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AIM: To study RESISTANCE TEMPERATURE DETECTORs.

THEORY OF OPERATION :

.Resistance thermometers, also called resistance temperature detectors or resistivethermal devices (RTDs), are Temperature sensors that exploit the predictable change inelectrical resistance of some materials with changing temperature. RTDs are verysimilar in appearance to thermocouples but they function completely different. Now aswe know, thermocouples produce a very small voltage when heated. An RTD does notproduce any voltage and so it relies on an instrument for power. RTDs are electricalresistors that change resistance as temperature changes.

The same year that Seebeck made his discovery about thermoelectricity, Sir HumphreyDavy announced that the resistivity of metals showed a marked temperaturedependence. Fifty years later, Sir William Siemens proffered the use of platinum as theelement in a resistance thermometer. His choice proved most propitious, as platinum isused to this day as the primary element in all high-accuracy resistance thermometers. Infact, the Platinum Resistance Temperature Detector, or PRTD, is used today as aninterpolation standard from the oxygen point (-182.96C) to the antimony point(630.74C).

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T E M P E R A T U R E D E T E C T O R | 1 0 / 2 5 / 2 0 1 0

Platinum is especially suited to this purpose, as it can withstand high temperatureswhile maintaining excellent stability. As a noble metal, it shows limited susceptibility tocontamination.

The classical resistance temperature detector (RTD) construction using platinum wasproposed by C.H. Meyers in 1932. He wound a helical coil of platinum on a crossedmica web and mounted the assembly inside a glass tube. This construction minimizedstrain on the wire while maximizing resistance.

Although this construction produces a very stable element, the thermal contact betweenthe platinum and the measured point is quite poor. This results in a slow thermalresponse time. The fragility of the structure limits its use today primarily to that of a

laboratory standard.

Another laboratory standard has taken the place of Meyers design. This is the bird-cageelement proposed by Evans and Burns. The platinum element remains largelyunsupported, which allows it to move freely when expanded or contracted bytemperature variations.

Strain-induced resistance changes over time and temperature are thus minimized, andthe bird-cage becomes the ultimate laboratory standard. Due to the unsupportedstructure and subsequent susceptibility to vibration, this configuration is still a bit toofragile for industrial environments.

A more rugged construction technique is shown in below Figure . The platinum wire isbifilar wound on a glass or ceramic bobbin. The bifilar winding reduces the effectiveenclosed area of the coil to minimize magnetic pickup and its related noise. Once thewire is wound onto the bobbin, the assembly is then sealed with a coating of molten

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glass. The sealing process assures that the RTD will maintain its integrity under extreme vibration, but it also limits the expansion of the platinum metal at hightemperatures. Unless the coefficients of expansion of the platinum and the bobbinmatch perfectly, stress will be placed on the wire as the temperature changes, resultingin a strain-induced resistance change. This may result in a permanent change in theresistance of the wire.

There are partially supported versions of the RTD which offer a compromise betweenthe bird-cage approach and the sealed helix. One such approach uses a platinum helixthreaded through a ceramic cylinder and affixed via glass-frit. These devices willmaintain excellent stability in moderately rugged vibrational applications.

The resistive property of the metal is called its resistivity. The resistive property defineslength and cross sectional area required to fabricate an RTD of a given value. Theresistance is proportional to length and inversely proportional to the cross sectionalarea:

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Where,R = Resistance (ohms)

= Resistivity (ohms)L = Length A = Cross sectional area

TEMPERATURE COEFFICIENT

Another common term used with RTDs is temperature coefficient . This refers to thechange in resistance vs. change in temperature. There are 2 common coefficients for platinum RTDs and several others for the copper and nickel types. The most commonplatinum RTD has a temperature coefficient of .00385 ohms/ohms/C. This means thata 100 ohm platinum RTD will increase in resistance .385 ohms for every 1C increase in

temperature. RTD Materials:

RTDs are manufactured using several different materials as the sensing element.The criterion for selecting a material to make an RTD is:

y The material must be malleable so that it can be formed into small wiresy The material should also be resistant to corrosion.y The material should be low costy It is preferred that the material have a linear resistance verses temperature

slope.

Metal Film RTDs

In the newest construction technique, a platinum or metal-glass slurry film is depositedor screened onto a small flat ceramic substrate, etched with a lasertrimming system,and sealed. The film RTD offers substantial reduction in assembly time and has thefurther advantage of increased resistance for a given size. Due to the manufacturingtechnology, the device size itself is small, which means it can respond quickly to stepchanges in temperature. Film RTDs are presently less stable than their hand-madecounterparts, but they are becoming more popular because of their decided advantagesin size and production cost. These advantages should provide the impetus for futureresearch needed to improve stability.

METALS

All metals produce a positive change in resistance for a positive change in temperature.This, of course, is the main function of an RTD. As we shall soon see, system error isminimized when the nominal value of the RTD resistance is large. This implies a metal

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wire with a high resistivity. The lower the resistivity of the metal, the more material wewill have to use.

Below Table lists the resistivities of common RTD materials:

METAL RESISTIVITY(OHM)

Gold 13

Silver 8.8

Copper 9.26

Platinum 59

Tungsten 30

Nickel 36

Because of their lower resistivities, gold and silver are rarely used as RTD elements.Tungsten has a relatively high resistivity, but is reserved for very high temperatureapplications because it is extremely brittle and difficult to work.

Copper is used occasionally as an RTD element. Its low resistivity forces the element tobe longer than a platinum element, but its linearity and very low cost make it aneconomical alternative. Its upper temperature limit is only about 120C.

The most common RTDs are made of either platinum, nickel, or nickel alloys. Theeconomical nickel derivative wires are used over a limited temperature range. They arequite non-linear and tend to drift with time. For measurement integrity, platinum is theobvious choice.

CONSTRUCTION:

RTDs are manufactured in 3 basic types of construction. Each of these different typeshas advantages and disadvantages.

Platinum Thin Film RTD

The thin film style of RTD is probably the most popular design because of their ruggeddesign and low cost. The thin film element is manufactured by coating a small ceramicchip with a very thin (.0001) film of platinum and then laser cutting or chemical etchinga resistance path in the platinum film. The element is then coated with a thin layer of glass to protect it from harmful chemicals and gases. Larger extension lead wires are

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spot welded to the chip and this junction is then covered with a drop of epoxy to helphold the wires to the element.

Inner Coil Wire Wound RTD

This type of element is normally manufactured using platinum wire. Very small platinumwire (.0002) is coiled and then slid into a small 2 hole ceramic insulator. Larger extension leads are then spot welded to the ends of the platinum wire and cemented inplace.

Some manufacturers backfill the bores of the insulator with ceramic powder once thecoils have been inserted. This keeps the coils from moving and shorting against eachother. The end opposite the extension leads is capped with ceramic cement also.

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Outer Wound RTD ElementThe outer wound RTD element is made by winding the sensing element wire around acenter mandrill, which is usually made of ceramic. This winding is then coated withglass or some other insulating material to protect and secure the windings. The windingwires are then spot welded to extension leads and secured to the body with ceramiccement or epoxy.

Each of the types has their advantages. The thin film is the least expensive tomanufacture and also the most rugged. They also can be manufactured in very smallsizes. The inner coil wire wound style is the most accurate. It is however,moreexpensive to manufacture and does not perform well in high vibration applications.

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The outer wound element is similar in cost to the inner coil element. It is not as accurateas the inner coil style but is more rugged.

Resistance Measurement

The common values of resistance for a platinum RTD range from 10 ohms for the bird-cage model to several thousand ohms for the film RTD. The single most common valueis 100 ohms at 0C. The standard temperature coefficient of platinum wire is = .00385.For a 100 ohm wire, this corresponds to + 0.385 ohms/C at 0C. This value for isactually the average slope from 0C to 100C. The more chemically pure platinum wireused in platinum resistance standards has an of +.00392 ohms/ohm/C.

Both the slope and the absolute value are small numbers, especially when we consider the fact that the measurement wires leading to the sensor may be several ohms or eventens of ohms. A small lead impedance can contribute a significant error to our temperature measurement.

A 10 ohm lead impedance implies 10/.385 26C error in measurement. Even thetemperature coefficient of the lead wire can contribute a measurable error. The classicalmethod of avoiding this problem has been the use of a bridge.

The bridge output voltage is an indirect indication of the RTD resistance. The bridgerequires four connection wires, an external source, and three resistors that have a zerotemperature coefficient. To avoid subjecting the three bridge-completion resistors to the

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same temperature as the RTD, the RTD is separated from the bridge by a pair of extension wires:

These extension wires recreate the problem that we had initially: The impedance of theextension wires affects the temperature reading. This effect can be minimized by using

a three-wire bridge configuration:

If wires A and B are perfectly matched in length, their impedance effects will cancelbecause each is in an opposite leg of the bridge. The third wire, C, acts as a sense leadand carries no current.

The Wheatstone bridge shown in above figure creates a non-linear relationship betweenresistance change and bridge output voltage change. This compounds the already non-linear temperature-resistance characteristic of the RTD by requiring an additionalequation to convert bridge output voltage to equivalent RTD impedance

4-Wire Ohms - The technique of using a current source along with a remotely senseddigital voltmeter alleviates many problems associated with the bridge.

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The output voltage read by the dvm is directly proportional to RTD resistance, so onlyone conversion equation is necessary. The three bridge-completion resistors arereplaced by one reference resistor. The digital voltmeter measures only the voltagedropped across the RTD and is insensitive to the length of the lead wires.

The one disadvantage of using 4-wire ohms is that we need one more extension wirethan the 3-wire bridge. This is a small price to pay if we are at all concerned with theaccuracy of the temperature measurement.

The CallendarVan Dusen equation is an equation that describes the relationshipbetween resistance (R) and temperature (t) of platinum resistance thermometers.

For the range between -200 C to 0 C the equation is

R(t) = R(0)[1 + A * t + B * + (t 100)C * ].

For the range between 0 C to 661 C the equation is

R(t) = R(0)(1 + A * t + B * ).

These equations are listed as the basis for the temperature/resistance tables for platinum resistance thermometers and are not intended to be used for the calibration of individual thermometers.The coefficients for individual thermometers (A(t) and B(t)) canbe obtained by calibration.

The equation was found by British physicist Hugh Longbourne Callendar, and refined byM. S. Van Dusen.

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TECHNICAL SPECIFICATIONS

It is important for users of PRTs to know and understand what these error sources areso they can make intelligent decisions related to PRT selection and use. The mostcommon error sources fall within the following categories: Hysteresis, Insulation

Resistance, Stability, Repeatability,Stem Conduction, Calibration and Interpolation,Lead Wire Resistance, Self-Heating, Time Response, and Thermal EMF.

Hysteresis: In general, hysteresis is a phenomena that results in a difference in an items behavior when approached from a different path. In PRTs, thermal hysteresis results in adifference in resistance at a given temperature based on the thermal history to whichthe PRT was exposed. More specifically, the resistance of the PRT will be differentwhen the temperature is approached from an increasing direction vs a decreasingdirection, and the magnitude of the difference will depend on the magnitude of the

temperature excursion and the design of the PRT.The most prominent factor that contributes to the hysteresis error in a PRT is strainwithin the sensing element caused by thermal expansion and contraction. Mostindustrial grade PRTs are manufactured using a sensing element made from a finediameter platinum wire, typically less than 0.001 inch diameter, or a thin film platinumelement. The other materials used to manufacture these elements are critical becausethey are in direct contact with the fragile platinum and must provide mechanical supportand protection while still allowing for free thermal expansion and contraction over a widetemperature range. These elements are then packaged into the final sensor

configuration, where the materials used must also allow for free thermal expansion and

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contraction or additional strain can occur .

How to Reduce Hysteresis Error

Hysteresis is controlled almost exclusively by the design and manufacture of the PRTand the temperature span to which the PRT is exposed. The best way to reducehysteresis error is to select a PRT that has a low specified hysteresis and minimize thetemperature span to which the PRT is exposed. Keep in mind that hysteresis is amaximum at the midpoint temperature and is zero at the end points, so using a sensor

near the end points can reduce the magnitude of this error.

RepeatabilityRepeatability refers to the ability of a PRT to maintain its Resistance vs. Temperature(R vs. T) relationship when measured under the same conditionsafter experiencingthermal cycling throughout a specified temperature range .

Causes of Repeatability Error Many factors can contribute to the inability of a PRT to repeat readings after thermal

cycling, but the most prominent factor is generally considered to be strain within thesensing element caused by thermal expansion and contraction. Most industrial gradePRTs are manufactured using a sensing element made from a fine diameter platinumwire, typically less than 0.001 inch diameter, or a thin film platinum element. The other materials used to manufacture these elements are critical because they are in directcontact with the fragile platinum and must provide mechanical support and protectionwhile still allowing for free thermal expansion and contraction over a wide temperaturerange. These elements are then packaged into the final sensor configuration, the

Temp Range: -200 to600C

Temp Range: 0 to400C

Temp Range: 0 to200C

Hysteresis Spec .02%of

span

.05%of

span

.10%of

span

.02%of

span

.05%of

span

.10%of

span

.02%of

span

.05%of

span

.10%of

spanError (C)

Temperatur e(C)

-200 0 0 0 - - - - - --100 .04 .10 .20 - - - - - -0 .08 .20 .40 0 0 0 0 0 0100 .12 .30 .60 .04 .10 .20 .04 .10 .20200 .16 .40 .80 .08 .20 .40 0 0 0300 .12 .30 .60 .04 .10 .20 - - -400 .08 .20 .40 0 0 0 - - -500 .04 .10 .20 - - - - - -600 0 0 0 - - - - - -

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materials used here must also allow for free thermal expansion and contractionadditional strain can occur.

How to Reduce Repeatability Error

Since repeatability is controlled almost exclusively by the design and manufacture of thePRT, the best way to reduce repeatability error is to select a high quality PRT that has alow specified repeatability. When selecting a PRT, the repeatability must be consideredfor the maximum temperature range of use, not necessarily the maximum ratedtemperature range of the PRT itself since many PRTs are not used over their maximumrated ranges. Never expose PRTs to temperatures in excess of their maximum ratedtemperature, or less than their minimum rated temperature, without consulting with themanufacturer first to determine the effect on repeatability.

Insulation Resistance

Insulation Resistance (IR) refers to the electrical resistance between the sensing circuitand the metallic sheath of a PRT. It is important for the sensing element circuit to beinsulated from the sheath because electrical leakage can cause an error whenmeasuring the resistance of the sensing element. Any error in measuring the resistancewill translate to an error in the indicated temperature.

Resistance is a parameter that cannot be measured directly, it is calculated by either

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applying a constant current and measuring the voltage drop, or by applying a constantvoltage and measuring the current. The typical method used to measure the resistanceof an industrial PRT is to apply a constant current, typically between .050 mA and 2 mA,and measure the voltage drop to determine the resistance. The formula required tomake this calculation is simply Ohms Law, however the details of this calculation are not

necessary for the purpose of understanding this concept. What is important to know isthat if a portion of the applied current has the opportunity to leak out of the circuit,through a low insulation resistance, then a false resistance reading will be obtained for the sensing element.

Estimating the Error Caused by Insulation Resistance

One method that has been used to estimate the magnitude of the error due to IR affectis to treat the PRT element resistance and the IR value as two resistors in parallel. Thismethod is not completely accurate however, since electrical leakage can occur not only

from lead wire to sheath, but from lead wire to lead wire. The lead to lead leakage alsoacts as a resistor in parallel and this type of leakage cannot be tested because thesensing element is in the circuit. Nevertheless, treating the PRT element resistance andIR as two resistors in parallel has become a common way to estimate the magnitude of the error due to IR. It is worthwhile noting that IR almost always results in a lower indicated temperature with few exceptions, such as installations where current mayactually leak into the circuit.

Stability

Stability refers to the ability of a PRT to maintain its Resistance vs. Temperature (R vs.T) relationship over time as a result of thermal exposure .

Causes of Stability Error

Many factors can contribute to the instability of a PRT, but the most prominent source of instability is contamination of the platinum in the sensing element. Contamination can

Rated Temperature(C)

Minimum IR (MQ )

Test Voltage(VDC)

EstimatedError for 100

ohm PRT (C)ASTM E1137 25 100 10 to 50 .0003ASTM E1137 300 10 10 to 50 .013ASTM E1137 650 2 10 to 50 .17

IEC 60751 25 100 10 to100 .0003IEC 60751 100 to 300 10 10 .013IEC 60751 301 to 500 2 10 .12IEC 60751 501 to 850 .5 10 1.0

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come from a variety of sources, such as metals that alloy with platinum at elevatedtemperatures, and very small amounts of these contaminants can have large effects onresistance. The materials and processes used to manufacture the sensor must becarefully selected and/or developed such that they have minimal affect on the platinumat temperatures up to the maximum rated temperature of the PRT. Cleanliness during

manufacture is also critical as any foreign substance may become a source of contamination.

How to Reduce Stability Error

Since stability is controlled almost exclusively by the design and manufacture of thePRT, the best way to reduce stability error is to select a high quality PRT that has a lowspecified stability. When selecting a PRT, the stability must be considered for themaximum temperature of use, not necessarily the maximum rated temperature of thePRT itself since many PRTs are not used to maximum rated temperatures. Never expose PRTs to temperatures in excess of their maximum rated temperature without

consulting the manufacturer first to determine the effect on stability. Also, avoidunnecessary exposure to elevated temperature, the less time the sensor is exposed toelevated temperature the smaller the cumulative effect

Industrial PRT Stability Example(Change at 0C (C))

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APPLICATIONS:

Mini RTDs for Small Places:

Small locations require small sensors. Freeze dryers, bearings,and motor windings, are just a few of the locations thatrequire a small diameter and short length RTD for arepeatableand stable temperature measurement. These types ofapplications may alsorequire high accuracy, durable extensioncable, and NIST traceable calibration to satisfyrequirements.

The Burns design group came up with a package that is 1/8diameter, and just 1 longthat has all the features and performanceyou would expect from a much larger sensor.Temperaturerange is 196C to 200C and the sensing element iscompletely sealedagainst moisture and can be completelyimmersed in water without degradation or lossof accuracy.

The Teflon encased cable and 316L SS sensor body are compatiblewith a wide varietyof chemicals and other agents.Sensors are available in 0.10% and 0.05%interchangeabilityand can be matched to a transmitter for even greater accuracyof up to 0.11C.

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Battery Powered Indicator

Winter is here and offloading a rail tanker of a cold thickfluid can be a problem when thetemperature drops outside.

Heating the fluid allows for pumping but getting it too hotcould ruin it. A portable andextremely rugged temperature indicator was required to monitor the fluid temperature tomaintain an optimal temperature.

Power was not available at the location and portability betweentanker cars was a must.The sensor had to be capableof being dropped and banged around as can be expected

when handling with bulky gloves and jackets impairing movement,not to mention up anddown a ladder.

The new Burns battery powered indicator and a Series 300sensor sporting the heavyduty sheath option was the perfectsolution. The 10 foot long heavy duty sheath providesthestrength to survive handling and the sensing element nestledin a proprietary packaging technique insures an accurate andrepeatable measurement.Connected to the sensor is our newbattery powered indicator. The LCD display is easily

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readablein any light condition and it provides accuracy to onedecimal point. Battery lifeis three years so theres not a lotof maintenance. Replacement is with a standard 3.7volt AAsize lithium ion available through Burns or at a variety of battery suppliers.

Surface Mount Sensor for Outdoor A gas manufacturer wants to measure the temperatureof a liquid nitrogen pipeline. Theywant to make the measurement without tapping into the line. They would like a sensor tomount tothe exterior of the pipe which is located outdoors.

Since the measurement is outdoors, this eliminates the use of traditional surface mountsensors. The packaging of many surface sensors is not waterproof and the cabling isnot protected from the elements.

In addition, measuring the temperature of liquid nitrogen requires the sensor to be ableto operate reliably from 196C to 50C and not be influenced by the ambient air temperature outside

In order to meet the weatherproof requirement and temperature range requirement, aSeries 200 probe was selected. The operational range of the Series 200 is 196C to500C and the construction is suitable for outdoor use. The probe, however, must beable to mount to the surface and make accurate pipeline temperature measurements.

A Series 200 B style probe was modified for the application.

A 90 degree bend was put in the probe to offset the connection head from the pipeline.

A stainless steel block was attached to the tip and radiused to match the outsidediameter of the pipe allowing the probe to simply be hose clamped into place.

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The connection head allowed the use of weatherproof PVC-coated armored cable for

the signal back to the control panel.

B URNS ENGINEERING COMPANY:

FOOD & BEVERAGE:

From the plant floor to the lab, Burns temperature measurement experts identify thebest approach to our most important and most challenging temperature measurementneeds.

Regulatory compliance, product quality and product safety are our top priorities.Experience with distillation, pasteurization, SIP, CIP, retort, cold storage, and dryingcombined with 3-A, ASTM, and MWFPA participation gives Burns the industryknowledge and awareness to ensure our process success.

It offers an extensive offering of standard sanitary sensors for both direct and indirectimmersion. Designed to optimize our process by providing accurate and reliableperformance over our entire temperature range.

It has also designed a high-accuracy probe for process validation in response to anFDA regulation that is now widely used in the dairy industry.Series S sanitary sensors are highly accurate and reliable temperature sensors. Perfectfor applications in pharmaceutical, biotech, chemical and food and beverage markets.

The SNI Series is ideally suited for use in small diameter piping where temperaturemeasurement is critical, but direct immersion temperature probes cannot be used.

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Non-Intrusive RTDs

Although designed primarily for use in autoclaves, the Burns Autoclave RTD can beused for any application in which moisture is a concern. An example? Measuringunderground pipeline temperatures and more.

Autoclave RTDs

Advantages of Resistance Temperature Detectors

The advantages of using RTD's include:

y Linear over wide operating rangey Wide temperature operating rangey High temperature operating rangey Interchangeability over wide rangey Good stability at high temperaturey High accuracyy Low drifty Suitable for precision applications

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Disadvantages of Resistance Temperature Detectors

The disadvantages of using RTD's include :

y Low sensitivityy Higher cost than thermocouplesy No point sensingy Affected by shock and vibrationy Requires three or four-wire operationy RTDs in industrial applications are rarely used above 660 C. At temperatures

above 660 C it becomes increasingly difficult to prevent the platinum frombecoming contaminated by impurities from the metal sheath of the thermometer.This is why laboratory standard thermometers replace the metal sheath with aglass construction.

y At very low temperatures, say below -270 C (or 3 K), due to the fact that thereare very few phonons, the resistance of an RTD is mainly determined byimpurities and boundary scattering and thus basically independent of temperature. As a result, the sensitivity of the RTD is essentially zero andtherefore not useful.

y Compared to thermistors, platinum RTDs are less sensitive to small temperaturechanges.

y RTDs are characterized by a slow response timey Because they require current excitation, they can be prone to self-heating

CONCLUSION:

Thus,resistance thermometers should be used when:

y When accuracy and stability are a requirement of the customers specificationy When accuracy must extend over a wide temperature rangey When area, rather than point sensing improves controly When a high degree of standardisation is desirable

Hence, we can conclude that resistance thermometers have become very popular because of their excellent stability, and exhibit the most linear signal with respect totemperature of any electronic temperature sensor.

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