
Temperature Sensors & Transmitters

Precise monitoring and control of temperature is critical in many industrial applications. Resistance temperature detectors (RTDs) and thermistors are devices that provide a reliable and accurate means of measuring temperature.
Resistance temperature detectors (RTDs) measure temperature based on the principle that as the temperature of a metal increases, so does the resistance to the flow of electricity. When an electrical current is applied to the RTD, the metallic element is used to measure the resistance of the passing electrical current. As the temperature increases, the resistance increases. The resistance value can be converted to temperature based on the known resistance characteristics of the metallic element. RTDs are adaptable to most industrial applications because of their wide temperature range (-330 degrees Fahrenheit to 1560 degrees Fahrenheit) and their exceptional accuracy to within 0.1 degree.
Thermistors exhibit a precise and predictable change in electrical resistance when exposed to a corresponding change in temperature. Positive temperature coefficient (PTC) thermistors exhibit an increase in electrical resistance when exposed to an increase in temperature and Negative temperature coefficient (NTC) thermistors exhibit a decrease in electrical resistance when exposed to an increase in temperature.
PTC thermistors can be used to protect devices or circuits from overcurrents. They have low resistance at room temperatures and as they allow current to flow through, the heat generated raises the temperature. Once a certain temperature is exceeded, high resistance results and limits the overcurrent. When the cause of the overcurrent is resolved, the PTC thermistor cools down and acts as a resettable fuse.
NTC thermistors can be used to protect devices from inrush current when powering them on. They have a high resistance at room temperature and when a current flows through, the heat generated causes the temperature to rise and the resistance to lower. The initial high resistance protects the device from inrush or abnormal currents.
Subcategories
What is an RTD? RTD stands for Resistance Temperature Detector. RTDs use a specific type of metal that, depending on the resistance measured in the metal, can determine temperature. Thin film element RTDs use a very thin layer of resistive metal adhered to a substrate, typically ceramic, and coated in glass or epoxy. Wire wound element RTDs provide greater accuracy and a wider temperature measurement range, however, over time the element wire can become stressed leading to slight measurement errors. Coiled element RTDs are the most common type of RTD and allow for strain free measurement on the wire element while still providing accurate measurement.
How do RTD elements work? As the temperature of a metal increases, the metal's resistance to the flow of electricity increases. As the temperature of the RTD resistive element increases, the electrical resistance, measured in ohms, also increases. Each metal has a certain resistance measurement at different temperatures. Based on the resistance that is measured in a given type of metal, it can be determined what the temperature is. The most common types of metals used in RTDs are platinum, nickel and copper.
What are some advantages and disadvantages of RTDs? RTDs provide a high level of accuracy over a wide operating range. Because of their accuracy, they are often used in situations where temperature measurement is critical. RTDs are rarely used at temperatures above 660 degrees Celsius and they do not provide the same level of accuracy or resolution as thermistors at lower temperatures.