To perform reliable temperature measurements, the first step is to select the correct temperature instrument, also known as a temperature sensor. Thermocouples, thermistors, platinum resistance thermometers (RTDs), and temperature ICs are among the most commonly used temperature sensors in testing.
The following is an introduction to the characteristics of thermocouples and thermistors.
Thermocouples: Thermocouples are the most commonly used temperature sensors in temperature measurement. Their main advantages are a wide temperature range and adaptability to various atmospheric environments. They are also robust, inexpensive, require no power supply, and are the cheapest option. A thermocouple consists of two different metal wires (metal A and metal B) connected at one end. When one end of the thermocouple is heated, a potential difference exists in the thermocouple circuit. The temperature can be calculated using the measured potential difference.
However, the relationship between voltage and temperature is non-linear. Because of this non-linear relationship, a second measurement is needed for a reference temperature (Tref). The voltage-temperature conversion is then processed internally by the testing equipment's software or hardware to finally obtain the thermocouple temperature (Tx). Both the Agilent 34970A and 34980A data acquisition units have built-in measurement and processing capabilities.
In short, thermocouples are the simplest and most versatile temperature sensors, but they are not suitable for high-precision measurements and applications.
Thermistors, on the other hand, use semiconductor materials and mostly have a negative temperature coefficient, meaning their resistance decreases as temperature increases. Temperature changes cause large resistance changes, making them the most sensitive temperature sensors. However, thermistors have extremely poor linearity and are highly dependent on the manufacturing process. Manufacturers do not provide standardized thermistor profiles.
Thermistors are very small and respond quickly to temperature changes. However, they require a current source, and their small size makes them extremely sensitive to self-heating errors.
Thermistors measure absolute temperature on two wires, offering good accuracy, but they are more expensive than thermocouples, and their measurable temperature range is smaller. A commonly used thermistor has a resistance of 5kΩ at 25°C, with a 1°C temperature change causing a 200Ω resistance change. Note that the 10Ω lead resistance introduces only a negligible error of 0.05℃. It is ideal for current control applications requiring rapid and sensitive temperature measurement. Its small size is advantageous for space-constrained applications, but self-heating errors must be prevented.
Thermistors also have their own measurement techniques. Their small size is an advantage; they stabilize quickly and do not create a thermal load. However, this also makes them less robust, and high currents can cause self-heating. Because a thermistor is a resistive device, any current source will generate heat due to power. Power equals the product of the square of the current and the resistance. Therefore, a small current source must be used. Exposure to high heat will result in permanent damage to the thermistor.
This introduction to two types of temperature instruments is intended to be helpful for your work and studies.