What do you know about the pros and cons of thermocouples

Advantages:

A. Wide temperature range: From low temperatures to jet engine exhaust, thermocouples are suitable for most practical temperature ranges. Thermocouples measure temperatures from -- 200°C to +2500°C, depending on the metal wire used.

B. Strong and durable: Thermocouple is a durable device with good shock and vibration resistance, suitable for dangerous and harsh environment.

C. Fast response: because of their small size and low heat capacity, the thermocouple responds quickly to temperature changes, especially when the induction junction is bare. They respond to temperature changes within hundreds of milliseconds.

D. No spontaneous heat: because the thermocouple does not need an excitation power supply, it is not easy to spontaneous heat, and its own is safe.

Disadvantages:

A. Complex signal conditioning: converting thermocouple voltage into usable temperature readings requires extensive signal conditioning. Signal conditioning has always consumed a lot of design time, improper processing will introduce errors, resulting in reduced accuracy.

B. Low accuracy: In addition to the inherent inaccuracy of the thermocouple due to the metal characteristics, the thermocouple measurement accuracy can only reach the measurement accuracy of the reference junction temperature, generally within 1°C to 2°C.

C. Susceptibility to corrosion: Because the thermocouple consists of two different metals, corrosion over time may reduce accuracy in some operating conditions. Therefore, they may need protection; And maintenance is essential.

D. Poor noise resistance: Noise generated by stray electric and magnetic fields may cause problems when measuring millivolt level signal changes. Stranded thermocouple wire pairs may significantly reduce magnetic field coupling. Electric field coupling can be reduced by using shielded cables or by wiring and shielding in metal conduits. The measuring device shall provide hardware or software signal filtering to strongly suppress the power frequency (50 Hz/60 Hz) and its harmonics.

Selection elements of thermocouple and thermal resistance

We can choose thermocouple and thermal resistance according to the following factors.

Need to measure the temperature range: 500℃ above the general choice of thermocouple, 500℃ below the application environment to choose.

Measurement range selection: thermocouple measurement generally refers to the "point" temperature, thermal resistance is usually used to measure space temperature.

Cold junction compensation

Due to the principle of thermoelectric effect. Therefore, an additional temperature sensor is required to measure the temperature of the reference point, also known as the cold end compensation point.

Several common cold end compensation sensors are as follows:

1. Thermistor: quick response, small package. But it requires linearity and limited accuracy, especially over a wide temperature range. Required excitation current, will produce spontaneous heat, causing drift. Combined with the signal conditioning function of the overall system precision is poor, only suitable for low measurement accuracy, low cost applications.

2. Resistance type temperature measuring device (RTD) : RTD compared to thermistor, more accurate, stable and linear characteristics, but the package size and cost, relatively high thermistor. Because of the need for well-matched excitation sources and sampling circuits, the design is relatively more complex and requires better peripherals. Thermocouple measurement systems using RTD as cold end compensation usually require higher precision at system level.

3. Integrated temperature sensor: integrated temperature sensor is a kind of integrated temperature measuring element made of semiconductor process. Through semiconductor technology, the information obtained from analog units such as temperature measurement can be digitally output, with high integration and system level accuracy of far less than 1°C. The peripheral circuit is simple in design and can communicate with MCU directly. The cold end compensation scheme for high-precision thermocouple acquisition system is also simple in use and design.