Resistance Temperature Detectors

Resistance Temperature Detectors or RTDs are sensors which measure the temperature by altering their resistance. >The Resistance temperature detector consists of an element made of a metal such as platinum  located in a metallic casing.

When the temperature increases, the resistance of the sensor increases (positive temperature coefficient of resistance) This increase in the resistance is measured through a wheatstone bridge. The relationship between temperature and the resistance is linear.  Thus, the temperature can be deduced from the measured resistance.

Platinum and Nickel are two metals used to construct the sensing elements.  

Some common types of RTDs are the Pt-100 and Pt-1000. 

The Pt stands for Platinum while the number 100 stands for the ohmic value at 0 degrees Celsius.

The resistance increases linearly with temperature.

For example, the Pt100 has an ohmic value of 100 ohms at 0 °C and a value of 161 ohms at 160°C  

Advantages of RTDs
Long Term Stability
Ability to withstand shock and vibration

Disadvantages of RTDs 
Errors due to lead resistance,
Slow response
Internal Self heating

2 Wire, 3 wire and 4 wire RTDs

One of the disadvantages of the RTD is the error caused by the lead resistance.  That is, the indicating device which measures the sensor resistance to calculate resistance also measures the resistance of the leads connecting the sensor to the device.  This is unavoidable, though the error can be minimized by running a wire in parallel to one or both the leads. (Refer diagram)