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RTD Sensors / Platinum Resistance Thermometer (RTD, PRT, Pt100 Sensors, Pt1000)

Platinum Resistance Thermometer Manufacture

A Pt100 probe or sensor is a type of Platinum Resistance Thermometer, also referred to as RTD Sensors and PRT Sensor, used for a wide variety of temperature measurement applications. There are many types of Resistance Thermometer, but typically Pt100 (made using a Platinum element with a resistance value of 100 ohms @ 0ºC and typically with a 38.5 ohm fundamental interval - change in resistance value over the range 0 to 100ºC), available in a wide range of designs and constructions as shown below. These Pt100's are available in class A or class B as well as higher tolerance values up to 1/10 DIN.

We are a large manufacturer of Pt100 Sensors and other Resistance Thermometers (RTD sensors). Having an enormous range of components in stock means we can make virtually any sensor you specify. We can ship custom built class A, class B and higher tolerance RTD Sensors to your specifications typically within 5 days or sooner.

Pt100 RTD Sensors - Mineral Insulated Various mineral insulated Pt100 sensors showing different terminations like terminal head, RTD wire and connectors

Our most popular style of Pt100 sensor and ideal for most applications. Vast choice of terminations e.g. pot seals, cables, connectors, heads etc.

Pt100 Sensors (Pt100, RTD, PRT) - Rigid Stem Basic Pt100 sensors with a rigid stem with cable and connectors fitted

Ideal for rigid stem applications or where the sensor is shorter than 50mm, limited to 250°C. Wide choice of terminations

Hand Held Pt100 Sensors (Pt100, RTD, PRT) various styles of hand held Pt100's for measuring air and surface temperatures

A range of hand held RTD Sensors to suit a variety of applications from general purpose to surface and air temperature measurements

Pt100 Sensors for Surface Measurements Clamp and magnetic RTD sensors for measuring the temperatures of surfaces

A wide range of RTD sensors for surface measurements including self adhesive patch, pipe, magnetic etc.

Miniature Pt100 Sensors (Pt100, RTD, PRT) Various styles of miniature Pt100 sensors showing pot seals and connector terminations

1.5 and 2.0mm diameter sensors ideal for precision temperature measurements where minimal displacement and a fast response is required

Swaged Tip Pt100 Sensors (Pt100, RTD, PRT) Diagram showing an example of a reduced tipe RTD probe

Fast response RTD sensors ideal for industrial applications. As with all our Pt100's these are available in class A or class B in as well as higher tolerances.

Autoclave Pt100 Sensors (Pt100, RTD, PRT) various styles of pt100 sensors designed for use in autoclaves and sterilisers

RTD sensors designed specifically for the harsh environments in autoclaves

Other Popular Styles of Pt100 Sensor Popular types of RTD sensors showing bayonet, bolts and simple element types of pt100 sensor

A wide range of RTD Sensors to suit many applications. Bayonet, bolt, stator slot, basic element styles etc. Built to specifications.

Pt100 Sensor (Resistance Thermometer) Theory

The resistance value 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.

Resistance Thermometer History

The first recorded proposal to use the temperature dependence of resistance value 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.

Resistance vs Temperature for a Platinum Resistance Thermometer

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:

Rt /R0 = 1 + At + Bt2
(above 0°C this second order approach is more than adequate) or Rt /R0 = 1 + At + 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)(Rt - 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.

Commercial Resistance Thermometers

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. The tables are available in IEC 60751, tolerance class A or class B.

Click here for RTD Pt100 Resistance vs Temperature Output Table
RTD Element Types incl tolerance for class A and Class B

Platinum Resistance Thermometer Wiring Configuration Diagram

Pt100 sensors are available with a 2 wire, 3 wire or 4 wire wiring connection as shown below. For an explanation of pt 100 wiring configuration, please click here.

Resistance Thermometer colour code Diagram

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