Resistance Temperature Detectors (RTDs) ¾ RTDs are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. ¾ The hotter they become, the larger or higher the value of their electrical resistance. ¾ Platinum RTD and PT100s are the most popular RTD types, nearly linear over a wide range of temperatures and some small enough to have response times of a fraction of a second. ¾ They are encapsulated in probes for temperature sensing and measurement with an external transmitter.

The Advantages of RTDs ¾ Long period of time

¾ Ease of recalibration

¾ Accurate readings over relatively narrow temperature spans. RTD Error Sources ¾ The lead wires used to connect the RTD to readout can contribute to their measurement

¾ Especially when there are long lead lengths involved, as often happens in remote temperature measurement locations.

¾ Often the lead error can be minimized through use of a temperature transmitter mounted close to the RTD.

Transmitters Convert the resistance measurement to an analog current or serial digital signal that can be sent long distances by wire or RF to a data acquisition or control system.

RTD Types, Calibration Tables, Construction

¾ Platinum RTD Output Equation The

temperature

of

Industrial

Platinum

Resistance

Thermometers can be described by the equation:

R(t) = R(0)[1 + α1t +α2t^2] Where:

t = temperature °C, R(t) = resistance at temperature t, R(0) = resistance at 0°C α1= temperature coefficient α2= temperature coefficient

Resistance Thermometer Wiring Configurations

Two Wire Configuration

¾ The simplest resistance thermometer configuration uses two wires ¾ It is only used when high accuracy is not required as the resistance of the connecting wires is always included with that of the sensor leading to errors in the signal ¾ Using this configuration you will be able to use 100 meters of cable

Three Wire Configuration

¾ In order to minimize the effects of the lead resistances a three wire configuration can be used ¾ Using this method the two leads to the sensor are on adjoining arms, there is a lead resistance in each arm of the bridge and therefore the lead resistance is cancelled out ¾ This configuration allows for up to 600 meters of cable

Four Wire Configuration

¾ The four wire resistance thermometer configuration even further increases the accuracy and reliability of the resistance being measured ¾ In the diagram above a standard two terminal RTD is used with another pair of wires to form an additional loop that cancels out the lead resistance