RTD (Resistance temperature detector)

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RTD (Resistance temperature detector) Varying resistance with

temperature Good for measurement of small

temperature differences

General equation for RTD material Rt = Ro(1+ At + Bt2 + Ct3 - - - - - Wt4 )B,C - - -W are negligibly small.Rt = Ro(1+ At)

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RTDs: Characteristics and Applications

Characteristics: Resistive device, linear Large range: -200 to +850oC for Platinum High accuracy: 0.001oC Low sensitivity: 0.39 % per oC Don’t need reference temperature

Applications: Industries and laboratories where high

accuracy of temperature measurements are required.

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Requirements for RTD material

High temperature coefficient High resistivity to ensure small wire

length Linearity of relation between

resistance and temperature Sufficient mechanical strength Stability of electrical characteristics

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Construction

A thin Platinum wire wound in form of a free spiral by an insulated carrier such as mica or ceramic.

Diameter of wire should be 0.02mm to 0.2mm.

Wire should be smooth, free from defects, ensurity of constancy of resistance to avoid resistance changes due to dimension changes.

Wire generally enclosed in a protective tube made of glass, quartz etc.

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Thin-Film RTDs

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Calendar-Van Dusen Equation

For platinum, the resistance temperature relationship is given by the following equation:

R R T T T T TT o { [ ( . )( . ) ( . )( . ) ]}1 0 0 1 1 0 0 1 0 0 1 1 0 0 1 3

w h ere an d a re co n s tan ts , d ep en d en t o n

th e p u rity o f p la tin u m . , an d fo r

an d fo r .

, ,

1 4 9 0 0

0 11 0

. = T>

= . T<

For the U. S. calibration curve, = 0.003851/°C

(U.S. calibration curve

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Platinum RTD: R versus T (U.S. Calibration)

0

50

100

150

200

250

300

-100 0 100 200 300 400

Temperature (C)

Res

ista

nce

(O

hm

)

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Graph for different materials

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RTD’s small resistance change requires

Bridge circuit: Can detect small resistance changes

R1 R2

RRTDR4

VoVs

“Supply”Voltage

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RTD Materials

Gold & Silver: Rarely used because of low resistivity

Tungsten: Very high resistivityGold Silver alloy: Give same characs

As Platinum below 1250 C.Phosphor Bronze alloy: Better for low

temperature measurementNickel: Most used, cheaper than platinum Platinum: More suitable but expensive, high

range, high precision

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Very accurate Very stable Standardized among vendors Large variety of packaging options

RTD AdvantagesRTD Advantages

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RTD Disadvantages

Costly Require current source or voltage

source Low resistance/small change 4-wire measurement Slow

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Latest development :Miniature RTDs