Pt100 Sensor / Resistance Thermometers (RTD, PRT, Pt100 Sensors, Pt1000)

Pt100 Sensor (Resistance Thermometer) Theory

The resistance that an electrical conductor exhibits to the flow of an electric current is related to its temperature, essentially because of electron scattering effects and atomic lattice vibrations. The basis of this theory is that free electrons travel through the metal as plane waves modified by a function having the periodicity of the crystal lattice. The only little snag here is that impurities and what are termed lattice defects can also result in scattering, giving resistance variations. Fortunately, this effect is largely temperature-independent, so does not pose too much of a problem.

In fact, the concept of detecting temperature using resistance is considerably easier to work with in practice than is thermocouple thermometry. Firstly, the measurement is absolute, so no reference junction or cold junction compensation is required. Secondly, straightforward copper wires can be used between the sensor and your instrumentation since there are no special requirements in this respect.

The first recorded proposal to use the temperature dependence of resistance for sensing was made in the 1860’s by Sir William Siemens, and thermometers based on the effect were manufactured for a while from about 1870. However, although he used platinum (the most widely used material in RTD thermometry today), the interpolation formulae derived were inadequate. Also, instability was a problem due mainly to his construction methods - harnessing a refractory former inside an iron tube, resulting in differential expansion and platinum strain and contamination problems. Callendar took up the reins in 1887, but it was not until 1899 that the difficulties were ironed out and the use of platinum resistance thermometers was established.

It is now accepted that as long as the temperature relationship with resistance is predictable, smooth and stable, the phenomenon can indeed be used for temperature measurement. But for this to be true, the resistance effects due to impurities must be small - as is the case with some of the pure metals whose resistance is almost entirely dependent on temperature. However, since in thermometry almost entirely is not good enough, the impurity-related resistance must also be (for all practical purposes) constant such that it can be ignored. This means that physical and chemical composition must be kept constant. An important requirement for accurate resistance thermometry is that the sensing element must be pure. It must also be (and remain) in an annealed condition, via suitable heat treatment of the materials such that it is not inclined to change physically. Then again, it must be kept in an environment protected from contamination so that chemical changes are indeed obviated.

Meanwhile, another challenge for the manufacturer is to support the fine, pure wire adequately, while imposing minimum strains due to differential expansion between the wire and its surroundings or former - even though the sensors may be attached to operating plant, with all the rigours of this characteristically arduous environment. Depending upon the accuracy you are after, the relationship governing platinum resistance thermometer output against temperature follows the quadratic equation:

R_t /R₀ = 1 + At + Bt²
(above 0°C this second order approach is more than adequate) or R_t /R₀ = 1 + A^t + Bt² + Ct³ (t-100)
(below 0°C, if you are looking for higher accuracy of representation, the third order provides it).

Therefore:

t = (1/α)(R_t - R₀)/R₀ + δ(t/100)(t/100 -1)

Where: Rt is the thermometer resistance at temperature t; R₀ is the thermometer resistance at 0°C; and t is the temperature in °C. A, B and C are constants (coefficients) determined by calibration. In the IEC 60751 industrial RTD standard, A is 3.90802 x 10^-3; B is -5.802 x 10^-7; and C is -4.2735 x 10^-12. Incidentally, since even this three term representation is imperfect, the ITS-90 scale introduces a further reference function with a set of deviation equations for use over the full practical temperature range above 0°C (a 20 term polynomial). The a coefficient, (R₁₀₀ - R₀)/100 . R₀, essentially defines purity and state of anneal of the platinum, and is basically the mean temperature coefficient of resistance between 0 and 100°C (the mean slope of the resistance vs temperature curve in that region). Meanwhile, δ is the coefficient describing the departure from linearity in the same range. It depends upon the thermal expansion and the density of states curve near the Fermi energy. In fact, both quantities depend upon the purity of the platinum wire. For high purity platinum in a fully annealed state the a coefficient is between 3.925x10^-3/°C and 3.928x10^-3/°C.

For commercially produced platinum resistance thermometers, standard tables of resistance versus temperature have been produced based on an R value of 100 ohms at 0°C and a fundamental interval (R₁₀₀ - R₀) of 38.5 ohms (α coefficient of 3.85x10^-3/°C) using pure platinum doped with another metal (see Part 2, Section 6). The tables are available in IEC 60751, tolerance classes A and B.

Mineral Insulated Pt100 Sensors

with Basic End Seal 3.0 to 8.0mm dia.

Internal basic end seal

with Pot Seal 1.5 to 8.0mm dia.

A large selection of plain and threaded pot seals supplied with tails or extension cable (PVC, FEP, Fibreglass etc.)

with Miniature Plug 1.5 to 3.2mm dia.

Fitted with a miniature thermocouple plug rated to either 220ºC or 350ºC

with Standard Plug 1.5 to 8.0mm dia.

Fitted with a standard thermocouple plug rated to either 220ºC or 350ºC

with Lemo Connector
1.5 to 6.0mm dia.

Fitted with a size 1 Lemo plug
or socket

with Terminal Entry Gland
1.5 to 8.0mm dia.

Terminated with a 16mm ISO x 1.5mm compression gland seal supplied with tails or extension cable (PVC, FEP Fibreglass etc.)

with Micro die cast alloy head 3.0 to 6.0mm dia.

Micro die cast alloy screw down terminal head with ceramic terminal block. Suitable for simplex and duplex assemblies

with Miniature IP67 die cast alloy head 3.0 to 8.0mm dia.

Weatherproof die cast alloy screw top terminal head with ceramic terminal block. Suitable for simplex and duplex assemblies

with Standard IP67 die cast head 4.5 to 8.0mm dia.

Weatherproof die cast alloy screw top terminal head with ceramic terminal block. Suitable for simplex, duplex and triplex assemblies

with IP67 heavy duty cast iron head 4.5 to 8.0mm dia.

Weatherproof cast iron screw top terminal head with ceramic terminal block. Suitable for simplex, duplex and triplex assemblies

with IP67 Bakelite head
4.5 to 8.0mm dia.

Weatherproof Bakelite screw top terminal head with Bakelite terminal block. Suitable for simplex, duplex and triplex assemblies

with alloy straight through head 4.5 to 8.0mm dia.

Die cast alloy straight through terminal head with Bakelite terminal block. Suitable for simplex and duplex assemblies

with 316 Stainless Steel head
4.5 to 8.0mm dia.

Weatherproof 316 stainless steel screw top terminal head with ceramic terminal block. Suitable for simplex and duplex assemblies

with BUZ-H style top hat head
4.5 to 8.0mm dia.

Weatherproof die cast alloy screw down 'flip' terminal head. Mainly used for duplex assemblies where up to two transmitters/blocks can be fitted

with spring loaded terminal block 3, 4.5, 6 and 8mm dia.

Spring loaded insert assemblies. The end seal is incorporated into a terminal block for mounting into any standard terminal head