temp sensor pdf

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TemperatureSensors

Faisal Ahmed

CIIT, Laohore

Instrumentation&

Process Control



Expansion of Materials

Electrical Resistance Change

(Thermistors)

Thermocouples

Pyrometers

Semiconductors

Temperature Measurement



• Most Commonly Mercury is used

• Mercury has high linear coefficient of expansion

• Doesn’t wet the glass b/c of high intermolecular

forces

•

The operating range: -35o

C to 450o

C• Inexpensive, Good accuracy (<0.1oC)

• High Linearity

(i) Liquid in Glass Thermometers

1. Expansion of Materials



(ii) Bimetallic Strip Thermometers

• Two strips of dissimilar metals(Brass & Invar)

• Different thermal coefficient ofthermal expansion

• Form an arc as Temp. changes

• Inexpensive, Compact,Rugged

• Usually configured as Spiralor Helix for compactness

•

Range: -180 to 430o

C



(iii) Pressure Spring Thermometer

• Metal bulb: low coefficient of thermal expansion

• Capillary tube: high coefficient of expansion

• Material in bulb may be liquid or gas

• As Temperature rises Pressure increases

• Calibration: Stem & Bourdon tube are Temp.sensitive

•

Range:up to 550

oC



• RTD’s are wire wound resistors

• Sealed in stainless steel shell

• Remaining space is filled with ceramic for goodheat conduction and poor electrical conduction

• Resistance of pure metals is positive andincreases linearly.

•

Range:-170 to 780oC

2. Resistance Temp. Devices



Thermistors

•

Belong to the resistance Temp. changedevices

• These are the class of metal oxides(Semiconductor material)

• High sensitivity:

up to 10% change per oC

• Range: -50 to 300oC



•

Two dissimilar metals are joined to make twojunctions

• Because of Temp. difference, EMF is producedat junctions

• The Potential difference is proportional to Temp.

3. Thermocouples



• Radiation is used to sense Temp.

(i) Radiation Pyrometer

4. Pyrometers

• Senses heat radiated from hot body through afixed lens

• Lens focuses heat energy on to a thermopile

• Non-contact device (Used in furnace)



• Incident radiationis matched with radiation

from internal filament

• Current through filament is adjusted until colorof radiation matches

• Current is then related to Temp. of source

• Range: 1100 to 2800oC

(ii) Optical Pyrometer



•There are number of parameters that varylinearly with temp.

•

Normally the reference voltage of a zener diodeor the junction voltage variations are used fortemp. sensing

• Range:-50 to 150oC

5. Semiconductor Devices

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