| Measuring Temperature - Basics
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:
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 used for absolute temperature measured on Kelvin scale.
The triple point of water is the temperature where water can
exist in all its three states.
Next to the Kelvin scale, 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.
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.