Thermometric property and temperature measurement

Thermometric property:

It is a physical property that varies continuously with temperature.

Thermometric properties can be used to measure the temperature. 

For example imagine a closed constant volume(no change in dimension) tube on a heater, as the temperature increases the  pressure also increases. later when you remove the tube temperature decreases steadily with decrease in pressure inside the tube. here as the temperature is varying with pressure, by measuring the pressure and by applying the value in ideal gas equation we can find value of temperature. As the temperature is measured with the help of pressure, pressure is considered as the Thermometric property.

similarly below are few thermometric properties for respective thermometers.

Type of thermometer	Principle	Thermometric property
Resistance	Wheat stone bridge	Resistance
Thermocouple	See back effect	E.M.F
Constant volume gas thermometer	Ideal gas equation (PV=MRT)	Pressure
Constant pressure gas thermometer	Ideal gas equation (PV=MRT)	Volume

for mercury thermometer length is considered to be thermometric property.

Method of temperature measurement

method used before 1954:

T=[100(P-Pi)] / (Ps-Pi) 

here T= temperature

P= pressure corresponding to temperature T
Pi= pressure corresponding to ice point (273K)
Ps= pressure corresponding to steam point (373K)

Using Ice point & Boiling point of water the thermometer can be calibrated.

method  after 1954:

T=273.16(P/Ptp)where T=temperature,P= Property(volume or pressure) corresponding to temperature T,Ptp= Property(volume or pressure) corresponding to triple point temperature.

Using concept of Triple point. As the temperature and pressure at triple point are known. 

Triple point of water:

It is the point where all the phases (gas,liquid,solid) coexist. for water its value is 273.16K or 0.01 degree centigrade.