Resistance thermometers, also called resistance temperature detectors or resistive thermal devices (RTDs), are sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material whose resistance at various temperatures has been documented. The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature.
As they are almost invariably made of platinum, they are often called platinum resistance thermometers (PRTs). They are slowly replacing the use of thermocouple
Thermocouple
A thermocouple is a device consisting of two different conductors that produce a voltage proportional to a temperature difference between either end of the pair of conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a...
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
, due to higher accuracy and repeatability.
R vs T relationship of various metals
Common RTD sensing elements constructed of platinum copper or nickel have a unique, and repeatable and predictable resistance versus temperature relationship (R vs T) and operating temperature range. The R vs T relationship is defined as the amount of resistance change of the sensor per degree of temperature change.
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
is a noble metal and has the most stable resistance to temperature relationship over the largest temperature range. Nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
elements have a limited temperature range because the amount of change in resistance per degree of change in temperature becomes very non-linear at temperatures over 572 ºF (300 ºC). Copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
has a very linear resistance to temperature relationship however copper oxidizes at moderate temperatures and cannot be used over 302ºF (150ºC).
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
is the best metal for RTDs because it follows a very linear resistance to temperature relationship and it follows the R vs T relationship in a highly repeatable manner over a wide temperature range. The unique properties of platinum make it the material of choice for temperature standards over the range of -272.5 ºC to 961.78 ºC, and is used in the sensors that define the International Temperature Standard, ITS-90. It is made using platinum because of its linear resistance-temperature relationship and its chemical inertness.
The basic differentiator between metals used as resistive elements is the linear approximation of the R vs T relationship between 0 and 100 ºC and is referred to as alpha, α. The equation below defines α, its units are ohm/ohm/ºC.
the resistance of the sensor at 0°Cthe resistance of the sensor at 100°C
Pure platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
has an alpha of 0.003925 ohm/ohm/ºC and is used in the construction of laboratory grade RTDs. Conversely two widely recognized standards for industrial RTDs IEC 6075 and ASTM E-1137 specify an alpha of 0.00385 ohms/ohm/ºC. Before these standards were widely adopted several different alpha values were used. It is still possible to find older probes that are made with platinum that have alpha values of 0.003916 ohms/ohm/ºC and 0.003902 ohms/ohm/ºC.
These different alpha values for platinum are achieved by doping; basically carefully introducing impurities into the platinum. The impurities introduced during doping become embedded in the lattice structure of the platinum and result in a different R vs.T curve and hence alpha value.
Calibration
To characterize the R vs T relationship of any RTD over a temperature range that represents the planned range of use, calibration must be performed at temperatures other than 0ºC and 100ºC. Two common calibration methods are the fixed point method and the comparison method.
Fixed point calibration, used for the highest accuracy calibrations, uses the triple point, freezing point or melting point of pure substances such as water, zinc, tin, and argon to generate a known and repeatable temperature. These cells allow the user to reproduce actual conditions of the ITS-90 temperature scale. Fixed point calibrations provide extremely accurate calibrations (within ±0.001°C) A common fixed point calibration method for industrial-grade probes is the ice bath. The equipment is inexpensive, easy to use, and can accommodate several sensors at once. The ice point is designated as a secondary standard because its accuracy is ±0.005°C (±0.009°F), compared to ±0.001°C (±0.0018°F) for primary fixed points.
Comparison calibrations, commonly used with secondary SPRTs and industrial RTDs, the thermometers being calibrated are compared to calibrated thermometers by means of a bath whose temperature is uniformly stable Unlike fixed point calibrations, comparisons can be made at any temperature between –100°C and 500°C (–148°F to 932°F). This method might be more cost-effective since several sensors can be calibrated simultaneously with automated equipment. These, electrically heated and well-stirred baths, use silicone oils and molten salts as the medium for the various calibration temperatures.
RTD Element Types
There are three main categories of RTD sensors; Thin Film, Wire-Wound, and Coiled Elements. While these types are the ones most widely used in industry there are some places were other more exotic shapes are used, for example carbon resistors are used at ultra low temperatures (-173 °C to -273 °C).
Carbon resistors Elements are widely available and are very inexpensive. They have very reproducible results at low temperatures. They are the most reliable form at extremely low temperatures. They generally do not suffer from significant hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
or strain gauge effects.
Strain Free Elements a wire coil minimally supported within a sealed housing filled with an inert gas. These sensors are used up to 961.78 °C and are used in the SPRT’s that define ITS-90. They consisted of platinum wire loosely coiled over a support structure so the element is free to expand and contract with temperature, but it is very susceptible to shock and vibration as the loops of platinum can sway back and forth causing deformation.
Thin Film Elements have a sensing element that is formed by depositing a very thin layer of resistive material, normal platinum, on a ceramic substrate
Plating
Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years, but it is also critical for modern technology...
; This layer is usually just 10 to 100 angstroms (1 to 10 nanometers) thick. This film is then coated with an epoxy or glass that helps protect the deposited film and also acts as a strain relief for the external lead-wires. Disadvantages of this type are that they are not as stable as there wire wound or coiled brethren. They are also can only be used over a limited temperature range due to the different expansion rates of the substrate and resistive deposited giving a "strain gauge
Strain gauge
A strain gauge is a device used to measure the strain of an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable...
" effect that can be seen in the resistive temperature coefficient. These Elements works with temperatures to 300 °C.
Wire-wound Elements can have greater accuracy, especially for wide temperature ranges. The coil diameter provides a compromise between mechanical stability and allowing expansion of the wire to minimize strain and consequential drift. The sensing wire is wrapped around an insulating mandrel or core. The winding core can be round or flat, but must be an electrical insulator. The coefficient of thermal expansion of the winding core material is matched to the sensing wire to minimize any mechanical strain. This strain on the element wire will result in a thermal measurement error. The sensing wire is connected to a larger wire, usually referred to as the element lead or wire. This wire is selected to be compatible with the sensing wire so that the combination does not generate an emf that would distort the thermal measurement. These Elements works with temperatures to 660 °C.
Coiled elements have largely replaced wire-wound elements in industry. This design has a wire coil which can expand freely over temperature, held in place by some mechanical support which lets the coil keep its shape. This “strain free” design allows the sensing wire to expand and contract free of influence from other materials in the This design is similar to that of a SPRT, the primary standard upon which ITS-90 is based, while providing the durability necessary for industrial use. The basis of the sensing element is a small coil of platinum sensing wire. This coil resembles a filament in an incandescent light bulb. The housing or mandrel is a hard fired ceramic oxide tube with equally spaced bores that run transverse to the axes. The coil is inserted in the bores of the mandrel and then packed with a very finely ground ceramic powder. This permits the sensing wire to move while still remaining in good thermal contact with the process. These Elements works with temperatures to 850 °C.
The current international standard which specifies tolerance, and the temperature-to-electrical resistance relationship for platinum resistance thermometers is IEC 60751:2008, ASTM E1137 is also used in the United States. By far the most common devices used in industry have a nominal resistance of 100 ohms at 0 °C, and are called Pt100 sensors ('Pt' is the symbol for platinum). The sensitivity of a standard 100 ohm sensor is a nominal 0.00385 ohm/°C. RTDs with a sensitivity of 0.00375 and 0.00392 ohm/°C as well as a variety of others are also available.
Function
Resistance thermometers are constructed in a number of forms and offer greater stability, accuracy and repeatability
Repeatability
Repeatability or test-retest reliability is the variation in measurements if they would have been taken by a single person or instrument on the same item and under the same conditions. A less-than-perfect test-retest reliability causes test-retest variability. Such variability can be caused by, for...
in some cases than thermocouples. While thermocouples use the Seebeck effect to generate a voltage, resistance thermometers use electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
and require a power source to operate. The resistance ideally varies linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
ly with temperature.
The platinum detecting wire needs to be kept free of contamination to remain stable. A platinum wire or film is supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration. RTD assemblies made from iron or copper are also used in some applications. Commercial platinum grades are produced which exhibit a temperature coefficient
Temperature coefficient
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the...
of resistance 0.00385/°C (0.385%/°C) (European Fundamental Interval). The sensor is usually made to have a resistance of 100 Ω at 0 °C. This is defined in BS EN 60751:1996 (taken from IEC 60751:1995). The American Fundamental Interval is 0.00392/°C, based on using a purer grade of platinum than the European standard. The American standard is from the Scientific Apparatus Manufacturers Association (SAMA), who are no longer in this standards field. As a result the "American standard" is hardly the standard even in the US.
Measurement of resistance requires a small current to be passed through the device under test. This can cause resistive heating, causing significant loss of accuracy if manufacturers' limits are not respected, or the design does not properly consider the heat path. Mechanical strain on the resistance thermometer can also cause inaccuracy.
Lead wire resistance can also be a factor; adopting three- and four-wire, instead of two-wire, connections can eliminate connection lead resistance effects from measurements (see below); three-wire connection is sufficient for most purposes and almost universal industrial practice. Four-wire connections are used for the most precise applications.
Advantages and limitations
Advantages of platinum resistance thermometers:
High accuracy
Low drift
Wide operating range
Suitable for precision applications
Limitations:
RTDs in industrial applications are rarely used above 660 °C. At temperatures above 660 °C it becomes increasingly difficult to prevent the platinum from becoming contaminated by impurities from the metal sheath of the thermometer. This is why laboratory standard thermometers replace the metal sheath with a glass construction. At very low temperatures, say below -270 °C (or 3 K), due to the fact that there are very few phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...
s, the resistance of an RTD is mainly determined by impurities and boundary scattering and thus basically independent of temperature. As a result, the sensitivity
Sensitivity (electronics)
The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.Sensitivity is...
of the RTD is essentially zero and therefore not useful.
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...
s, platinum RTDs are less sensitive to small temperature changes and have a slower response time. However, thermistors have a smaller temperature range and stability.
Sources of error:
The common error sources of a PRT are:
Interchangeability: the “closeness of agreement” between the specific PRT's Resistance vs. Temperature relationship and a predefined Resistance vs. Temperature relationship, commonly defined by IEC 60751.
Insulation Resistance: Error caused by the inability to measure the actual resistance of element. Current leaks into or out of the circuit through the sheath, between the element leads, or the elements.
Stability: Ability to maintain R vs T over time as a result of thermal exposure.
Repeatability: Ability to maintain R vs T under the same conditions after experiencing thermal cycling throughout a specified temperature range.
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
: Change in the characteristics of the materials from which the RTD is built due to exposures to varying temperatures.
Stem Conduction: Error that results from the PRT sheath conducting heat into or out of the process.
Calibration/Interpolation: Errors that occur due to calibration uncertainty at the cal points, or between cal point due to propagation of uncertainty
Propagation of uncertainty
In statistics, propagation of error is the effect of variables' uncertainties on the uncertainty of a function based on them...
or curve fit errors.
Lead Wire: Errors that occur because a 4 wire or 3 wire measurement is not used, this is greatly increased by higher gauge wire.
2 wire connection adds lead resistance in series with PRT element.
3 wire connection relies on all 3 leads having equal resistance.
Self Heating: Error produced by the heating of the PRT element due to the power applied.
Time Response: Errors are produced during temperature transients because the PRT cannot respond to changes fast enough.
Thermal EMF: Thermal EMF errors are produced by the EMF adding to or subtracting from the applied sensing voltage, primarily in DC systems.
RTDs vs Thermocouples
The two most common ways of measuring industrial temperatures are with resistance temperature detectors (RTDs) and thermocouples. Choice between them is usually determined by four factors.
What are the temperature requirements? If process temperatures are between -200 C, an industrial RTD is the preferred option. Thermocouples have a range of -180 C, so for temperatures above 500 °C (932 °F) they are the only contact temperature measurement device.
What are the time-response requirements? If the process requires a very fast response to temperature changes—fractions of a second as opposed to seconds (e.g. 2.5 to 10 s)—then a thermocouple is the best choice. Time response is measured by immersing the sensor in water moving at 1 m/s (3 ft/s) with a 63.2% step change.
What are the size requirements? A standard RTD sheath is 3.175 to 6.35 mm (0.125 to 0.25 in) in diameter; sheath diameters for thermocouples can be less than 1.6 mm (0.062992125984252 in).
What are the accuracy and stability requirements? If a tolerance of 2 °C is acceptable and the highest level of repeatability is not required, a thermocouple will serve. RTDs are capable of higher accuracy and can maintain stability for many years, while thermocouples can drift within the first few hours of use.
Construction
These elements nearly always require insulated leads attached. At temperatures below about 250 °C PVC, silicon rubber or PTFE insulators are used. Above this, glass fibre or ceramic are used. The measuring point, and usually most of the leads, require a housing or protective sleeve, often made of a metal alloy which is chemically inert to the process being monitored. Selecting and designing protection sheaths can require more care than the actual sensor, as the sheath must withstand chemical or physical attack and provide convenient attachment points.
Two-wire configuration
The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is not required, as the resistance of the connecting wires is added to that of the sensor, leading to errors of measurement. This configuration allows use of 100 meters of cable. This applies equally to balanced bridge and fixed bridge system.
Three-wire configuration
In order to minimize the effects of the lead resistances, a three-wire configuration can be used. Using this method the two leads to the sensor are on adjoining arms. There is a lead resistance in each arm of the bridge so that the resistance is cancelled out, so long as the two lead resistances are accurately the same. This configuration allows up to 600 meters of cable.
Error on the schematic : A three wire RTD is connected in the following manner.
One lead is connected to R1. That wire's lead resistance is measured as a part of the RTD resistance.
One wire (of the two on the other end of the RTD) is connected to the lower end of R3. This wire's lead resistance is measured with R3, the reference resistor.
One wire (of the two on the other end of the RTD) is connected to the supply return. (ground) This resistance is normally considered too low to matter in the measurement and is in series with the currents through the RTD and the reference resistor (R3) The lead resistance effects are all translated into common mode voltages that are rejected (common mode rejection) by the instrumentation amplifier.
The wires are typically the same gauge and made of the same wire, to minimise temperature coefficient issues.
Four-wire configuration
The four-wire resistance thermometer configuration increases the accuracy and reliability of the resistance being measured: the resistance error due to lead wire resistance is zero. In the diagram above a standard two-terminal RTD is used with another pair of wires to form an additional loop that cancels out the lead resistance. The above Wheatstone bridge
Wheatstone bridge
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer. It was invented by Samuel Hunter Christie in 1833 and...
method uses a little more copper wire and is not a perfect solution. Below is a better configuration, four-wire
Four-terminal sensing
Four-terminal sensing , 4-wire sensing, or 4-point probes method is an electrical impedance measuring technique that uses separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurements than traditional two-terminal sensing...
Kelvin connection. It provides full cancellation of spurious effects; cable resistance of up to 15 Ω can be handled.
Classifications of RTDs
The highest accuracy of all PRTs is the Standard platinum Resistance Thermometers (SPRTs). This accuracy is achieved at the expense of durability and cost. The SPRTs elements are wound from reference grade platinum wire. Internal lead wires are usually made from platinum while internal supports are made from quartz or fuse silica. The sheaths are usually made from quartz or sometimes Inconel depending on temperature range. Larger diameter platinum wire is used, which drives up the cost and results in a lower resistance for the probe (typically 25.5 ohms). SPRTs have a wide temperature range (-200°C to 1000°C) and approximately accurate to ±0.001°C over the temperature range. SPRTs are only appropriate for laboratory use.
Another classification of laboratory PRTs is Secondary Standard platinum Resistance Thermometers (Secondary SPRTs). They are constructed like the SPRT, but the materials are more cost-effective. SPRTs commonly use reference grade, high purity smaller diameter platinum wire, metal sheaths and ceramic type insulators. Internal lead wires are usually a nickel based alloy. Secondary SPRTs are limited in temperature range (-200°C to 500°C) and are approximately accurate to ±0.03°C over the temperature range.
Industrial PRTs are designed to withstand industrial environments. They can be almost as durable as a thermocouple. Depending on the application industrial PRTs can use thin film elements or coil wound elements. The internal lead wires can range from PTFE insulated stranded nickel plated copper to silver wire, depending on the sensor size and application. Sheath material is typically stainless steel; higher temperature applications may demand Inconel. Other materials are used for specialized applications.
Applications
Sensor assemblies can be categorized into two groups by how they are installed or interface with the process: immersion or surface mounted.
Immersion sensors take the form of an SS tube and some type of process connection fitting. They are installed into the process with sufficient immersion length to ensure good contact with the process medium and reduce external influences. A variation of this style includes a separate thermowell that provides additional protection for the sensor. These styles are used to measure fluid or gas temperatures in pipes and tanks. Most sensors have the sensing element located at the tip of the stainless steel tube. An averaging style RTD however, can measure an average temperature of air in a large duct. This style of immersion RTD has the sensing element distributed along the entire probe length and provides an average temperature. Lengths range from 3 to 60 feet.
Surface mounted sensors are used when immersion into a process fluid is not possible due to configuration of the piping or tank, or the fluid properties may not allow an immersion style sensor. Configurations range from tiny cylinders to large blocks which are mounted by clamps, adhesives, or bolted into place. Most require the addition of insulation to isolate them from cooling or heating affects of the ambient conditions to insure accuracy.
Other applications may require special water proofing or pressure seals. A heavy-duty underwater temperature sensor is designed for complete submersion under rivers, cooling ponds, or sewers. Steam autoclaves require a sensor that is sealed from intrusion by steam during the vacuum cycle process.
Immersion sensors generally have the best measurement accuracy because they are in direct contact with the process fluid. Surface mounted sensors are measuring the pipe surface as a close approximation of the internal process fluid.
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
with rising temperature was first described by Sir William Siemens
Carl Wilhelm Siemens
Carl Wilhelm Siemens was a German born engineer who for most of his life worked in Britain and later became a British subject.-Biography:...
The Bakerian Lecture is a prize lecture of the Royal Society, a lecture on physical sciences.In 1775 Henry Baker left £100 for a spoken lecture by a Fellow on such part of natural history or experimental philosophy as the Society shall determine....
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
. The necessary methods of construction were established by Callendar
Hugh Longbourne Callendar
Hugh Longbourne Callendar FRS was a British physicist. He was born at Hatherop as the eldest son of the Reverend Hugh Callendar, a local Anglican rector...
, Griffiths, Holborn and Wein between 1885 and 1900.
Standard resistance thermometer data
Temperature sensors are usually supplied with thin-film elements. The resisting elements are rated in accordance with BS EN 60751:2008 as:
Tolerance Class
Valid Range
F 0.3
-50 to +500 °C
F 0.15
-30 to +300 °C
F 0.1
0 to +150 °C
Resistance thermometer elements can be supplied which function up to 1000 °C. The relation between temperature and resistance is given by the Callendar-Van Dusen equation
Callendar-Van Dusen equation
The Callendar–Van Dusen equation is an equation that describes the relationship between resistance and temperature of platinum resistance thermometers.It is also used in the international standard DIN EN 60 751...
,
Here, is the resistance at temperature T, is the resistance at 0 °C, and the constants (for an alpha=0.00385 platinum RTD) are
Since the B and C coefficients are relatively small, the resistance changes almost linearly with the temperature.
Values for various popular resistance thermometers
Values for various popular resistance thermometers
Temperature in °C
Pt100 in Ω
Pt1000 in Ω
PTC in Ω
NTC in Ω
NTC in Ω
NTC in Ω
NTC in Ω
NTC in Ω
Typ: 404
Typ: 501
Typ: 201
Typ: 101
Typ: 102
Typ: 103
Typ: 104
Typ: 105
−50
80.31
803.1
1032
−45
82.29
822.9
1084
−40
84.27
842.7
1135
50475
−35
86.25
862.5
1191
36405
−30
88.22
882.2
1246
26550
−25
90.19
901.9
1306
26083
19560
−20
92.16
921.6
1366
19414
14560
−15
94.12
941.2
1430
14596
10943
−10
96.09
960.9
1493
11066
8299
−5
98.04
980.4
1561
31389
8466
0
100.00
1000.0
1628
23868
6536
5
101.95
1019.5
1700
18299
5078
10
103.90
1039.0
1771
14130
3986
15
105.85
1058.5
1847
10998
20
107.79
1077.9
1922
8618
25
109.73
1097.3
2000
6800
15000
30
111.67
1116.7
2080
5401
11933
35
113.61
1136.1
2162
4317
9522
40
115.54
1155.4
2244
3471
7657
45
117.47
1174.7
2330
6194
50
119.40
1194.0
2415
5039
55
121.32
1213.2
2505
4299
27475
60
123.24
1232.4
2595
3756
22590
65
125.16
1251.6
2689
18668
70
127.07
1270.7
2782
15052
75
128.98
1289.8
2880
12932
80
130.89
1308.9
2977
10837
85
132.80
1328.0
3079
9121
90
134.70
1347.0
3180
7708
95
136.60
1366.0
3285
6539
100
138.50
1385.0
3390
105
140.39
1403.9
110
142.29
1422.9
150
157.31
1573.1
200
175.84
1758.4
The function for temperature value acquisition (C++)
The following code provides a temperature sensor Pt100 or Pt1000 from its current resistance.
float GetPt100Temperature(float r)
{
float const Pt100[] = { 80.31, 82.29, 84.27, 86.25, 88.22, 90.19, 92.16, 94.12, 96.09, 98.04,
100.0, 101.95, 103.9, 105.85, 107.79, 109.73, 111.67, 113.61, 115.54, 117.47,
119.4, 121.32, 123.24, 125.16, 127.07, 128.98, 130.89, 132.8, 134.7, 136.6,
138.5, 140.39, 142.29, 157.31, 175.84, 195.84};
int t = -50, i, dt = 0;
if (r > Pt100[i = 0])
while (250 > t) {
dt = (t < 110) ? 5 : (t > 110) ? 50 : 40;
if (r < Pt100[++i])
return t + (r - Pt100[i-1]) * dt / (Pt100[i] - Pt100[i-1]);
t += dt;
};
the resistance of the sensor at 0°C
the resistance of the sensor at 100°C
is the resistance at temperature T,
is the resistance at 0 °C, and the constants (for an alpha=0.00385 platinum RTD) are
