What is an RTD Sensor - Reference Information on Temperature Sensors

What is an RTD Sensor?

An RTD sensor is a type of temperature sensor that changes resistance in response to temperature changes. Usually a 100 ohm platinum element, known as a Pt100, is used where the relationship between resistance and temperature is standardised and repeatable. RTDs are passive, meaning they do not generate an output on their own but use electronic devices (indicators) to measure the sensor's resistance by passing a small current (typically 1mA) through the sensor, which in turn generates a voltage. The resistance of the sensor increases as the temperature increases and are provided in our resistance vs temperature tables.

RTD is simply an acronym of Resistance Temperature Detector, a type temperature sensor known as a resistance thermometer which is used for a wide variety of temperature measurement applications. RTD sensors are also commonly referred to as PRT and Pt100 Sensors. There are many styles of RTD Sensor and typically they are Pt100, although Pt1000 is also popular, both are available in a wide range of designs and constructions.

How does an RTD Probe (Pt100 Sensor) work?

RTD Sensors, usually Pt100, rely on a resistive element with a resistance of 100ohms at 0ºC. This element is generally encased in a stainless steel sheath suitable for most temperature measurement applications. As the temperature changes the value of this resistance also changes providing a reliable and predictable resistance value which can then be measured and converted to display in ºF or ºC by appropriate instrumentation.

RTD Sensors are reliable and accurate especially when higher grade elements are selected. A full explanation of the working principles of RTD sensors is detailed below.

What is the difference between 2-wire, 3-wire and 4-wire RTD Pt100 Sensors?

When connected as a 2 wire system the resistance of the 2 wires connecting the RTD sensor to the instrument will be included in any measurement, thus introducing an error equivalent to this lead resistance. A 3 wire RTD Sensor will (via bridge networks) compensate for one leg of this lead resistance and a 4 wire system will compensate for both leads.

A full explanation of the working principles of RTD sensors is detailed below.

What is the color code and wiring configuration for RTD Pt100 Sensors?

2-wire RTD Pt100 Sensor = 1 x red wire and 1 white wire

3-wire RTD Pt100 Sensor = 2 x red wires and 1 white wire

4-wire RTD Pt100 Sensor = 2 x red wire and 2 white wires

A colour code and wiring diagram is shown below.

For a detailed explanation of RTD wiring and bridge networks, please click here.

Resistance Thermometer color codes
Click here for Resistance vs Temperature Output Tables for Pt100 Sensors
Free Guide to Thermocouple and Resistance Thermometry

How do RTD Temperature Sensors work?

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:

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).


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.

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.

RTD Sensor Materials

Several materials are available to fulfil the basic requirements of providing a predictable, smooth and stable temperature with resistance relationship. They include copper, gold, nickel, platinum and silver. Of these, copper, gold and silver have inherently low electrical resistivity values, making them less suitable for resistance thermometry - although copper does exhibit an almost linear resistance relationship against temperature.

In fact, because of this and its low price, copper is used in some applications, with the caveat that above moderate temperatures it is prone to oxidation and is generally not all that stable or repeatable. This notwithstanding, it can come into its own in applications where average temperatures over a long stem length are required, especially within the range -100°C to +180°C.

Nickel and nickel alloys are also relatively low cost and have high resistivities and high resistance versus temperature coefficients, making them very sensitive. However, they suffer from being non-linear with temperature and sensitive to strain. They also exhibit an unfortunate inflexion around the Curie point (358°C) which makes the derivation of the resistance to temperature expressions rather more complicated. These materials are therefore restricted to the temperature range of about -100°C to +180°C.

This leaves platinum, which has considerable advantages that make it well suited to resistance thermometry. Firstly, being a noble metal, it has a wide and unreactive temperature range. Secondly, its resistivity is more than six times that of copper. Thirdly, it has a reasonable, simple and well understood, although not entirely linear, resistance vs temperature relationship. Finally it can be obtained in a very pure form, and drawn into fine wires or strips very reproducibly, making the production of interchangeable detectors relatively easy.

Although platinum is not cheap, only very small amounts are needed for resistance thermometer construction its expense is therefore not a significant factor in calculating the overall cost. On the down side, it is contaminated by a number of materials, particularly when heated, so support and sheath materials have to be chosen carefully. Furthermore, heat treatment of the material is particularly important in view of the presence of vacancy defects which are present at all temperatures unless annealed out.

Beyond these materials, molybdenum film resistors are available, with useful, stable ranges around the -50 to +200°C mark. Semiconductor materials, like thermistors made from various metal oxides, are also available, which with the advent of better manufacturing methods and improved linearisers, can cover a very wide temperature range. However, no standardisation of these devices has been achieved, and they are beyond the scope of this guide. Meanwhile, there is a place for germanium RTD’s below 100K, and especially 10K where the resistivity of platinum is too small for practical purposes. However, the resistance/temperature relationship is not exactly trivial and neither is calibration. Then we get into the realms of carbon-glass RTD’s which exhibit negative temperature coefficients and high sensitivity at very low temperatures. Beyond this, there is the rhodium-iron alloy for temperatures right down to 0.5K.

Click here for RTD Pt100 Resistance vs Temperature Output Tables

RTD Sensor Elements - wire wound, thin film etc.

RTD Pt100 Sensor Wiring.
See our range of RTD Sensors.

RTD Sensor Diagram - RTD Colour Codes

Resistance Thermometer colour codes

RTD Tolerances

Resistance Thermometer Tolerances
Mineral Insulated Pt100 Sensors
with Pot Seal 1.5 to 8.0mm dia. Mineral Insulated RTD Sensor With Pot Seal

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. Mineral Insulated RTD Sensor With Miniature Plug

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

with Standard Plug 1.5 to 8.0mm dia. Mineral Insulated RTD's With Standard Plug

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

with Terminal Entry Gland
1.5 to 8.0mm dia.
Mineral Insulated Resistance Sensor With Terminal Entry Gland

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. Mineral Insulated RTDs with Micro Head

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.Mineral Insulated Pt100 Sensor With Miniature Head

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. Mineral Insulated Pt100's With Standard Head

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. Mineral Insulated RTDs with Cast Iron HeadWeatherproof cast iron screw top terminal head with ceramic terminal block. Suitable for simplex, duplex and triplex assemblies with 316 Stainless Steel head
4.5 to 8.0mm dia.
Mineral Insulated RTD Pt100 With Stainless Steel Head

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

with spring loaded terminal block 3, 4.5, 6 and 8mm dia.Mineral Insulated RTD Temperature Sensor With Spring Loaded Terminal Block

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


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