Resistance Temperature Device works on the principles that the resistance of the material changes as its temperature changes. Temperature is determined by measuring resistance and then using the RTD Resistance vs Temp characteristic to detect temperature. Typical elements used for RTD are Nickel, Copper and Platinum. Platinum is widely used in RTDs because of accuracy. PT 100 means at 0 deg temp 100 ohms resistance. A typical RTD consists of a fine platinum wire wrapped around a mandrel and covered with a protective coating (glass or ceramic).