If reliable temperature measurement is to be carried out, the first step is to choose the correct temperature instrument, that is, temperature sensor. Thermocouples, thermistors, platinum resistors (RTDs), and temperature ICs are the most commonly used temperature sensors in testing.
The following is an introduction to the characteristics of thermocouple and thermistor temperature instruments.
1. Thermocouple
Thermocouples are the most commonly used temperature sensors in temperature measurement. Its main advantages are a wide temperature range and adaptability to various atmospheric environments, as well as being sturdy, affordable, requiring no power supply, and also the cheapest. A thermocouple is composed of two different metal wires (metal A and metal B) connected at one end. When one end of the thermocouple is heated, there is a potential difference in the thermocouple circuit. Temperature can be calculated using the measured potential difference.
However, there is a non-linear relationship between voltage and temperature, and temperature requires a second measurement for the reference temperature (Tref) due to the non-linear relationship between voltage and temperature. Testing equipment software or hardware is used to process the voltage temperature transformation inside the instrument to ultimately obtain the thermocouple temperature (Tx). Both Agilent 34970A and 34980A data collectors have built-in measurement and computation capabilities.
In short, thermocouples are the simplest and most versatile temperature sensors, but they are not suitable for high-precision measurements and applications.
2. Thermistor
Thermistors are made of semiconductor materials, mostly with a negative temperature coefficient, meaning that the resistance decreases with increasing temperature. Temperature changes can cause significant changes in resistance, making it the most sensitive temperature sensor. However, the linearity of thermistors is extremely poor and is closely related to the production process. The manufacturer cannot provide a standardized thermistor curve.
Thermistors have a very small volume and respond quickly to temperature changes. But thermistors require the use of a current source, and their small size makes them extremely sensitive to self heating errors.
Thermistors measure absolute temperature on two lines with good accuracy, but they are more expensive than thermocouples and can measure temperatures within a smaller range. A commonly used thermistor has a resistance of 5k Ω at 25 ℃, and every 1 ℃ temperature change causes a resistance change of 200 Ω. Note that a lead resistance of 10 Ω only causes a negligible error of 0.05 ℃. It is highly suitable for current control applications that require fast and sensitive temperature measurement. Small size is advantageous for applications with space requirements, but attention must be paid to preventing self heating errors.
Thermistors also have their own measurement techniques. The advantage of thermistors is their small size, as they can quickly stabilize without causing thermal loads. However, it is also very weak as high currents can cause self heating. Due to the fact that thermistors are resistive devices, any current source will generate heat on them due to power. Power is equal to the product of the square of current and resistance. Therefore, a small current source should be used. If the thermistor is exposed to high heat, it will cause permanent damage.
