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SUBJECT : To Study And VerifyCharacteristic OF ResistorTemp. Detector

Presented By :Satvik ThakerVishal SanziraMeet Dave

Guided By :

Pratik Patel

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History

Fifty years later, Sir William Siemens proffered theuse of platinum as the element in a resistancethermometer. His choice proved most propitious,as platinum is used to this day as the primaryelement in all high-accuracy resistancethermometers. In fact, the Platinum ResistanceTemperature Detector, or PRTD, is used today as

an interpolation standard from the oxygen point(-182.96C) to the antimony point (630.74C).

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Methods of Temperature

Measurement

1. Thermocouples2. Thermistors

3. Electrical resistance change (RTD)4. Pyrometers5. Expansion of materials

PEC UET Lahore Dr. Shahid Naveed 13. August 2013 / Dr. Ing Naveed Ramzan 3

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What Is an RTD?resistance thermometers , also called resistancetemperature detectors or resistive thermaldevices (RTDs), are temperature sensors that exploit thepredictable change in electrical resistance of somematerials with changing temperature. As they are almostinvariably made of platinum, they are often called platinum

resistance thermometers (PRTs). They are slowlyreplacing the use of thermocouples in many industrialapplications below 600 C, due to higher accuracy andrepeatability.

It is temperature coefficient device, which means that theresistance increases with temperature. The resistance of the metal increases with temperature. The resistiveproperty of the metal is called its resistivity.

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The resistive propertydefines length and cross sectional arearequired to fabricate an RTD of a givenvalue.The resistance is proportional tolength and inversely proportional to thecross sectional area :

r X LR= ------------

AWhere

R = Resistance (ohms)r = Resistivity (ohms)L = LengthA = Cross sectional area

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Why use an RTD instead of a

thermocoupleor thermistor sensor?Each type of temperature sensor has a particularset of conditions for which it is best suited. RTDsoffer several advantages: A wide temperature range ( -50 to 500C forthin-film and -200 to 850C for wire-wound) Good accuracy (better than thermocouples)

Good interchangeability Long -term stabilityWith a temperature range up to 850C, RTDs canbe used in all but the highest-temperature industrialprocesses. When made using metals such asplatinum, they are very stable and are not affected

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the material must be malleable sothat it can be formed into smallwires. it must have a repeatable andstable slope or curve. the material should also beresistant to corrosion. the material should be low cost

it is preferred that the materialhave a linear resistance versestemperature slope

RTD Materials

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Other materials such asnickel, copper, and nickel-ironalloy have also been used forRTDs. However, thesematerials are not commonly

used since they havelower temperaturecapabilities and are not asstable or

repeatable as platinum .

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Platinum is especially suited to this purpose, as it canwithstand high temperatures while maintainingexcellentstability. As a noble metal, it shows limitedsusceptibilityto contamination.

The classical resistance temperature detector (RTD)construction using platinum was proposed by C.H.Meyers in 1932.7 He wound a helical coil of platinumon

a crossed mica web and mounted the assembly insidea glass tube.

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When discussing RTDs, severalspecifications must be considered :

Wiring configuration (2, 3 or 4-wire)

Self-heating AccuracyStability

RepeatabilityResponse time

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Resistance temperaturedetector type

Two wire RTD

Three Wire RTDFour Wire RTD

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Figure 3.Lead wires have resistance

that is a function of thematerial used, wire size, andlead length. This resistancecan add to the measured RTDresistance, and improper wirecompensation can result insignificant errors. The

common configurations of RTDs are two (A), three (B) orfour wires (C).

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1. Wiring Configuration : 2-wire RTD :

Serious lead-wire resistance errors can occur when using a two-

wire RTD (see Fig. 3A ), especially in a 100 sensor. In a two -wirecircuit, a current is passed through the sensor. As the temperatureof the sensor increases, the resistance increases. This increase inresistance will be detected by an increase in the voltage (V = IR).The actual resistance causing the voltage increase is the totalresistance of the sensor and the resistance introduced by the leadwires. As long as the lead wire resistance remains constant, it canbe offset and not affect the temperature measurement. The wireresistance will change with temperature, however, so as the

ambient conditions change, the wire resistance will also change,introducing errors. If the wire is very long, this source of errorcould be significant. Two-wire RTDs are typically used only withvery short lead wires, or with a 1000 element.

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3-wire RTD :In a 3-wire RTD (see Fig. 3B ), there are three leads coming

from the RTD instead of two. L1 and L3 carry the measuringcurrent, while L2 acts only as a potential lead. Ideally, theresistances of L1 and L3 are perfectly matched and thereforecanceled. The resistance in R3 is equal to the resistance of the

sensor Rt at a given temperature usually the begining of thetemperature range. At this point, V out = zero. As the temperatureof the sensor increases, the resistance of the sensor increases,causing the resistance to be out of balance and indicated at V out.

Resistances L1 and L3 in leads up to tens of feet long usually match well enough for 100 ohm three-wire RTDs. The worst case isresistance offset equal to 10% of single-lead resistance.

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4- wire RTD :The optimum form of connection for RTDs is a

four-wire circuit (see Fig. 3C). It removes the errorcaused by mismatched resistance of the lead wires.A constant current is passed through L1 and L4; L2and L3 measure the voltage drop across the RTD.With a constant current, the voltage is strictly afunction of the resistance and a true measurementis achieved. This design is slightly more expensive

than two or three-wire configurations, but is thebest choice when a high degree of accuracy isrequired.

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READILY AVAILABLE RTDINSTRUMENTS:

Digital Temperature Indicators12-Inch Round Chart RecordersBarnum Steam Control Systems and Multipoint Rtd

IndicatorsCompton and Jewell Rtd Analog Meters and SetpointControllersRed Lion digital Indicators and Controllers

R.I.S. Transmitters and Trips and 36 Point Alarm MonitorsRustrak Miniature RecordersWest Rtd Controllers: On-Off, Hi-Lo Limit, or PID

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TYPICAL APPLICATIONS

PRECISION PROCESSTEMPERATURE CONTROL

TextileChemical

Food

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Resistance Temperature Detectors (RTDs)are increasingly becoming the sensor of choice for temperature measurementapplications due to the accuracy and longterm stability that platinum elements offer.

These devices are very linear over a widetemperature range. Due to advances intechnology, resistance temperature detector

elements are very robust and cost effective.They can be designed into a wide variety of packages that suit applications up to1200 F.

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AUTOMATIC TEMPERATURECONTROL

Test ChambersOven TemperaturePlastic ExtrudersInjection Molders

Solder PotsBearing Temperature

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ADVANTAGES

Absolute Measurement - Resistancethermometers, unlike thermocouples,do not require a reference point. No ice

baths or compensation circuits.High Output - With an output of 50 to 200times that of a thermocouple, resistancethermometers permit the use of simplerindication and control instruments. Noamplifiers are needed and the resultingsystem is less expensive and morereliable.

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Greatest Accuracy - The pre-eminent position of the resistance

thermometer as a precision temperaturemeasuring instrument is demonstrated by itsselection to define the InternationalTemperature scale from -260 deg C to +660deg C.

The main reasons for its selection are:

1) the exceptional stability

2) the repeatability of the resistancethermometer.

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Disadvantages of Resistance

Temperature Detectors

Low sensitivityHigher cost than thermocouplesNo point sensingAffected by shock and vibrationRequires three or four-wire operation

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Discussion &Question s?

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THANK YOU..!!