The form of temperature measurement that most people are familiar with is the mercury thermometer. German physicist Gabriel Daniel Fahrenheit was the first person to use mercury as a thermometric fluid when he developed the mercury thermometer in 1714. Mercury is a good conduit for measuring temperature because its freezing point is substantially below that of water, and its boiling point is significantly higher. Mercury also expands smoothly with changes in temperature.

When Fahrenheit created his thermometer, he did not use the freezing and boiling points of water as reference points. Instead he used a mixture of ice and salt as a reference for a cold point, and the temperature of the human body as a reference for the warm point. Fahrenheit associated zero degrees with his mixture of ice, water, and salt. The ice and water mixture without the salt read 32 degrees. The temperature of the human body read 96 degrees.

Fahrenheit’s thermometer has since been recalibrated, and while 32 degrees Fahrenheit is still recognized as the freezing point, the temperature of the human body is now recognized as 98.6 degrees Fahrenheit. The boiling point of water is 212 degrees on the Fahrenheit scale.

Swedish astronomer Anders Celsius created his temperature scale in 1742. In this scale, zero degrees was the boiling point of water and 100 degrees was the freezing point of water. These points were later reversed, so the freezing point is now recognized as zero degrees and the boiling point of water is 100 degrees. For many years, these degrees were called degrees centigrade, until in 1948 the National Conference on Weights and Measures decreed that degrees centigrade should be called degrees Celsius.

While glass thermometers are sufficiently precise to handle many of the temperature measurement needs of daily life, they are not suited to the needs of most industrial environments, primarily because they do not have an output and require a manual reading. Also, glass thermometers are not designed to survive the temperature extremes found in many industrial applications, and their range of measurement is rather limited.

For industrial environments, there are five main types of temperature sensors prevalent in today’s marketplace. These include thermocouples, resistance temperature detectors (RTDs), thermistors, infrared thermometers, and fiber-optic temperature sensors.

Thermocouples are the most widely used temperature sensor in industrial manufacturing environments. Thermocouples consist of two wires made of different metals that are joined at one end. This point is called the measurement junction. The other end of the conductors is called the reference junction. When the measurement junction and the reference junction have different temperatures, a continuous current flows in the circuit. The resulting voltage is a function of the difference in temperature between the measurement and the reference junctions. The amount of voltage depends on the types of metals used. A voltmeter or other device is required to interpret the voltage reading as a temperature value.

Thermocouples are the workhouse of the temperature sensor world, as they are the most widely used type of temperature sensor. They come in a multitude of types that are composed of different mixtures of metals and designed for different temperature ranges and conditions.

Resistance temperature detectors, or RTDs, monitor the flow of electricity through a wire under different temperature conditions to arrive at a temperature measurement. Platinum is the most commonly used wire material in RTDs. There are two types of RTDs — wirewound and thin-film. Wirewound RTDs consist of wire wound on a bobbin, which is enclosed in glass or metal. For thin-film RTDs, a film is etched onto a ceramic substrate and sealed. RTDs are more accurate and stable than thermocouples, but cannot be used to measure extremely high temperatures.

Thermistors, like RTDs, also change resistance with changing temperatures, but they are more sensitive than RTDs, and they are highly nonlinear. Because of their extreme sensitivity and nonlinearity, thermistors are limited to measuring temperatures of a few hundred degrees Celsius. They are also less rugged than RTDs, which further limits their application.

Thermistors have a narrower temperature range than either RTDs or thermocouples, but they are more capable of registering temperature changes within those narrower ranges. Thermistors are widely used in food service, food transportation, and HVAC industries. Increased concern with food safety is leading to more government regulations that require food temperature to be monitored and recorded during shipping and storage. This is increasing the demand for thermistor probes that monitor food temperature.

Infrared thermometers are primarily used to measure temperature when other temperature measurement methods aren’t feasible. For example, infrared thermometers are used to measure the temperature of moving objects, such as moving machinery on a conveyor belt. They are also used in hazardous environments where contamination is present, or in applications where the distance is too great for contact sensors. Infrared sensors detect the infrared energy given off by materials. The most common design includes a lens to focus the infrared energy onto a detector. The amount of infrared energy is then converted into a temperature measurement, according to specifiable units.

The infrared thermometer market is undergoing rapid growth for several reasons, much of which has to do with end-user familiarity. Infrared is actually not a new technology, and infrared thermometers have been around for more than 35 years. Even though infrared thermometers have a number of technical advantages, there are many situations in which infrared cannot easily replace thermocouples and RTDs. Infrared thermometers are mainly used when contact temperature sensing is impractical. For example, infrared thermometers are often used to measure the temperature of rotating machinery or to make measurements in environments where extremely high voltages are present.

Fiber-optic temperature sensors are a form of temperature measurement that uses optical fibers to arrive at a temperature measurement. Most types of fiber-optic temperature sensors work by placing a temperature-sensing component on one tip of the optical fiber. The other end is attached to a measuring system that collects radiation and processes it into a temperature value.

Fiber-optic temperature sensors are the newest kid on the temperature-sensing block. These temperature sensors function well in harsh environments, including radio-frequency (RF), microwave, and high-voltage environments. While they are more expensive than other technologies, fiber-optic temperature sensors succeed in some applications where temperature cannot be reliably measured by other means.

Technology Shifts the Market
The temperature sensors market in 2007 is marked by several important shifts in technology. One is the shift away from contact to noncontact measurement. This involves a shift away from thermocouples, RTDs, and thermistors to infrared thermometers and fiber-optic temperature sensors. Both infrared thermometers and fiber-optic temperature sensors are forms of noncontact temperature measurement.

Another technology shift that is occurring in the temperature sensor market is a shift within contact-type sensors from thermocouples to RTDs. Users are increasingly selecting RTDs over thermocouples because of their higher accuracy and stability. Also, an increasing number of high-end RTDs are being produced. As a result, thermocouple revenues are projected to decline over the next several years.
Still another technology shift in the temperature market is the shift from wirewound to thin-film RTDs. Thin-film

RTDs are becoming more popular because their technology has improved significantly and because they cost less than wirewound RTDs. Wirewound RTDs are still preferred for the most rugged applications. Sensitivity to vibration has traditionally been a problem for RTDs, and thin-film RTDs are now made that are less sensitive to vibration. Other improvements include improvements to the lead wires to make them less likely to break off. This has been a problem in the past for some thin-film RTDs.

Expect continued improvements in temperature sensing technologies, as companies continue to battle for market share. There has been a substantial amount of consolidation among temperature sensor suppliers in the past five years, and this trend is likely to continue as well.

Jesse Yoder, Ph.D., is president of Flow Research Inc. (Wakefield, Mass.), a company he founded in 1998. He has 20 years of experience as an analyst and writer in process control. Dr. Yoder specializes in flowmeters and other field devices, including pressure, level, and temperature products. He has written over 70 market research studies in industrial automation and control and has published numerous journal articles. Dr. Yoder can be reached at jesse@flowresearch.com or 781 245-3200.

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This article is based on Flow Research’s study, “The Market for Temperature Sensors in the Americas.” For more information on this study visit Flow Research’s temperature portal www.tempflows.com.