Platinum Takes the Gold

Thermistors and thermocouples serve well in many applications, but when you need high-accuracy temperature measurements, platinum resistor-temperature detectors (RTDs) get the top prize. These detectors can provide an accuracy of better than 0.03C (at 0C) and operate between about -200C and 800C.

The resistance of a platinum RTD varies almost linearly with temperature and manufacturers specify a temperature coefficient, ?å, which represents an average resistance change per ohm-?‹C over the range from 0 to 100C. International groups have established standard ?å values for platinum thermocouples: IEC-751 specified ?å = 0.003850 ohm/ohm?‹C (0.3850%/?‹C) and the American standard gives ?å = 0.003911 (0.3911%/?‹C) for a 100 ohm platinum RTD. A 1990 International Temperature Scale (ITS-90) spec defines ?å = 0.003926 (0.3962%/?‹C).

The nearly linear relationship between temperature and resistance lets you approximate a temperature from the relationship shown below. In most cases, the reference temperature, T₀, equals 0 degrees C.

R_T = R₀ + ?åR₀(T_x - T₀) = R₀(1 + ?å(T_x - T₀))

So, a platinum RTD with a 100 ohm resistance (R₀) at 0C (T₀) will have a resistance of about 119.3 ohm (R_T) at 50C (T_x). (For IEC-751 ?å.)

To obtain better temperature values, you must apply the Callendar-Van Dusen equations, which use two coefficients for measurements above 0 degrees C and three coefficients at or below 0 degrees C. RTD manufacturers?f data sheets and application notes supply the equations and coefficients that correspond to the IEC-751, American and ITS-90 standards. Given a resistance change, you can use these equations to solve for a temperature value. Or, you can create a look-up table of temperature values for various resistances. (See Notes.)

Like a thermistor, an RTD requires an electrical stimulus so an instrument can measure its resistance. Measurements with simple two-wire RTDs may yield poor results due to the resistance of the RTD?fs leads. A three-wire configuration, though, lets you include an RTD is a standard bridge circuit that produces a voltage. The equation for the output voltage includes terms for the connection resistances and a ƒ¢R term for the change in the RTD?fs resistance from R₀, or 0 degrees C, to (T_x). You can use the ƒ¢R value to calculate or look up the RTD?fs temperature.

A four-wire arrangement delivers an excitation current to an RTD through wires separate from those used for measurements. A high-impedance voltmeter can measure the potential without drawing much current, so the connection wires contribute a negligible error to the RTD?fs resistance. Many data-acquisition systems accept three- and four-wire RTD connections.

NOTES:

Several documents refer to a 20th-order polynomial that better fits a platinum RTD?fs resistance vs. temperature data, but Internet searches failed to uncover it.