What is an RTD Sensor? - Also known as Pt100 Sensors, Pt1000s or PRT Sensor
RTD is simply an acronym of Resistance Temperature Detector, a type of resistance thermometer (usually Pt100) used for a wide variety of temperature measurement applications. RTD sensors are also referred to as Pt100 Sensors and PRTs. 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 Sensor (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.
Typical RTD Sensors - Pt100 Resistance Thermomeres
Mineral Insulated
RTD Sensors
Our most popular style of RTD sensor and ideal for most applications. Wide choice of terminations e.g. pot seals, wire, connectors
Rigid Stem
RTD Sensors
Ideal for rigid stem applications or where the sensor is shorter than 2" long, limited to 480°F. Large choice of terminations
Hand HeldRTD Sensors
A range of hand held RTD Sensors to suit a variety of applications from general purpose to surface and air temperature measurements
RTD Sensors for
Surface Measurements
A wide range of RTD sensors for surface measurements including self adhesive patch, pipe, magnetic etc.
Miniature Pt100RTD Sensors
0.062" and 0.080" diameter sensors ideal for precision fast response measurements or minimal displacement is required
Other Popular Styles
of RTD Sensors
A wide range of RTD Sensors to suit many applications. Bayonet, bolt, stator slot, basic element styles etc.
Reduced TipRTD Sensors
Fast response RTD sensors ideal for industrial and other applications
AutoclaveRTD Sensors
RTD sensors designed specifically for the harsh environments in autoclaves
RTD Sensor Resistance / Temperature Calculations
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.
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.
A Typical Industrial RTD Sensor
Typical RTD Probe Construction - Probe and Head
Hand Held RTD Probe
Platinum Resistance Thermometers
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 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.
RTD (Resistance Thermometer Detector)
The industry-wide acronym for resistance thermometer detector is widely used to describe the RTD sensor which is a device comprising a resistive element (usually Pt100) which relies on the inherent change in resistance with temperature of the wire or material in the sensing element.
Resistance Thermometer
An instrument or system incorporating a length of wire or film having predictable resistance vs temperature characteristics, forming a temperature sensor. Measurement of the resistance of the device yields its temperature.
PRT
Platinum Resistance Thermometer
Pt100
A generic term usually used to describe a Pt100 Sensor. It reallty refers to the resistance of the element at 0ºC (100 ohms) and the material it is made from (Platinum).
RTD Element
The sensing part of an RTD sensor. Usually a suspended wire wound coil of Platinum wire within a ceramic cylinder or a Platinum film deposited on a substrate. Can be Pt100, Pt1000 or Cu.
Fundamental Interval
The Fundamental Interval is the value of resistance change in the element over the temperature 0 to 100ºC which is usually 38.5ohms for a Pt100 element to IEC 60751.
Alpha Value (coefficient)
Linked to Fundamental Interval, the alpha value represents the change in resistance per ºC step (over the range 0 to 100ºC) for a resistance element. The industry standard is IEC 60751 Pt100, where the α coefficient is 3.85x10-3/°C.
