Temp Sensor PDF

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 1/11

TemperatureSensors

Faisal Ahmed

CIIT, Laohore

Instrumentation&

Process Control

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 2/11

Expansion of Materials

Electrical Resistance Change

(Thermistors)

Thermocouples

Pyrometers

Semiconductors

Temperature Measurement

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 3/11

• 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

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 4/11

(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

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 5/11

(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

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 6/11

• 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

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 7/11

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

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 8/11

•

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

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 9/11

• 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)

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 10/11

• 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

8/6/2019 Temp Sensor PDF

http://slidepdf.com/reader/full/temp-sensor-pdf 11/11

•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