Measuring temperature is a requirement in a myriad of applications. Accurate measurements are crucial to surveillance, helping the individual monitoring them to prevent adverse outcomes. For this task, a number of different sensors are available. Each uses a different approach to gauge the temperature of an object or process.

The various sensors work by identifying select changes in the physical characteristics of a gas, liquid or material. Activity of molecules or atoms influence the level of heat energy produced by the target element. The array of instruments available to measure that heat energy include resistive temperature devices, thermocouples, bimetallic thermometers, infrared sensors and other options.

Below, we'll briefly describe each one. The details that follow will help you to select the measuring device that's best-suited for your application.

Resistive Temperature Detectors

Oftentimes made from metals, such as copper and platinum, resistive temperature detectors (RTDs) note changes in temperature by monitoring electrical resistance. Changes in the level of resistance present are positively correlated with changes in heat.

A common type of RTD is a thermistor. It's usually made of ceramic or related materials (i.e. nickel, cobalt, etc.). A current is passed through the device to generate electrical resistance. That resistance can be measured to determine temperature. In contrast to most RTDs, increasing levels of resistance are correlated to falling temperature levels.


Thermocouples are the most common type of temperature measuring devices used in industrial applications. They're simple to use and deliver quick results that can be easily read. In addition, they can be used to measure temperature across an expansive range.

These devices employ two conductors, which are constructed of different metals. A variance in temperature between the two elements generates voltage. The higher the voltage, the higher the temperature.

Change-Of-State Devices

Change-of-state (COS) temperature gauges measure temperature by watching for physical changes in the target element. Once the element reaches a certain temperature, it signals so by changing one of its physical characteristics. For example, a sensor label might turn from white to red.

Ease of use is one of the advantages to using a COS temp gauge. Due to how it works, temperature levels are simple to monitor. The downside is that the device lacks the same degree of accuracy and precision seen with other instruments. It may also require several minutes to signal that a certain temperature level has been reached.

Liquid Expansion Devices

Most people know these instruments as thermometers. Traditional models used mercury. However, because mercury is now considered to be harmful to the environment, its transport and use is limited. Newer devices employ alternative liquids to the same effect: heat causes them to expand. That makes them viable temperature gauges.

There are several advantages to using liquid expansion devices. The base liquid is stable and the measurements are accurate. In addition, the devices can be operated without an electric power source.

Infrared Sensors

This type of temperature measurement device relies on thermal radiation emitted by the target element. There's no need for it to make contact with the element. For that reason, it's often called a non-contact thermometer.

A basic infrared sensor uses a lens to focus the thermal radiation coming off the target element. That radiation is converted into an electrical signal. The signal can then be displayed as a numeric value.

Bimetallic Thermostats

Bimetallic thermostats measure temperature by noting the differences in rates of expansion between two metals. The thermostat is designed with two metals placed side by side (or one on top of the other). When exposed to heat, one of the metals undergoes a faster rate of thermal expansion than the other. The thermostat uses that difference to determine the temperature of the target element.

Choosing an option based on the application is simpler when you work with a supplier that is motivated to help you find the right solution. Applied Engineering's experienced support staff can match your needs to the temperature measurement device that's best suited for the job.