49951852 RTD Resistance Temperature Detector

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MADE BY :- ABHISHEK SINGHEE

ROLL NO. :-26

IV SEM

INDUSTRIAL ENGG.

RESISTANCE

TEMPERATUREDETECTOR (RTDs)


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WHATARE RTDs ???

Resistance thermometers, also calledresistance temperature detectors or resistive

thermal devices (RTDs), are temperature

sensors that exploit the predictable change in

electrical resistance of some materials withchanging temperature. As they are almost

invariably made of platinum, they are often

called platinumresistance 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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CONSTRUCTION

These elements nearly always require insulated leadsattached. 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 beingmonitored. 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.


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Advantages

Advantages of platinum resistance thermometers:

High accuracy

Low drift

Suitable for precision applications

Linear over wide operating range

Wide temperature operating range(-200 to 660OC)

High temperature operating range Interchangeability over wide range

Good stability at high temperature


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Disadvantages & Limitations

The disadvantages of using RTD's include: Higher cost than thermocouples

No point sensing

Affected by shock and vibration

Requires three or four-wire operation 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.

Compared to thermistors , platinum RTDs are less

sensitive to small temperature changes and have a

slower response time .


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Sources Of ErrorThe 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, commonlydefined 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.[4]

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: 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 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.

SelfHeating: 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


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Calculations

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 the B and C coefficients are relatively small, the resistance

changes almost linearly with the temperature.


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ApplicationsApplications of RTD's include Air conditioning and refrigeration servicing

Food Processing

Stoves and grills

Textile production

Plastics processing

Petrochemical processing

Micro electronics

Air, gas and liquid temperature measurement

Exhaust gas temperature measurement


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Film thermometers

Wire-wound thermometers

Coil elements

Types OfRTDs


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Documents

49951852 RTD Resistance Temperature Detector