There are many considerations to take into account when selecting a temperature sensor. Firstly, the application environment must be considered. What parameter of the device needs to be measured? Is it the ambient temperature inside or outside of a casing? Can electronic components with plastic or metal enclosures withstand high voltages? Does it need to be installed in a specific part of an automotive engine, such as an intake or exhaust port? The budget and characteristics of each sensor will be influenced by environmental and safety factors, which will impact our selection. There are many different types of sensors available for temperature measurement. The three most common temperature sensors are resistance temperature detectors (RTDs), thermocouples (TC), and thermistors. Their respective advantages determine their outstanding performance in different locations.
A resistance temperature detector (RTD) is the most accurate type of temperature sensor. In an RTD, the device’s resistance is proportional to the temperature. The most common material used in RTDs is platinum, but some RTDs are made from other metals such as nickel and copper. The temperature measurement range of an RTD is wide, depending on how it is constructed, with RTDs capable of measuring temperatures from -270°C to +850°C. However, an RTD requires external excitation (usually a current source) to function properly.
Advantages: RTDs have certain advantages that other temperature sensors do not. Firstly, an RTD is the most stable and accurate temperature measuring device. Like a thermocouple, an RTD is nonlinear, which means it also requires linearization, usually achieved through calibration factors.
Disadvantages: RTDs are more expensive than thermistors and thermocouples, and they require a current source. RTDs have a smaller change in resistance. For example, to change 1 degree Celsius, an RTD might change 0.1Ω.
Sources of Error: When using an RTD, certain issues are often overlooked, such as self-heating. The self-heating produced by the current flowing through the RTD can cause measurement errors. If measuring low temperatures (below 0°C), the heat generated by the RTD can cause the expected temperature to decrease. Additionally, if test leads are not compensated, more errors can be introduced into the measurement. Another common mistake is not selecting the appropriate RTD temperature range. Measuring temperatures outside of the measurement range can cause greater errors or even damage the sensor.
Thermocouple sensors (TCs) are currently the most commonly used temperature sensors. The main reason is that thermocouples are low cost, extremely durable, and can work over long distances. Thermocouple sensors can self-power, and there are many types that can cover a wide range of temperatures. Low cost is an important reason why this type of sensor is widely used. Their durability means they can continue to work in many different environments, including outdoor and harsh factory environments. Providing TCs with metal shielding helps protect them in harsh or corrosive environments, or when TCs are working in conduits.
Different alloys can achieve different temperature ranges and measurement sensitivities. Some common types of TCs include J, K, T, E, R, S, B, and N, which refer to the construction material type of the TC. An important characteristic of a thermocouple is its nonlinearity, which means that the output voltage of the thermocouple is nonlinear with temperature. Its working principle is the thermoelectric effect.
Advantages: This type of temperature sensor does not require an external power source. And they can withstand harsh environments. Thermocouples are cheaper and more diverse in types and have a wider range of temperature measurement than RTDs and thermistors.
Disadvantages: Thermocouples are nonlinear. In addition, the voltage signal is low, usually only a few tens to hundreds of millivolts, and requires careful elimination of noise and drift in low-voltage environments.
Sources of Error: If compensation is not set or set incorrectly, this can result in inaccurate or nonlinear temperature measurements.
Thermistors are another common type of temperature sensor. Like RTDs, the resistance of a thermistor changes with temperature. Thermistors have higher sensitivity than RTDs, meaning the change in resistance with temperature is greater for a thermistor than for an RTD.
Advantages: Thermistors are simpler to use and operate, and have a faster response time.
Disadvantages: Thermistors have a limited temperature range. As they are semiconductors, they are more prone to uncalibrated behavior at high temperatures.
Sources of Error: Self-heating can lead to measurement errors.
There are many types of sensors to choose from when measuring temperature. Thermocouples, RTDs, and thermistors are the most common today. Each type of sensor has its own advantages and disadvantages. Choosing the right sensor is crucial for achieving accurate and reliable temperature measurement.