RTD and Thermistors are simplest device in these devices resistance is increase when temperature is increase or decrease. when temp. is increase , resistance is increase  is call positive temp coefficient and vise versa.

Thermistors are devices made of metal oxide which either increase in resistance with increasing  temperature.RTDs are devices made of pure metal wire (usually platinumor copper) which always increase in resistance with increasing temperature. The major difference between thermistors and RTDs is linearity: thermistors are highly sensitive and nonlinear, whereas RTDs are relatively insensitive but very linear. For this reason, thermistors are typically used where high accuracy is unimportant. Many consumer-grade devices use thermistors for temperature sensors.

A Resistive Temperature Detector (RTD) is a special temperature-sensing element made of fine metal wire, the electrical resistance of which changes with temperature as approximated by the following formula:
RT = Rref [1 + α(T − Tref )]
Where,                   RT = Resistance of RTD at given temperature T (ohms)
Rref = Resistance of RTD at the reference temperature Tref (ohms)
α = Temperature coefficient of resistance (ohms per ohm/degree)

Water’s melting/freezing point is the standard reference temperature for most RTDs. Here are some typical values of α for common metals:

   Nickel = 0.00672
• Tungsten = 0.0045
• Silver = 0.0041
• Gold = 0.0040
• Platinum = 0.00392
• Copper = 0.0038

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