Temperature sensor

TEMPERATURE SENSOR
PRESENTED
BY
RAJ KUMAR S
DEPARTMENT OF MECHANICAL
ENGINEERING
PSG COLLEGE OF TECHNOLOGY

Temperature - A measure of hotness and coldness in degrees.
Sensor - is a device that detects and responds to some
type of input (light,temperature, motion, moisture and pressure)
from the physical parameter
Mechanical temperature sensors
1. Thermometer
Electrical temperature sensors
INTRODUCTION
1. Voltage - Thermocouple
2. Resistance - Resistance thermometer
Thermistor
3. Radiation - pyrometer

THERMOCOUPLE
Principle:
It is based on the Seeback effect.
This Peltier effect is opposite to the Seebeck effect.
This effect states that the difference of the temperature can be formed
among any two dissimilar conductors by applying the potential variation
among them.
Objective:
1. Unknown temperature --Hot or measuring junction.
2. Known temperature --Cold or reference junction.
Thermocouple produces a temperature-dependent voltage.
Thermocouples are suitable for measuring over a large temperature range,
from −270 °C up to 1800 °C.

WORKING
1. If the temperatures difference between the cold
and hot junction Temperature will create an
emf.
2. This emf production is measured in the
millivolt meter.
3. Thermocouple is never measure the absolute
temperature.
4. Always measure the differential temperature of
those junction.
.

STANDARD CONFIGURATION
OF THERMOCOUPLE
IEC is International Electrotechnical
Commission
• Alumel -- 94% nickel , 2%aluminum,
3%manganese, 1%silicon.
• Constantan -- 45% nickel, 55%copper.
• Chromel -- 90% nickel, 10%chromium.

THERMOCOUPLE LAWS
FORMULA
The e.m.f. generated in a
thermocouple :
E = a T +1/2 bT2
T = difference in temperature
between two junctions (oC)
a, b = constants depending upon
the thermocouple material
1.Law of intermediate metal
2.Law of intermediate temperature

APPLICATION
1. Plastic injection molding machinery .
2. Food process equipment.
3. Steel industry-(carbon content of molten
steel)
4. Gas appliance safety(water heaters )
5. Thermopile radiation sensors
6. Power production
7. Thermocouple as vacuum gauge
8. gas turbine exhaust, diesel engines, other
industrial processes and fog machines.

RTD
PRINCIPLE:
Resistance thermometers, also called resistance temperature detectors (RTD).
RTD are increases Resistance with Temperature increases.
OPERATIVE TEMPERATURE:
1. RTDs typically operate temperature range of (– 200 °C to + 850 °C)
2. More accurate then thermocouple.
There are major types of RTD:
1. Thin-Film RTD
2. Wire Wound Element RTD

Resistance Temperature Formula for an RTD
Rt = RO(1 + ɑΔT)
1. Rt is the resistance temperature.
2. RO is the resistance at 0 °C.
3. ɑ is the Temperature Coefficient of Resistance (TCR) .
4. ΔT is the Temperature Change.
RTD sensors are usually manufactured with a resistance RO of 100 Ω at
0 °C and an alpha value ɑ (TCR) at 0.00385ohms/ohm.°C

RTD made OF Material
Platinum
1. withstand oxidation and is effective over a range of -200 to
+ 850 degrees C.
2. Platinum has a linear increase of 0.385 ohms per degree
over a wide temperature range .
3. Platinum has the best accuracy and stability .
Nickel
1. good compromise between copper and platinum.
2. has a higher output and is slightly less expensive than
platinum. non-linear at temperatures above 300 °C
Copper
1. Copper has a very linear resistance.
2. Performs poorly in oxidizing atmospheres
3. Copper oxidizes and it cannot be used over 150°C

Thin-Film RTD
1. Consists a very thin layer of resistive material
platinum on a ceramic substrate.
2. This layer is usually just 1 to 10 nanometers
thick.
3. This film is then coated with an epoxy or glass
that helps protect the deposited film and also acts
as a strain relief for the external lead wires.
4. Strain gauge effect that can be seen in the
resistive temperature coefficient.
5. Temperatures up to 600 °C when suitably
encapsulated in glass or ceramic.
6. Special high-temperature RTD elements can be
used up to 900 °C with the right encapsulation.
7. Disadvantages of this type are that they are not as
stable as their wire-wound .

Wire Wound Element RTD
1. Built by winding a small diameter
platinum sensing wire around a non-
conducting, cylindrical ceramic rod
2. Greater accuracy, especially for wide
temperature ranges.
3. The coefficient of thermal expansion of
the winding core material is the sensing
wire to minimize any mechanical strain.
4. These elements work with temperatures
to 660 °C.

PT100
1. The most common RTD element is
made of Platinum with a resistance of
100 Ω at 0 °C.
2. The resistance increases “linearly” by
0.385 ohms per degree.
3. This is called the Alpha “co-efficient”
and is specified as 0.00385ohms/ohm.°C

THERMISTOR
Principle:
Thermistor is opposite to RDT
Decrease Resistance with Temperature increases.
Negative temperature coefficient.
Made of semiconductor.
Specification:
1. More sensitive then RDT.
2. 1oC rise in temperature, there will be a 5% decrease in their resistance. So their
sensitivity is very high.
3. Resistance thermistor vary 0.5 ohm to 0.75 mega ohm.
4. Temperature range highest accuracy is within the -50 to 300°C .
.

DIFFERENT MODELS:
bead type - diameter from 0.15 to 1.5 millimeters.
rod type - high pressure
disc type - 3 millimeters to 25 millimeters.
WORKING
A Wheatstone Bridge is a network of four resistances an
Electromotive Force (emf) source and millivolt meter
connected such that when the four resistances are matched, the
millivolt meter will show a zero deflection.
Vout =Vc - Vd
RX =R3 (R2/R1 )
APPLICATION
1. Thermostats(Engine , battery)
2. Household appliances (like microwaves, fridges, ovens)
3. Circuit protection
4. Food handling and processing

PYROMETER
Greek word PYRO -fire, meter meaning to measure ???
PRINCIPLE:
A pyrometer is a type of remote-sensing thermometer used
to measure the temperature of a surface from the amount of
the thermal radiation.
Types:
1. Total Radiation pyrometer
2. Infrared pyrometer
3. Optical radiation

TOTAL RADIATION PYROMETER
Principle:
Senses the heat radiation from a
targeted hot body and reads and records its
temperature, depending upon the intensity
of radiation.
• Temperature ranging 1200 to 3500 °C
• wavelength range of 0.1-100 μm.
Formula
Stefan-Boltzmann law
Eb = σT 4
• Eb = emissive power of a blackbody
• σ =5.67 x 10 -8 (W. m. K -4 )
Application:
It is used for measurement of temperature in
exhaust, flues etc. in heating system.

INFRARED PYROMETER
Principle
1. Detect infrared radiation of objects to
determine the temperature.
2. Temperature ranging 400 to 3500 °C.
3. Distance of 120cm the spot size is 5 mm.
Working:
1. Photo-voltaic cell respond to wavelength in
infrared region.
2. Protective window is made of thin glass use to
Product cell from physical damage..
3. The filter is used to reduce on the range of 1000
°C to 1200 °C.
Application:
infrared temp gun,weapon.

OPTICAL PYROMETER
Principle:
1. Matching the brightness of an object to
the brightness of the filament which is
placed inside the pyrometer.
2. Optical pyrometer has a range of
measuring temperature of 700°C to
4,000°C
Advantage:
1. The optical pyrometer has high
accuracy.
2. The temperature is measured without
contacting the heated body
Application:
Measure furnaces temperature.

QUERIES
Why ice is called as black body

Temperature sensor

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