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Temperature measurements with an RTD

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.

All metals increase in resistance with temperature, so their resistance change can be used to measure temperature. For different temperature ranges different metals are sometimes used, including copper, nickel and platinum. Passing current through an RTD generates a voltage by measuring which you can determine its resistance and calculate its temperature with a simple algorithm.

Most general-purpose RTDs are standardized to a value of 100 Ω at 0°C, but they are also manufactured in other values, such as 50, 200, 300, 1000, and 2000 Ω.

Measuring RTD resistance with Mosaic's 24/7 Data Acquisition Wildcard

RTDs need sensitive instrumentation optimized for low resistance measurements. Because RTD voltage is so small, a high-gain, low-noise analog input is required to measure this voltage. The 24/7 Wildcard is ideal for this purpose - it provides programmable gain and filtering against power line noise.

This tiny data acquisition module can extract highly accurate data, even from noisy sources with imperfect signal conditioning in electrically harsh environments. You can add up to 8 stackable W-DA24/7 modules to most of Mosaic's embedded controllers for programmable gain, high resolution RTD measurements.

Relating resistance to temperature

The resistance of an RTD changes almost linearly with temperature. Tables are widely available for converting temperature to resistance and vice versa. Conversion of resistance to temperature via an equation offers more flexibility. RTDs are characterized by their temperature coefficient, α, defined as the average fractional change in resistance per degree Centigrade over a temperature interval of 0°C to 100°C. The most common values are α = 0.00385 for the European standard and α = 0.003916 for the less used US Industrial Standard.

Computing temperature from RTD resistance

As an example of a simple equation you might use, for the case of measurements between 0°C and 100°C, you could use a linear approximation as,

T = (R/R0 - 1) / α where R0 = 100, and α = 0.00385

If you need to measure temperature over a much wider range you can use a cubic fit. A cubic fit over the range of -100°C to +600°C provides an rms error of only 0.038°C over the entire range, and 0.026°C in the range of 0°C to 400°C. The equation is,

T = -247.29 + 2.3992 R + .00063962 R² + 1.0241E-6 R³

If you need greater accuracy, you may calibrate the RTD at one or two fixed temperatures. In that case you will need an independent, accurate temperature measurement at the time of calibration.

See more details in Mosaic Application Note: "Measuring Temperature with RTDs".

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