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Resistance thermometer

From Wikipedia, the free encyclopedia

Resistance thermometers, also called resistance temperature detectors orresistive thermaldevices (RTDs), are sensors 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 placedinside 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 ofthermocouples in many industrial

applications below 600 C, due to higher accuracy and repeatability.

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Contents

[hide]

1 R vs T relationship of various metals

2 Calibration

3 RTD Element Types

4 Function 5 Advantages and limitations

o 5.1 RTDs vs Thermocouples

6 Construction

7 Wiring configurationso 7.1 Two-wire configuration

o 7.2 Three-wire configuration

o 7.3 Four-wire configuration

8 Classifications of RTDs

9 Applications

10 History

11 Standard resistance thermometer data 12 Values for various popular resistance thermometers

13 The function for temperature value acquisition (C++)

14 See also

15 References

16 External links
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[edit] 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 operatingtemperature 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 noble metal and has the most stable resistance to temperature relationship over the

largest temperature range.Nickel elements have a limited temperature range because the amountof change in resistance per degree of change in temperature becomes very non-linear at

temperatures over 572 F (300 C). Copperhas a very linear resistance to temperature

relationship, however copper oxidizes at moderate temperatures and cannot be used over 302F(150C).

Platinum 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 fortemperature standards over the range of -272.5 C to 961.78 C, 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 ofthe R vs T relationship between 0 and 100 C and is referred to as alpha, . The equation below

defines , its units are ohm/ohm/C.

R0 = the resistance of the sensor at 0C

R100 = the resistance of the sensor at 100C

Pureplatinum has an alpha of 0.003925 ohm/ohm/C and is used in the construction oflaboratory grade RTDs. Conversely two widely recognized standards for industrial RTDs IEC

6075 and ASTM E-1137 specify an alpha of 0.00385 ohms/ohm/C. Before these standards were

widely adopted several different alpha values were used. It is still possible to find older probesthat are made with platinum that have alpha values of 0.003916 ohms/ohm/C and 0.003902

ohms/ohm/C.

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 thelattice structure of the platinum and result in a different R vs.T curve and hence alpha value.[2]
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[edit] 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 0C and 100C.Two common calibration methods are the fixed point method and the comparison method.[3]

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 reproduceactual conditions of the ITS-90temperature scale. Fixed point calibrations provide

extremely accurate calibrations (within 0.001C) A common fixed point calibration

method for industrial-grade probes is the ice bath. The equipment is inexpensive, easy touse, and can accommodate several sensors at once. The ice point is designated as a

secondary standard because its accuracy is 0.005C (0.009F), compared to 0.001C

(0.0018F) 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 100C and 500C (148F to932F). 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.

[edit] 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 othermore exotic shapes are used, for example carbon resistors are used at ultra low temperatures (-

173 C to -273 C).[4]

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 extremelylow temperatures. They generally do not suffer from significanthysteresis 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 SPRTs thatdefine 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 causingdeformation.
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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; This layer is usually just

10 to 100 angstroms (1 to 10 nanometers) thick. [5] This film is then coated with an epoxyor 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 temperaturerange due to the different expansion rates of the substrate and resistive deposited giving a

"strain gauge" 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 iswrapped 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 onthe 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 work with

temperatures to 660 C.
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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 mechanicalsupport 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 issimilar to that of a SPRT, the primary standard upon which ITS-90is based, whileproviding 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 spacedbores 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 nominalresistance of 100 ohmsat 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.

[edit] Function
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Resistance thermometers are constructed in a number of forms and offer greater stability,

accuracy and repeatability in some cases than thermocouples. While thermocouples use the

Seebeck effectto generate a voltage, resistance thermometers use electrical resistance andrequire a power source to operate. The resistance ideally varieslinearly with temperature.

The platinum detecting wire needs to be kept free of contamination to remain stable. A platinumwire 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 fromiron or copper are also used in some applications. Commercial platinum grades are produced

which exhibit a temperature coefficient of resistance 0.00385/C (0.385%/C) (European

Fundamental Interval).[6] The sensor is usually made to have a resistance of 100 at 0 C. Thisis defined in BS EN 60751:1996 (taken from IEC 60751:1995). The American Fundamental

Interval is 0.00392/C,[7] 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 smallcurrent to be passed through the device under test.

This can cause resistive heating, causing significant loss of accuracy if manufacturers' limits arenot 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 leadresistance effects from measurements (seebelow); three-wire connection is sufficient for most

purposes and almost universal industrial practice. Four-wire connections are used for the most

precise applications.

[edit] 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 becomingcontaminated by impurities from the metal sheath of the thermometer. This is whylaboratory 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

phonons, the resistance of an RTD is mainly determined by impurities andboundaryscattering and thus basically independent of temperature. As a result, thesensitivity of the

RTD is essentially zero and therefore not useful.
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Compared to thermistors, 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.[8]

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.[9]

Stability: Ability to maintain R vs T over time as a result of thermal exposure. [10]

Repeatability: Ability to maintain R vs T under the same conditions after experiencing

thermal cycling throughout a specified temperature range.

[11]

Hysteresis: Change in the characteristics of the materials from which the RTD is built

due to exposures to varying temperatures.[12]

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 topropagation of uncertainty or curve fit errors.

Lead Wire: Errors that occur because a 4 wire or 3 wire measurement is not used, this isgreatly increased by higher gauge wire.

o 2 wire connection adds lead resistance in series with PRT element.

o 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.

[edit] 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 to 500

C (-328 to 932 F), an industrial RTD is the preferred option.Thermocoupleshave a

range of -180 to 2,320 C (-292 to 4,208 F),[13]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 changesfractions of a second as opposed to seconds (e.g. 2.5 to 10 s)
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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 (0.1250 to0.250 in) in diameter; sheath diameters for thermocouples can be less than 1.6 mm

(0.063 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 arecapable of higher accuracy and can maintain stability for many years, while

thermocouples can drift within the first few hours of use.

[edit] 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.

[edit] Wiring configurations

[edit] Two-wire configuration

The simplest resistance thermometer configuration uses two wires. It is only used when highaccuracy 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.
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[edit] 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 resistancesare 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 isconnected 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'slead 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 thereference 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.

[edit] 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 diagramabove 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 bridgemethod uses a little more

copper wire and is not a perfect solution. Below is a better configuration,four-wireKelvin

connection. It provides full cancellation of spurious effects; cable resistance of up to 15 can behandled.
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[edit] 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 woundfrom 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 (-200C to 1000C) and approximately accurate to

0.001C 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 nickelbased alloy. Secondary SPRTs are limited in temperature range (-200C to 500C) and are

approximately accurate to 0.03C over the temperature range.

Industrial PRTs are designed to withstand industrial environments. They can be almost asdurable as a thermocouple. Depending on the application industrial PRTs can use thin filmelements 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. Othermaterials are used for specialized applications.

[edit] 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 connectionfitting. They are installed into the process with sufficient immersion length to ensure

good contact with the process medium and reduce external influences.[14] A variation of

this style includes a separate thermowell that provides additional protection for thesensor.[15]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
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averaging style RTD however, can measure an average temperature of air in a large duct.[16] 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 animmersion style sensor. Configurations range from tiny cylinders[17] to large blocks which

are mounted by clamps[18], adhesives, or bolted into place. Most require the addition ofinsulation 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 directcontact with the process fluid. Surface mounted sensors are measuring the pipe surface as a close

approximation of the internal process fluid.

[edit] History

The application of the tendency ofelectrical conductors to increase theirelectrical resistancewith rising temperature was first described by SirWilliam Siemens at the Bakerian Lectureof

1871 before the Royal Society ofGreat Britain. The necessary methods of construction were

established by Callendar, Griffiths, Holborn and Wein between 1885 and 1900.

[edit] 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 relationbetween temperature and resistance is given by the Callendar-Van Dusen equation,
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Here,RT is the resistance at temperature T,R0 is the resistance at 0 C, and the constants (for an

alpha=0.00385 platinum RTD) are

Since theB and Ccoefficients are relatively small, the resistance changes almost linearly with

the temperature.

[edit] Values for various popular resistance thermometers

Values for various popular resistance thermometers

Temperaturein C

Pt100in

Pt1000in

PTCin

NTCin

NTCin

NTCin

NTCin

NTCin

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
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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


14/15

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

[edit] 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)
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{

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;

};

return t;

}

float GetPt1000Temperature(float r)

{

return GetPt100Temperature(r / 10);

}

[edit] See also

Thermowell Thermistor

Thermostat

Thermocouple platinum
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res thermo platinum

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