RTD PT100 - General Information0 pages

Data sheet
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INFO RTD
06/2011
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GENERAL INFORMATION ABOUT RESISTANCE THERMOMETERS

WORKING PRINCIPLE
The working principle for metal resistance thermometers, normally called thermo resistances, is based on the variation of
the electrical resistance of a metal with variations in the surrounding temperature. In the industrial field the materials most
frequently used are platinum and nickel which, due to their high resistivity and stability, permit the production of thermo
elements which are highly reproducible, small and with excellent dynamic characteristics. The temperature measurements
carried out with thermo resistances are far more precise and reliable than those carried out with other types of sensor
such as thermocouples. Normally resistance thermometers are identified with the code of the material used to construct
them (platinum = Pt, nickel = Ni etc.) followed by their nominal resistance at a temperature of 0°C. The range of use for
industrial resistance thermometers is between -200 and +850°C.
PLATINUM (Pt) RESISTANCE THERMOMETERS
PCI platinum resistance thermometers comply with the international standard IEC 751; sensitive elements which conform
to other standards, for example JIS C 1604 etc., may be supplied on request. According to standard IEC 751 the platinum
used for the manufacture of resistance thermometers should have a temperature coefficient of alpha = 3,85x10-3
Standard IEC 751 allows for thermo resistances with a nominal value at 0 °C (Ro) of between 5 and 1000 ohm; however,
the values most commonly used are 100 ohm, 500 ohm and 1000 ohm. The equation linking resistance at temperature t°
(Rt) and resistance at 0° (Ro) is a follows:
in the range -200°C / 0 °C
Rt = Ro [ 1+At+Bt²+C ( t-100 ) t³ ]
in the range 0 °C / 850 °C
Rt = Ro ( 1+At+Bt² )

Where the coefficients A, B and C have the following values:
-3

A = 3,9083 x 10

-7

B = -5,775 x 10

-12

C = -4,183 x 10
The classes of precision for platinum resistance thermometers refer to temperature and are standardized as follows:
Class A = 0,15+0,002 | t | ( °C )
Class B = 0,3+0,005 | t | ( °C )
Class A covers up to a maximum temperature of 650°C and thermometers connected to three or four wires. It is, however,
possible to have more precise thermo resistances which are classified as a fraction of class B, for example class 1/3B
equals 1/3 (0.3+0.005 | t |).

_______________________________________________________________________________________________
PCI (Process System Solutions) Ltd, Unit 7 Slaidburn Crescent, Southport, Merseyside, PR9 9YF, UK,
Tel: +44 (0) 1704 509090, Fax: +44 (0) 1704 509091,
email: sales@pci-instruments.com, Website: www.pci-instruments.com