There are a variety of reasons we need to know the temperature of an object or a process — to prevent product damage, ensure sterilization, determine biological health, ensure mixture blending, control chemical reactions, or ensure drying, curing, and outgassing, to name just a few. Temperature measurement can also be a regulatory requirement; for example, the Food and Drug Administration (FDA) requires temperature monitoring of food and drug products.
Selecting the sensor and measurement device to match a specific process is extremely important, and knowing the various options is the first step to optimizing temperature measurement. For example, temperature measurement sensors play an extremely important role in heat-treatment of metals such as structural steel used in buildings and metals used in aircraft. In these cases, a manufacturer must be able to guarantee that the metal was heat-treated in a particular way to ensure the metal has the desired properties. Another example is automobile parts. To be tough enough to withstand wear and tear, certain surfaces on some parts must be heat-treated in accordance with a specific ASTM schedule. It is simply not possible to physically look at something (or even conduct X-ray fluorescence) to tell if it is properly heat-treated.
Temperature Measurement Sensors
Sensors used in temperature measurement have an electrical property that is sensitive to temperature changes, as shown in Table 1. The temperature sensor chosen for a particular process depends on its cost, range of operation, sensitivity, response time, repeatability, and its ability to survive its environment. There is usually some measurement range overlap, and more than one sensor type may be suitable for an application. For example, biological systems can often be monitored with non-contact infrared (IR) detectors, thermistors, or silicon diodes.
Occasionally, the only practical way to monitor an internal temperature is to embed a temperature sensor into a product during the manufacturing process. An example is an electric motor with an embedded resistive temperature detector (RTD), or a thermistor on one of the motor’s copper windings.
A thermistor is a type of resistor, generally made of ceramic or polymer, whose resistance varies significantly with temperature. Unfortunately, thermistors require more complicated software to account for their very nonlinear temperature response. RTDs use such metals as platinum or nickel. RTDs are useful over larger temperature range, while thermistors achieve a higher precision within a limited temperature range, typically -90 °C to 130 °C.
A few industrial processes operate at such high temperatures that only a few sensor types can survive and give repeatable results after temperature cycling. Steel making requires measuring up to 1700 ºC, requiring use of either thermocouples, which can be immersed in molten metals for internal measurements, or IR thermometry. IR optical sensors are sometimes used for high-temperature measurements, but they can only measure hot surface temperatures. Non-contact IR thermometry may be the only choice if the product is moving or if penetrations are not allowed in the product; for example, ceramic firing in a kiln. Non-contact methods can only measure surface temperatures; internal temperatures cannot be measured. Table 2 provides an overview of temperature sensor applications, including the process, approximate sensor temperature ranges, and appropriate sensor types.
Temperature Measurement Applications
Manufacturing plastic products often requires thermocouples due to the high operating temperatures of the machines that make the plastic parts. The sensors are very close to the electric heaters that extrude and form the finished products.
Excessive drying temperatures can cause damage and waste energy, which makes it desirable to control how products, objects, and even people, are dried. For example, streams of hot air drying human hair and hands must be controlled and limited to prevent injury. In the case of food preparation, insufficient drying can cause product damage or allow harmful bacteria to grow. Thermistors often monitor a product’s temperature directly or the atmosphere around the product.
The relative humidity of the atmosphere is an important variable related to drying. A thermistor in the humidity sensor of a chilled mirror hygrometer (CMH) helps measure the dewpoint, which is related to the moisture content of the nearby atmosphere. Figure 1 shows a thermistor monitoring a temperature-controlled chilled mirror to regulate the mirror temperature.