck effect).
② Principle
- Based on the thermoelectric effect (Seebeck effect).
When two different metals/semiconductors are connected in a circuit and the two contact points are at different temperatures, a thermoelectric potential will be generated in the circuit.
Calculate the temperature difference by measuring this electromotive force.
2.Materials
It is composed of two different metals/alloys. Common types include K-type (nickel-chromium - nickel-silicon), J-type (iron-copper-nickel), T-type (copper-copper-nickel), etc.
3.Application Circuit
High input impedance amplifier, cold junction compensation, analog-to-digital conversion, temperature table lookup or calculation
4.Characteristics
① Wide temperature range: Thermocouples are suitable for temperature measurement from -200℃ to 1800℃.
② Fast response speed: Thermocouples have a relatively fast response speed to temperature changes.
③ No external power supply required: Thermocouples generate electromotive force based on temperature differences and do not require an external power supply.
④ Lower accuracy: The accuracy of thermocouples is relatively low, usually ranging from ±1℃ to ±2℃.
⑤ Linearity difference: The temperature-electromotive force characteristic curve of the thermocouple is nonlinear, and temperature conversion needs to be carried out through table lookup or calculation.
⑥ Poor stability: Its characteristics may change after long-term use, and regular calibration is required.
5.Signal generation
The temperature difference at the connection of two different metals generates an electromotive force (voltage).
The temperature is calculated by measuring the electromotive force.
① Microvolt voltage P: The voltage signal is usually within the range of microvolts to millivolts.
② Nonlinearity: The relationship between voltage signals and temperature is nonlinear, and it is necessary to look up tables or use specific algorithms for temperature conversion.
③ Cold end compensation: When measuring temperature, cold end (reference end) compensation is required to accurately calculate the actual temperature.
6.Signal processing
A high-precision amplifier is required to amplify the tiny thermoelectric potential, and a cold junction compensation circuit is needed to compensate for the temperature at the reference end.
Thermal resistors and thermocouples each have their advantages and disadvantages. There is no absolute "better", only choices that are more suitable for specific applications. After understanding their core differences, engineers and technicians can select the most suitable temperature sensing solution based on factors such as measurement range, accuracy requirements, environmental conditions and budget. With technological advancement, the performance boundaries between the two may change, but the fundamental principle differences will persist for a long time, making it a basic knowledge in the field of temperature measurement to understand their distinctions.