What is an RTD Sensor? - Also known as Pt100 Sensors, Pt1000s or PRT 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 of resistance thermometer (usually Pt100) used for a wide variety of temperature sensing 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 do RTD Sensors 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 monitoring 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 lead wires.
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
An insulation 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 Thermometers
RTDs 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 Temperature Sensor Ideal for rigid stem applications or where the sensor is shorter than 2" long, limited to 480°F. Simplex and duplex. Hand Held
RTD 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, bearing etc. Miniature Pt100
RTD 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 Tip
RTD Sensors Fast response RTD sensors ideal for industrial and other applications Autoclave
RTD Sensors RTD sensors designed specifically for the harsh environments in autoclaves
RTD Pt100 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).
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 Sensor Tolerances
Application Methods and Equipment - RTD’s
As with thermocouples, RTD outputs measuring temperature change are small - we are looking at less than 0.5 ohms per °C for an IEC standard device. However, the resulting signals are not quite as minute - 1mA energizing current with a 100 ohm nominal resistance RTD sensor yields 5mV output for a change of 10°C. Move the current up to 5mA and the output is 25mV for 10°C change - at least an order of magnitude better signal strength than with thermocouples. However, bridge amplifiers (or equivalent) are still required to provide signal levels suitable for most purposes.
There are two main instruments for determining RTD temperature probe resistance - measuring bridges (null-balance or fixed-bridge direct-deflection), in which the supply current can vary, and potentiometers, where the current has to be known and constant. Both can use AC or DC currents, although a smooth, stable low voltage power supply is the norm.
Early measuring instrumentation relied on null balance bridges (resistive, capacitive, or inductive). In fact, balanced measuring bridges are still used extensively in laboratories where the bridge elements might be resistance decades, or tapped inductances in AC versions. Today, fixed bridge systems are more common - where the imbalance itself is a direct measure of the changing sensing resistance.
However, high accuracy can also be achieved using today’s precision potentiometers, digital voltmeters and the like to measure voltage drop directly across the sensor. Stable, constant energizing current circuits are available, and these tend to favor potentiometric instrumentation, particularly for industrial use. Notably, they lend themselves to high accuracy, high speed RTD sensor scanning applications.
Also, there is now a plethora of direct reading equipment covering both instrumentation types, interpolating from the quadratic resistance (and therefore voltage, if current is constant) vs temperature relationship to give a direct temperature output. The following provides some insight into methods and equipment available.
Bridge Measuring Systems - RTD Sensors
Commercially available industrial bridge measuring systems use one of several circuit arrangements relying mainly on two versions of the Wheatstone bridge - balanced, or fixed bridge, both resistive. Incidentally, it is worth just noting that inductive ratio bridges can also be used, in which precision wound transformers are used for the ratio arms of the bridge. These can offer several advantages in terms of robustness, portability and stability.
Returning to resistive bridges, whatever the circuit format selected, all bridges can be made self-balancing using servo mechanisms controlled from the balance detector. In industrial applications, the bridge is not normally balanced (by altering variable resistances). Instead, as stated above, the imbalance voltage in a fixed element bridge tends to be used as a measure of the sensor resistance - and hence of temperature.
Irrespective of bridge style, all the bridge resistors, except, of course, the sensor, are set to exhibit negligible resistance change with temperature, and in AC bridges are designed to be non-inductive. Also, bridge arm resistance errors due to sliding contacts on variable resistors (where applicable) are normally prevented by introducing these into the current supply line itself, or the balance detector circuit where they can clearly have no influence on the bridge balance.
The sensing resistor, which may well be some distance away from the bridge in industrial applications, is then attached to the bridge using copper cable - whose resistance is low compared with that of the bridge, but which will obviously vary with temperature, particularly nearer to the measurement point. When the conductors are long, or of small cross section, these resistance changes can be large enough to cause significant errors in the temperature reading. There are several wiring configurations to compensate for this potential problem.
Two Wire RTD Sensor Configuration
The simple two wire connection shown in Figure 3.1 is used only where high accuracy is not required - the resistance of the connecting wires is always included with that of the sensor, leading to errors in the signal. In fact, a standard restriction with this arrangement is a maximum of 1 - 2 ohms resistance per conductor - which is typically about 300 feet of cable. This applies equally to balanced bridge and fixed bridge systems. The values of the lead resistance can only be determined in a separate measurement (without the RTD sensor) and therefore a continuous correction during the temperature measurement is not possible.
Figure 3.1: Wheatstone Bridge with RTD in Two Wire Configuration
Three Wire RTD Configuration
A better wiring configuration is shown in Figure 3.2. Here, the two leads of the sensor are on adjoining legs. Although there is lead resistance in each leg of the bridge, the lead resistance is cancelled out from the measurement. It is assumed that the two lead resistances are equal, therefore demanding high quality connection cables.This allows an increase to 10 ohms - usually allowing cable runs of around 1500 feet or more, if necessary.
Also, with this wiring configuration, if fixed bridge measurement is being made, compensation is clearly only good at the bridge balance point. Beyond this, errors will grow as the imbalance increases. This, however, can be minimized by using larger values of resistance in the opposite bridge circuits to reduce bridge current changes.
Figure 3.2: Wheatstone Bridge with RTD in Three Wire Configuration
Four Wire RTD Sensor Bridge Configurations
However, this approach is a little more costly on the copper wiring. An alternative, better version of the four wire configuration uses full four wire terminal RTDs, and as depicted in figure 3.4. This provides for full cancellation of spurious effects with the bridge type measuring technique. Cable resistance of up to 15 ohms can be handled with this arrangement, accommodating cable runs of around 3,000 feet. Incidentally, the same limitation as for three wire connections applies if the fixed-bridge, direct-reading approach is being used.
Figure 3.3: Wheatstone Bridge with RTD in Four Wire Configuration
Figure 3.4: Alternative Four Wire Bridge Connection
Differential Temperature - RTD’s
To measure differential temperatures using bridge circuitry, a second RTD is simply introduced into the bridge circuit alongside the first sensor. A twin two-wire arrangement is adequate for this purpose if the cables used are both of similar resistance (see Figure 3.5).
If, however, high accuracy is required and the two sensing cable lengths, or resistances are dissimilar, then a four wire equivalent is preferable (see Figure 3.6) in which both sensors are equipped with compensating pairs (one per sensing leg of the bridge).
Figure 3.5: Differential Temperature Measurement - Two Wire, Bridge Configuration
Potentiometric Measuring Systems - RTD’s
As described above, the resistance thermometer temperature probe can be energized from a constant current source, and the potential difference developed across it measured directly by some kind of potentiometer. An immediate advantage is that here, incidentals like conductor resistance and selector switch contact resistance are irrelevant. The essentials for this voltage-based method are simply a stabilized and accurately known current supply for the RTD sensor (giving a direct relationship of voltage to resistance and thus to temperature) and a high impedance voltmeter (DVM, or whatever) to measure the voltage developed with negligible current flow.
With this approach, absolute temperature can be derived as long as the current is known. Even where it is not known, if it is stable, differential resistance (and thus temperature) is provided. Also, a number of RTDs can be connected in series using the same current source. Voltage signals from each can then be scanned by high impedance measuring instrumentation.
Four Wire Potentiometric Systems - RTD’s
Again, a four wire configuration is appropriate, although clearly somewhat different to that used with bridge systems. Using the configuration in Figure 3.7 the resistance of the leads has a negligible effect on measurement accuracy.
Having looked at the circuitry and measurement methods in detail, it is time to take a look at the measuring instrumentation itself - detecting the null or measuring the imbalance in bridge systems, or sensing the voltage drop in potentiometric systems. The detector can, of course, take the form of a simple galvanometer - this is appropriate to balanced and fixed bridge arrangements. Deflection will indicate resistance (either directly, or indirectly through voltage as described), and the scale can be configured for direct temperature reading should this be required.
Sophistication can be added, with limit detectors set to provide on-off controls or alarms.
However, in general, low power electronic amplifiers, signal converters or transmitters are used. With the fixed bridge and potentiometric systems, they provide both a high input impedance and adequate power to drive more robust local or remote indicators, recorders or recorder/controllers. For null balance bridges, they are used to drive a servo system to balance the bridge, the system often forming part of an indicator, recorder or controller.
They are usually positioned close to the RTD, and give the added advantage of minimizing sensor cable resistance and providing a large, relatively RFI-immune signal for transmission to the signal reading instrumentation. The amplifier power supply is remote, and we’re back in the realms of standard transmitter technology and process 4-20mA signalling.
Potentiometric Measuring Instruments - RTD’s
Then again, self-balancing direct potentiometric indicators and recorders can also be used to measure either the bridge imbalance voltage, or the direct sensor voltage drop. Constant current supply, bridge resistors, etc are all self-contained in these devices.
Another more modern alternative involves either the bridge voltage imbalance, or the RTD potential drop being measured using a digital voltmeter. This clearly provides the opportunity for applying digital linearizing techniques for direct temperature reading. In fact, there is a range of direct reading instrumentation today which operates more than adequately for industrial grade accuracy temperature measurement in ranges from -200 to +850°C.
Equipment is self-balancing, and the most straightforward comprises basically high resolution digital multimeter technology, with resistance or voltage signals being converted into direct temperature readings. The devices use linearizing techniques following the RTD relationship (Part 1, Section 4) to, say, two or three orders. Linearization is usually generalized to the RTD (as per the IEC 60751 standard quadratic expression), or specific to the sensor, with empirical calibration data taken into account.
In the former case, specifications and tolerances will be to IEC 60751 and accuracy will be to within a few hundredths of a degree. With individual calibration, accuracies to 10mK or better are available. Calibration characteristics can be provided on EEPROM, which is plugged into the linearizing and indicating system together with the sensor, or data can be programmed into the instrument, either directly via the front panel keypad, or remotely, with configuration performed typically in a PC and then downloaded via a serial port into the instrument.
More Information about RTD Pt100 SensorsDo I need an RTD or Thermocouple? RTD Pt100 Output Tables RTD Element Types
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.
An temperature probe 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.
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).
The sensing part of an RTD probe. 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.
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.