In the context of process automation, temperature is one of the most critical items to measure. It plays a significant role in the safety of an operation as well as the quality of that operation's output.

Consumers tend to think of temperature as either hot or cold. Little effort is made to define specifically where those two "measurements" begin and end. With regard to temperature control in commercial applications, exact measurements must be noted in order to optimize the efficiency and safety of a given process.

There are a number of methods and devices used to accomplish this task. The following article will briefly describe them, and explain how they function.


Methods And Sensors Used To Measure Temperature

Temperature measurement devices use sensors to monitor a range of physical attributes evident in a given medium. Some are known as contact devices. They employ sensors that come into direct contact with the medium. Others are known as non-contact devices. They rely on other factors, such as infrared radiation, to determine the medium's temperature.

Most people are familiar with a simple thermometer. It's a contact measurement device that was traditionally filled with mercury. The mercury expanded and contracted as temperature rose and fell (respectively). Today, thermometers are commonly filled with alcohol due to the fluid's lower toxicity.

Commercial applications require more complex measurement systems. Among the most common are thermocouples and resistance temperature devices, known as RTDs. Bimetallic and infrared devices are also used, though somewhat less commonly.


Thermocouple Temperature Measurement

Thermocouples are designed with two different conductors in the form of metal wires. The wires are connected at one or more points. The device identifies changes in voltage as signaling changes in temperature. As the former increases, so too does the latter. The relationship is nearly, but not quite, linear.

This measurement device is used in many different applications. For example, companies use them to monitor and control temperature in kilns, diesel engines, and water heaters. They also use them to help optimize a broad range of chemical production and manufacturing processes.


Infrared Temperature Measurement Sensors

Infrared thermometers use sensors that detect the level of thermal radiation given off by the medium under observation. As a general rule, the more infrared radiation a medium discharges, the higher the medium's temperature. (It's worth noting that the relationship between emitted thermal radiation and temperature is positive, but nonlinear.)

Different mediums have varying abilities to radiate heat via radiation. This fact needs to be taken into account by the engineer who calibrates and monitors the infrared temperature measurement system. It is also important for the engineer to determine whether any of the energy discharged by the observed medium stems from other surfaces or objects in close proximity to it.


Resistance Temperature Devices

Resistance thermometers (RTDs) are designed with a piece of coiled wire, typically made of copper or nickel. The wire, called the RTD element, is wound around an insulating core and used to monitor changes in thermal resistance levels. The greater the resistance, the greater the temperature of the observed element.

To the extent that the relationship between the two parameters is linear, much depends on the type of metal used for the RTD element and the temperature of the medium. For example, the relationship when copper is used is very linear as long as the temperature remains below 300 °F. The relationship when nickel is used is non-linear, particularly at temperatures above 572 °F.


Thermistors For Measuring Temperature

Thermistors are sometimes discussed in the same context as resistance temperature detectors (RTDs). The reason is because they use a similar design to measure temperature, namely an element that conveys resistance levels. However, there are marked differences between the two devices.

Thermistors typically employ ceramic or some other non-metal element (rather than copper or nickel, as found in RTDs). The correlation between the level of resistance expressed by the element and the observed material's temperature is positive, but very non-linear. Large changes in the former are associated with small changes of the latter. Having said that, select manufacturers produce thermistor elements that pose a more linear relationship, which is useful in certain applications.

Temperature measurement devices are an essential part of a wide array of processes involving nearly every imaginable industry. The type of device chosen by a company should accommodate the conditions in which the device is to be used as well as the company's budget and performance goals.