Analog Input Channels
Temperature is a measure of the average kinetic energy of the particles in a sample of matter expressed in units of degrees on a standard scale. You can measure temperature in many different ways that vary in equipment cost and accuracy. The most common types of sensors are thermocouples, RTDs, and thermistors.
Figure 1. Thermocouples are inexpensive and can operate over a wide range of temperatures.
Thermocouples are the most commonly used temperature sensors because they are relatively inexpensive yet accurate sensors that can operate over a wide range of temperatures. A thermocouple is created when two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature. You can use this thermoelectric voltage, known as Seebeck voltage, to calculate temperature. For small changes in temperature, the voltage is approximately linear.
You can choose from different types of thermocouples designated by capital letters that indicate their compositions according to American National Standards Institute (ANSI) conventions. The most common types of thermocouples include B, E, K, N, R, S, and T.
For more information on thermocouples, read The Engineer's Toolbox for Thermocouples.
Figure 2. RTDs are made of metal coils and can measure temperatures up to 850 °C.
A platinum RTD is a device made of coils or films of metal (usually platinum). When heated, the resistance of the metal increases; when cooled, the resistance decreases. Passing current through an RTD generates a voltage across the RTD. By measuring this voltage, you can determine its resistance and, thus, its temperature. The relationship between resistance and temperature is relatively linear. Typically, RTDs have a resistance of 100 Ω at 0 °C and can measure temperatures up to 850 °C.
For more information on RTDs, read The Engineer's Toolbox for RTDs.
Figure 3. Passing current through a thermistor generates a voltage proportional to temperature.
A thermistor is a piece of semiconductor made from metal oxides that are pressed into a small bead, disk, wafer, or other shape and sintered at high temperatures. Lastly, they are coated with epoxy or glass. As with RTDs, you can pass a current through a thermistor to read the voltage across the thermistor and determine its temperature. However, unlike RTDs, thermistors have a higher resistance (2,000 to 10,000 Ω) and a much higher sensitivity (~200 Ω/°C), allowing them to achieve higher sensitivity within a limited temperature range (up to 300 °C).
For information on thermistors, read The Engineer's Toolbox for Thermistors.