RESISTANCE THERMOMETER THEORY AND PRACTICE
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BASIC THEORY
The electrical conductivity of a metal depends on the movement of electrons through its
crystal lattice. Due to thermal excitation, the electrical resistance of a conductor varies
according to its temperature and this forms the basic principals of resistance thermometry.
The effect is most commonly exhibited as an increase in resistance with increasing
temperature, a positive temperature coefficient of resistance.


ADOPTION OF Pt100 THERMOMETERS
The practical range of Pt100 based thermometers extends from –200°C to 650°C although
special versions are available for up to 962°C. Their use has in part taken over form
thermocouples in many applications for a variety of reasons:
a) Installation is simplified since special cabling and cold junction considerations are not
relevant. Similarly, instrumentation considerations are less complex in terms of input
configuration and enhanced stability.
b) Instrumentation developments have resulted in high accuracy, high resolution and high
stability performance from lower cost indicators and controllers; such accuracy can be
better exploited by the use of superior temperature sensors.
c) The availablility of a growing range of sensing resistor configurations include miniature,
flat-film fast response versions in addition to the established wirewound types with
alternative tolerance bands.


RESISTANCE THERMOMETER PRACTICE
3.1 Terminating the Resistance Thermometer
Fundamentally, every sensing resistor is a two wire device. When terminating the resistor
with extension wires, a decision must be made as to whether a 2, 3 or 4 wire arrangement
is required for measurement purposes.
In the sensing resistor, the electrical resistance varies with temperature. Temperature is
measured indirectly by reading the voltage drop across the sensing resistor in the
presence of a constant current flowing through it using Ohm’s Law:
V= R.I


Transmitters
The problems of the 2 or 3 wire configuration as described can be resolved in large part
by using a 4-20mA transmitter. If the transmitter is located close to the Pt100, often in the
terminal head of the thermometer, then the amplified “temperature’ signal is transmitted
to the remote instrumentation. Cable resistance effects are then no applicable other than
those due to the relatively short leadwires between the sensor and transmitter