Why the PRT platinum resistance thermometer can be used as a

Why the PRT (platinum resistance thermometer) can be used as an accurate reference sensor?

Solution

Platinum Resistance Thermometers (PRT) :-

A PRT is a thermometer constructed from a high purity platinum element 9Wire-wound coil or thin film) placed in a tube of metal or glass and sealed with an inert atmospherre and/or mineral insulator. Two, three or four leads are connected to the element and are used to provide for the measurement of the electrical resistance of the element.Some of these characteristics are :

PRTs offer excellent accuracy over a wide temperature range (from -200 to +850 C). Standard sensors are available from many manufacturers with various accuracy specifications and numerous packaging options to suit most applications.

The Principle of operation is to measure the resistance of a platinum element. The most common type (PT100) has a resistance of 100 ohms at 0C and 138.4 ohms at 100 C. There are also PT1000 sensors that have a resistance of 1000 ohms at 0 C.

The relationship between temperature and resistance is approximately linear over a small temperature range: for example, if you assume that it is linear over the 0 to 100 C range, the error at 50 C is 0.4 C. For precision measurement, it is necessary to linearise the resistance to give an accurate temperature. The most recent definition of the relationship between resistance and temperature is International Temperature Standard 90 (ITS-90).

The linearization equation is :

Rt = Ro * (1 + A8t + B8t2 + C8(t-100)*t3)

Where :

Rt is the resistance at temperature t, Ro is the resistance at 0 C, and

A= 3.9083 E - 3

B = -5.775 E - 7

C = -4.183 E - 12 (below 0 C) or

C = 0 (above 0 C).