Definition Temperature is the main physical quantity which describes the state of a thermodynamic system. The measurement of temperature is based on the fact that all objects – and consequently also temperature sensors – are exchanging energy with the environment. There are three ways to exchange energy:
- Conduction
- Convection
- Radiation
For an ideal temperature measurement, the temperature sensor and its environment are in thermal equilibrium, there is no energy transfer to or from the sensor. The temperature of the sensor is equal to the temperature of the surroundings. To determine the absolute temperature, a reference value is necessary. A scale for temperature can be defined knowing the reference value and the temperature behavior of the sensor. The Kelvin scale is based on the absolute minimal temperature 0 K and the triple point of water 273.16 K. The unit is 1 Kelvin = 1 K. T is the symbol for the absolute temperature measured on Kelvin scale. The triple point of water is the single combination of pressure and temperature at which pure water, pure ice, and pure water vapor can coexist in a stable equilibrium at exactly 273.16 K (0.01 °C) and a pressure of 611.73 pascals. Next to the Kelvin scale is the Celsius scale widely accepted. The unit of the Celsius scale is 1 °C = 1 K. The Celsius scale is shifted by 273.15 units with respect to the Kelvin scale. At normal atmospheric pressure of 1013.25 mbar the melting of ice is at 273.15 K (resp. 0 °C). All temperature scales (IPTS-68 or TTS) were replaced in 1990 by the International Temperature Scale ITS-90. On this new Celsius scale the boiling temperature of water (was 100 °C) is no longer a reference point. The boiling temperature of water according to ITS-90 is at 99.974 °C at normal atmospheric pressure. 
Measuring Methods The temperature can be measured mechanically or electronically. Mechanical methods are based e.g. on bimetal, liquid thermometers or gas thermometers. In the industrial measurement technology the electronic methods are preferred. These are mainly based on the variation of an electrical resistor as function of temperature. The PTC elements (positive temperature coefficient) are used for thermal protection switches, NTC-elements (negative temperature coefficient) for low accuracy requirements.
For high accuracy applications metal resistors are used. The metal used can be platinum, molybdenum or nickel in form of wire or thin film on a substrate. E+E temperature transmitters are using platinum-temperature sensors with characteristics according IEC751 and EN60751.