Data Acquisition (DAQ)
Types of Temperature Sensors
Believe it or not, temperature sensors are everywhere; they’re in our thermometers, refrigerators, cars, water heaters, and washing machines. These vital components can be found in many applications, such as consumer, industrial, and medical electronics.
Of course, each application has different temperature sensing needs. Medical devices require highly precise and very responsive temperature sensors, while the thermometer in your water tank could be a degree or two off, and nobody would care.
So, it’s safe to assume that not all temperature sensors are made equal, and in this guide, we’ll discuss the main types of temperature sensors and what sets them apart.
What is a Temperature Sensor?
In their simplest forms, the various temperature sensor types are simple instruments that measure the coldness or hotness of an object or environment and convert said measurement into a readable unit. However, more modern sensors—those developed after the 1930s—are electromechanical devices that detect and measure temperature and convert it into an electrical signal.
As it’s measuring an analog physical value of an object, the signal output is usually analog as well. The exact nature of this signal largely depends on the type of temperature sensor, which we’ll discuss later.
Depending on the application, the measurement of the analog signal can be converted into units of temperature that are readable on an analog readout unit, like a dial. Or, in more modern consumer and industrial applications, the analog signal is converted into a digital one through analog-to-digital conversion (ADC).
The digital signal is then converted into units of temperature and displayed on a digital readout in numeric form or some other type of quantification.
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Types of Temperature Sensors
Before we dive into the specifics of different temperature sensor types, it’s important to note that temperature sensors come in two main temperature sensor classes:
- Contact Temperature Sensors – As their name implies, these sensors have to be in direct contact with the object whose temperature is being measured. This makes contact temperature sensors great for temperature detection in liquids, solids, or even gasses, covering a wide range of temperatures.
- Non-Contact Temperature Sensors – Unlike conventional contact sensors, non-contacts don’t require physical contact with the object. Instead, these sensors measure the temperature radiation, or lack thereof, that’s emitted by the object and convert it into readable temperature units.
Now that we’ve covered the basic classes of temperature sensors, let’s go over their different types:
The Resistance Temperature Detector (RTD)
Resistance temperature detectors change their resistance with temperature. An RTD consists of a film or a wire wrapped around a ceramic or glass core. They’re mostly made of platinum, in configurations that either have a 100Ω or 1000Ω resistance at 0°C—referred to as PT100 and PT1000, respectively.
Platinum is most commonly used for RTD sensors as it offers a near-linear response to temperature fluctuations. This makes them very stable and accurate, as they provide repeatable responses within a rather wide temperature range of -200° to 600°C. Other metals, such as copper and nickel, can also be used for RTDs; while they’re more accessible price-wise, they’re also less accurate.
The resistance of the RTD changes with the temperature, which implies the application of an excitation current onto the element. This means that the RTD needs some current flow, as it allows the measurement unit of a Data Acquisition System (DAQ) to measure resistance and convert it into temperature. Without the flow of current, the resistance is measured as infinite.
Thermistors
Thermistors are resistors whose resistance changes with the temperature, and in that regard, they’re very similar to RTDs. However, they are made of polymer or ceramic material, which makes them cheaper and somewhat less accurate compared to RTDs. However, there are some significant differences between the two types of temperature sensors.
For example, the negative temperature coefficient (NTC) thermistor’s resistance decreases as the temperature increases, while the positive temperature coefficient (PTC) thermistor’s resistance increases along with the temperature.
Additionally, unlike RTDs, which have a linear response to temperature changes, thermistors’ resistance changes much more exponentially. So, despite their high sensitivity to temperature changes, deriving the actual temperature from the resistance reading requires using a more complex equation and calculation method.
Thermocouples
Thermocouples are among the most frequently used temperature sensors in industrial, automotive, and consumer applications, as they’re self-powered, require no excitation, and operate over a wide temperature range.
They consist of two wires made of different metals, which are electrically bonded at two points. When the metals are exposed to temperature fluctuations, the different responses generate a voltage difference between the two metals. This difference is measured to derive the temperature.
It’s worth mentioning that thermocouples, just like thermistors, don’t have a linear response to temperature fluctuation. This means that they require a more complex signal interpretation.
Semiconductor Sensor ICs
These sensors are typically integrated into ICs and rely on two diodes with temperature-sensitive voltage vs current characteristic, which is used to measure temperature. They’re the least accurate and the least responsive among the basic types of temperature sensors, but they offer a linear response, albeit at a very limited temperature range of -70° to 150°C.
Infrared Sensors
Infrared sensors are non-contact temperature sensors that capture the heat of an object (which is reflected as IR light) and convert it into an electrical signal, which is then digitized, processed, and displayed as temperature measurement. Every object emits infrared rays according to its temperature and emissivity. The higher the temperature and emissivity, the more infrared rays emitted. A radiation temperature sensor detects the amount of infrared radiation. They’re rather accurate and cover vast temperature measurements—up to 3000°C—but require adequate calibration to provide surgical accuracy.
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Applications of Temperature Sensors
The primary function of a temperature sensor is to measure temperature in a variety of applications. This included monitoring the temperature in various environments, machinery, and manufacturing. But their industrial uses don’t end there; they’re great for interpreting temperature-related or temperature-generating stress and changes in volume, as well as studies of thermal effects on various instruments.
Additionally, they’re used to monitor patients and medical devices, refrigeration, heating, air conditioning, the automotive industry, etc. The use of these devices is so ubiquitous that it’s really hard to imagine an industry and application in which these sensors don’t play a role.
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Summary
In summary, choosing the right type of temperature sensor mostly comes down to your application. Thermocouples are inexpensive and quite durable, RTDs offer a wide temperature range, but they’re smaller and require excitation current, and thermistors are small and durable, but they’re not as accurate and may require data corrections.
And while there are other types of temperature sensors present, the basic types fit most applications. To learn more about measurement sensors, visit our KEYENCE DAQ page.
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