Industrial Temperature Sensor Types and How They Work

Temperature also influences an electronic device's physical properties. Fortunately, we can monitor these influences using a variety of tools and methodologies. This article explores some of the most common temperature sensor types  and methods and the fundamentals of how they work.

Resistance-based Temperature Sensors

As the temperature of any conductor changes, so does its resistivity. In complex circuitry, resistivity can be harmful ― and it often sets electronic components' temperature rating limits. However, we can also use this physical characteristic to determine the temperature of a component or environment. Two common devices that utilize resistivity are Resistance Temperature Detectors (RTD) and thermistors.

Resistance Temperature Detectors

RTDs utilize metal conductors like platinum ― which has a well-known temperature-to-resistivity relationship ― to create a measurable signal that's directly influenced by the conductor's temperature. The temperature-to-resistivity relationship of some metallic materials is relatively linear and consistently repeatable, making RTDs desirable in high-accuracy applications.

RTDs have a wide array of temperature ranges and are even used for sensing ultra-low temperatures, down to 10 milliKelvins. However, platinum is often the primary material in RTDs, making them less cost-effective than most other temperature-sensing solutions. Cheaper RTDs tend to utilize copper or film materials, but these devices are less linear and repeatable than platinum RTDs.

Thermistors

Thermistor sensors are a significantly cheaper alternative to RTDs, but they don't boast the same temperature range, linearity, or accuracy as RTDs. The main difference between thermistors and RTDs is their construction material ― thermistors are usually made from a ceramic or polymer blend. Thermistors are primarily negative temperature coefficient (NTC), which means their resistivity decreases with an increase in temperature. NTC thermistors have an exponential output relationship ― which requires linearization circuitry ― but they also have minimal latency. These qualities make them desirable in environments with highly fluctuating temperatures.

Thermostats: Bimetallic Devices

While we still call our 'smart' thermostats by their original names, true thermostats employ a specific technology. Thermostats utilize two conductors mechanically fastened together. When these conductors are exposed to various temperatures, they expand and contract at different rates. This expansion and contraction creates a 'push' or 'pull' force that forms a binary on/off signal.

When traditional thermostats in residential heating and cooling applications contract, the device triggers the furnace to run until the ambient temperature rises enough to cause the conductors to expand, thus turning it off. Some modern thermostats still use this technology, notably in circuit protection for temperature-sensitive electronics. Users can set a thermostat to automatically cut power to circuitry if the ambient environment gets too high, ultimately protecting the circuitry from harm.

Thermocouples: Most Common Industrial Temperature Sensor

Thermocouples are the most common temperature sensor in industrial settings. You'll find thermocouples in nearly every industry where temperature sensing is required, such as:

·      Manufacturing

·      Automotive

·      Aerospace

·      Agriculture

Thermocouples utilize the Seebeck effect, which is when heat causes two dissimilar conductors to create a voltage differential. The heated conductor animates the conductor's electrons, which ― given the metals' unique properties ― creates a measurable difference in electron activity.

Based on this voltage difference and the known relative differences between the metals, we can calculate the thermocouple's temperature. The relationship between voltage and temperature in thermocouples is non-linear, so linearization is important in the temperature calculation as well. The Seebeck effect's low-voltage nature also requires amplification of the voltage signal for many applications. If this amplification isn't performed correctly, the thermocouples may be prone to low tolerances. However, the simple, intrinsically reliable, and low-cost nature of thermocouples have made them a popular choice across the electronics industry.  

IR Temperature Sensors

Until this point in the discussion, all the temperature sensors we referenced are intended for localized temperature sensing. That means the sensor's temperature must be local (or even integral) to the device's temperature.

On the other hand, infrared temperature sensors can detect an object's surface temperature remotely by utilizing thermopile technology. Thermopiles utilize the same principles as a thermocouple to detect the infrared energy that an object emits. Thermopiles accomplish this task using an array of cold and hot junctions ― often made from a semiconductor material ― to change the signal they emit.

Infrared temperature sensors can even consist of thermopile matrices ― also referred to as infrared or thermal cameras ― to detect a visualizable 'image' of varying infrared intensity. Infrared sensors, also called thermal sensors, require robust amounts of supporting circuitry to power and interpret the sensor-created signal. Still, their flexibility and applicability have made infrared temperature sensors a revolutionary technology in the last 50 years. Additionally, infrared technology offers contactless temperature sensing, making it widely popular in medical and health-related fields.

Temperature Sensor Design Guide

Selecting the right temperature sensor for your design comes down to five factors:

·      Temperature-sensing range

·      Longevity

·      Accuracy

·      Latency

·      Cost 

Once you've determined your parameters, head to Arrow.com to select the right components for your application.