Using RTDs
This application note shows you how to measure temperature using RTDs connected to the Mosaic 24/7 Data Acquisition Wildcard. As many as three RTDs can be interfaced to a single 24/7 Wildcard, allowing you to measure temperatures with errors of less than ±0.1°C.
What is an RTD?
An RTD (Resistance Temperature Device) is a temperature-sensitive resistor made out of platinum, either as a coil of wire or a thin film, usually encapsulated on a glass or ceramic substrate.
Because RTDs are simply resistors, they can be fabricated in all shapes and sizes, or embedded in all sorts of probes. For industrial use, you usually purchase them mounted in probes that include connectors, sheaths, and handles for convenient mechanical placement. But you can also purchase RTDs as simple components, and easily incorporate them into your design.
How do RTDs work?
All metals increase in resistance with temperature, so their resistance change can be used to measure temperature. The resistance of metals is almost directly proportional to absolute temperature, although many metals have physical processes that may distort that proportionality a little. For different temperature ranges different metals are sometimes used, including copper, nickel and platinum. The best is platinum because it has a very high melting point and it doesn't readily corrode. Its resistance is quite linear with temperature; in fact, it is almost perfectly proportional to absolute temperature.
If the proportionality were exact, the temperature coefficient of resistance, defined as the change in resistance per °C over the 0°C to 100°C range, would be,
α = (373.15°K-273.15°K)/273.15°K/100°C = 0.003661 Ω/Ω/°C 1)
In fact, it is close to this ideal value. Depending on the alloy and purity of the platinum, the temperature coefficient, α, is 0.00385 Ω/Ω/°C (the European curve) to 0.003916 Ω/Ω/°C (the American curve).
RTDs are most often manufactured to have a resistance of 100Ω at 0°C, but varieties are available with baseline resistances of 200Ω, 1KΩ, 2KΩ, 4KΩ, 10KΩ, and other values.
Most RTDs available commercially in the US use the European Standard (0.00385 Ω/Ω/°C coefficient), so that their resistance increases from 100 Ω at 0°C to 138.5 Ω at 100°C, as shown in the following graph. You can see that the response is quite linear, but for precision work the slight curvature of the graph should be taken into account.
Data for the above graph is available in an RTD Resistance vs Temperature Table.
For more information
You can find lots more information on RTDs, including accuracy tables and resistance vs temperature tables, at,