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HEAT & TEMPERATUREBy

Dr. Karamjit SinghSenior Lecturer

Govt. Polytechnic College For Girls Patiala

Email: karamjit_gpw@yahoo.com

Mobile:- 9914029020; 9501029020

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What is Heat

The Universe is made up of matter and energy. Matter is made up of atoms and molecules and energy causes the atoms and molecules to always be in motion - either bumping into each other or vibrating back and forth.

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The motion of atoms and molecules creates a form of energy called heat or thermal energy which is present in all matter. Even in the coldest voids of space, matter still has a very small but still measurable amount of heat energy.

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Energy can take on many forms and can change from one form to another. Many different types of energy can be converted into heat energy. Light, electrical, mechanical, chemical, nuclear, sound and thermal energy itself can each cause a substance to heat up by increasing the speed of its molecules. So, put energy into a system and it heats up, take energy away and it cools. For example, when we are cold, we can jump up and down to get warmer.

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Examples of various types of energy being converted into thermal energy

(1) Mechanical energy is converted into thermal energy whenever you bounce a ball. Each time the ball hits the ground, some of the energy of the ball's motion is converted into heating up the ball, causing it to slow down at each bounce.

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(2) Thermal energy can be transfered to other objects causing them to heat up. When you heat up a pan of water, the heat from the stove causes the molecules in the pan to vibrate faster causing the pan to heat up. The heat from the pan causes water molecules to move faster and heat up. So, when you heat something up, you are just making its molecules move faster.

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• 3) Electrical energy is converted into thermal energy when you use objects such as heating pads, electrical stove elements, toasters, hair dryers, or light bulbs.

A thermal infrared image of a hair dryer and a flourescent light bulb

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(4) Chemical energy from the foods we eat is converted into heating our bodies. (5) Light from the sun is converted to heat as the sun's rays warm the earth's surface. (6) Energy from friction creates heat. For example when you rub your hands, sharpen a pencil, make a skid mark with your bike, or use the brakes on your car, friction generates heat.

A thermal infrared image of a pencil after being sharpened

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A thermal infrared image of hot brakes in a car

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The more energy that goes into a system, the more active its molecules are. The faster molecules move, the more heat or thermal energy they create. So, the amount of heat a substance has is determined by how fast its molecules are moving, which in turn depends on how much energy is put into it.

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What is Temperature

The atoms and molecules in a substance do not always travel at the same speed. This means that there is a range of energy (the energy of motion) among the molecules. In a gas, for example, the molecules are traveling in random directions at a variety of speeds - some are fast and some are slow.

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Temperature is a measure of the average heat or thermal energy of the particles in a substance. Since it is an average measurement, it does not depend on the number of particles in an object. In that sense it does not depend on the size of it. For example, the temperature of a small cup of boiling water is the same as the temperature of a large pot of boiling water. Even if the large pot is much bigger than the cup and has millions and millions more water molecules.

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Heat and Temperature

We have all noticed that when you heat something up, its temperature rises. Often we think that heat and temperature are the same thing. However, this is not the case. Heat and temperature are related to each other, but are different concepts.

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Perhaps the reason the two are usually and incorrectly thought to be the same is because as human beings on Earth our everyday experience leads us to notice that when you add heat to something, say like putting a pot of water on the stove, then the temperature of that something goes up. More heat, more temperature - they must be the same, right? Turns out, though, this is not true.

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• Temperature is a number. That number is related to energy, but it is not energy itself.

• Temperature is a number that is related to the average kinetic energy of the molecules of a substance.

• Read that last sentence carefully. It does not say that temperature is kinetic energy, nor does it state exactly what is the relation between temperature and kinetic energy.

• Here is the relation: If temperature is measured in Kelvin degrees, then its value is directly proportional to the average kinetic energy of the molecules of a substance. Note that temperature is not energy, it is a number proportional to a type of energy.

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Heat, on the other hand, is actual energy measured in Joules or other energy units. Heat is a measurement of some of the energy in a substance. When you add heat to a substance, you are adding energy to the substance. This added heat (energy) is usually expressed as an increase in the kinetic energies of the molecules of the substance. If the heat (energy) is used to change the state of the substance, say by melting it, then the added energy is used to break the bonds between the molecules rather than changing their kinetic energy.

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• When heat (energy) goes into a substance one of two things can happen:1. The substance can experience a rise in temperature. The heat (the added energy) can be realized as an increase in the average kinetic energy of the molecules. The molecules now, on average, have more kinetic energy. This increase in average kinetic energy is registered as a number called temperature that changes proportionally with it. Note that this increase in the average kinetic energy of the molecules means that they will now, on average, be traveling faster than before the heat arrived.

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2. The substance can change state. For example, if the substance is ice, it can melt into water. Perhaps surprisingly, this change does not cause a rise in temperature. At the exact moment before melting, the average kinetic energy of the ice molecules is the same as the average kinetic energy of the water molecules at the exact moment after melting. That is, the melting ice and the just melted water are at the same temperature. Although heat (energy) is absorbed by this change of state, the absorbed energy is not used to change the average kinetic energy of the molecules, and thus proportionally change the temperature. The energy is used to change the bonding between the molecules.

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• So, when heat comes into a substance, energy comes into a substance. That energy can be used to increase the kinetic energy of the molecules, which means an increase in their temperature which means an increase in their speed. Or at certain temperatures the added heat could be used to break the bonds between the molecules causing a change in state that is not accompanied by a change in temperature.

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Heat is the total energy of molecular motion in a substance while temperature is a measure of the average energy of molecular motion in a substance.

Heat energy depends on

1. the speed of the particles,

2. the number of particles i.e. size or mass, and

3. the type of particles in an object.

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Temperature does not depend on the size or type of object. For example, the temperature of a small cup of water might be the same as the temperature of a large tub of water, but the tub of water has more heat because it has more water and thus more total thermal energy.

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• It is heat that will increase or decrease the temperature. If we add heat, the temperature will become higher. If we remove heat the temperature will become lower.

• Higher temperatures mean that the molecules are moving, vibrating and rotating with more energy. If we take two objects which have the same temperature and bring them into contact, there will be no overall transfer of energy between them because the average energies of the particles in each object are the same. But if the temperature of one object is higher than that of the other object, there will be a transfer of energy from the hotter to the colder object until both objects reach the same temperature.

• Temperature is not energy, but a measure of it. Heat is energy

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Molecular Model of an Ideal Gas

• Macroscopic properties of a gas were pressure, volume and temperature

• Can be related to microscopic description– Matter is treated as a collection of molecules– Newton’s Laws of Motion can be applied

statistically• The model shows that the pressure that a gas

exerts on the walls of its container is a consequence of the collisions of the gas with the walls

• It is consistent with the macroscopic description developed earlier

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Ideal Gas Assumptions

The number of molecules in the gas is large, and the average separation between the molecules is large compared with their dimensions– The molecules occupy a negligible volume

within the container. – This is consistent with the macroscopic model

where we assumed the molecules were point-like

• The molecules obey Newton’s laws of motion, but as a whole they move randomly– Any molecule can move in any direction with

any speed.

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• The molecules interact only by short-range forces during elastic collisions– This is consistent with the macroscopic

model, in which the molecules exert no long-range forces on each other

• The molecules make elastic collisions with the walls– These collisions lead to the macroscopic

pressure on the walls of the container– The gas under consideration is a pure

substance– All molecules are identical

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Ideal Gas

• An ideal gas is often pictured as consisting of single atoms

• However, the behavior of molecular gases approximate that of ideal gases quite well

• Molecular rotations and vibrations have no effect, on average, on the motions considered

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Pressure and Kinetic Energy

• Assume a container is a cube– Edges are length d

• Look at the motion of the molecule in terms of its velocity components

• Look at its Impulse -momentum theorem and the average force

• Assume perfectly elastic collisions with the walls of the container

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Pressure and Kinetic Energy

• The relationship between the pressure and the molecular kinetic energy comes from momentum and Newton’s Laws

• Total force on the wall can be written as

• The relationship will be:

22

3x

m N mvF Nv

d d

___22 1

3 2

NP mv

V

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Pressure and Kinetic Energy, • So, pressure is proportional to the number of

molecules per unit volume (N/V) and to the average translational kinetic energy of the molecules

• This equation also relates the macroscopic quantity of pressure with a microscopic quantity of the average value of the square of the molecular speed

• One way to increase the pressure is to increase the number of molecules per unit volume

• The pressure can also be increased by increasing the speed (kinetic energy) of the molecules

21

2mv

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Molecular Interpretation of Temperature

– Now PV = NkBT

Also

• Therefore, the temperature is a direct measure of the average molecular kinetic energy

___ ___2 2

B

2 1 2 1

3 2 3 2B

PV N mv Nk T T mvk

___22 1

3 2

NP mv

V

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Molecular Interpretation of

Temperature, contd.

• Simplifying the equation relating temperature and kinetic energy gives

• This can be applied to each direction,

• with similar expressions for vy and vz

___2

B

1 3

2 2mv k T

___2

B

1 1

2 2xmv k T

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Thermometers

• A thermometer is a device that is used to measure the temperature of a system

• Thermometers are based on the principle that some physical property of a system changes as the system’s temperature changes.

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Thermometers contd.

• These properties include:– The volume of a liquid– The dimensions of a solid– The pressure of a gas at a constant volume– The volume of a gas at a constant pressure– The electric resistance of a conductor– The color of an object

• A temperature scale can be established on the basis of any of these physical properties

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Calibrating a Thermometer-Celsius Scale

• A thermometer can be calibrated by placing it in contact with some natural systems that remain at constant temperature

• Common systems involve water– A mixture of ice and water at atmospheric

pressure called the ice point of water 0oC– A mixture of water and steam in equilibrium

called the steam point of water 100o C• The length of the column between these two

points is divided into 100 increments, called Celsius degrees

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• Put thermometer into pure melting ice. After few minutes, marks the position of mercury level; thus 00 C is obtained.• Put thermometer into steam, hence 1000C is obtained.• Divides the length obtained into 100 divisions.

Calibrating an unmarked thermometer

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The Constant-Volume Gas Thermometer & The Absolute Temperature Scale

• The physical change exploited is the variation of pressure of a fixed volume gas as its temperature changes

• The volume of the gas is kept constant by raising or lowering the reservoir B to keep the mercury level at A constant

• The thermometer is calibrated by using an ice water bath and a steam water bath

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Constant Volume Gas Thermometer contd.

• The pressures of the mercury under each situation are recorded– The volume is kept constant by

adjusting A.– The information is plotted

• To find the temperature of a substance, the gas flask is placed in thermal contact with the substance

• The pressure is found on the graph• The temperature is read from the graph

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Absolute Zero• The thermometer readings are virtually independent

of the gas used• If the lines for various gases are extended, the

pressure is always zero when the temperature is –273.15o C

• This temperature is called Absolute Zeroro

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Absolute Temperature Scale

• Absolute zero is used as the basis of the absolute temperature scale

• The size of the degree on the absolute scale is the same as the size of the degree on the Celsius scale

• To convert: TC = TA – 273.15 (19.1)

• Because ice and steam points are experimentally difficult to duplicate (depend on atmospheric pressure):

– An absolute temperature scale is now based on two new fixed points

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• One point is absolute zero• The other point is the triple point of water

– Combination of temperature and pressure where ice, water, and steam can all coexist

• The triple point of water occurs at 0.01o C and 4.58 mm of mercury

This temperature was set to be 273.16 on the absolute temperature scale– This made the old absolute scale agree closely

with the new one– The units of the absolute scale are KELVINS

Absolute Temperature Scale contd.

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Absolute Temperature Scale contd.

• The absolute scale is also called the Kelvin scale– Named after William Thomson, Lord

Kelvin

• The triple point temperature is 273.16 K– No degree symbol is used with Kelvin

• The Kelvin is defined as 1/273.16th of the difference between absolute zero and the temperature of the triple point of water

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Some Examples of Absolute Temperatures

• The figures to the right give some absolute temperatures at which various physical processes occur

• The scale is logarithmic• The temperature of

absolute zero cannot be achieved

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Fahrenheit Scale

• Fahrenheit Scale a common scale in everyday use.– Named for Daniel Fahrenheit– Temperature of the ice point is 32oF– Temperature of the steam point is 212oF– There are 180 divisions (degrees) between

the two reference points• Celsius and Kelvin have the same size

degrees, but different starting points

TC = TK – 273.15

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Temperature ScalesTemperature Scales

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Comparison of Scales

• Celsius and Fahrenheit have different sized degrees and different starting points

• To compare changes in temperature

• Ice point temperatures0o C = 273.15 K = 32o F

• Steam point temperatures100oC = 373.15 K = 212o F

F C

9 532 32

5 9C FT T F or T T

C F

5

9KT T T

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Thermometer, Liquid in Glass

• A common type of thermometer is a liquid-in-glass• The material in the capillary tube expands as it is

heated• The liquid is usually mercury or alcohol

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Mercury Thermometers• The mercury thermometer is a common type of thermometer in everyday use.• Narrow bore of capillary tube makes the thermometer more sensitive.• Range : -100C to 1100C (or 00C to 1000C).• Round or oval glass stem serve as magnifying lens. • the bulb is made by thin glass.

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• Typical clinical thermometer is liquid-in-glass thermometer.

• Range : 350C to 420C.

• It has a constriction for preventing liquid fall back to the bulb immediately after taking the reading. • When taking reading, the bulb is gently held under the patient’s tongue.

Clinical ThermometerClinical Thermometer

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Clinical Thermometers

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Most of the liquid-in-glass widely use mercury, because

1. more uniform expansion,2. does not stick to glass.3. visible meniscus,4. react quickly to temperature changes,5. boiling point: 3570C, freezing point = -390C.

But its weak points are:

1. expensive,2. poisonous liquid3. High freezing point

Mercury Thermometer

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Alcohol Thermometers

This is a very cheap liquid-in-glass thermometer.It can measure very low temperature.

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Alcohol is used for making thermometers because:1. Low freezing point ---- -1150C,2. Safe liquid,3. Cheap.

Its disadvantages:Its disadvantages:

1. Non-uniform expansions,2. Sticks to glass,3. Slow reaction to temperature changes,4. Low boiling point 780C.

Alcohol Thermometer

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PLATINUM RESISTANCE THERMOMETER

The platinum resistance thermometer-in which the principle of measurement is the variation in the resistance of a platinum wire as a function of temperature-is generally accepted as the most accurate temperature measuring instrument available. Its sensitivity and reliability are evident from the fact that it was first used in 1928 to define the International Temperature Scale from -190°C to 660°C.

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But it has other advantages that find many and increasing applications in industry. It is particularly suitable where measurements are to be made over a relatively narrow range of temperature, where the point of measurement is some distance from the recording instrument, and where there are several measuring

points and readings are required at one central instrument panel. In addition to the measurement of elevated temperatures, the platinum resistance thermometer is also finding a number of applications where the accuratedetermination or control of sub-zero temperatures is needed.

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The operation of the resistance thermometer depends upon two characteristics of platinum-first the simple relationship between its resistance and its temperature, andsecondly the high purity, stability andreproducibility of the specially preparedplatinum employed for this purpose.R = R0 [1 + α (t - t0 )]

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The platinum resistance thermometer, the most accurate and sensitive means of temperature measurement, was introduced and developed by the late Professor H. L. Callendar, C.B.E., F.R.S.

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Advantages

The advantages of platinum resistance thermometers include:

• High accuracy

• Low drift

• Wide operating range

• Suitability for precision applications.

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Limitations

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. This is why laboratory standard thermometers replace the metal sheath with a glass construction. At very low temperatures, say below -270 °C (or 3 K). Compared to thermistors, platinum RTDs are less sensitive to small temperature changes and have a slower response time.

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Thermocouple ThermometersThermocouple ThermometersA thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control. They are inexpensive and interchangeable, are supplied fitted with standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one degree Celsius (C) can be difficult to achieve.

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Thermocouple thermometers with two different metals are placed in contact, a voltage develops between them.

The voltage varies with temperature.

One of the junctions is kept in melting ice at 00C while the other one is used as temperature probe.

Thermocouple is used for measuring very high

temperature and temperatures which varies

greatly. The voltmeter can be calibrated directly in 0C.

Thermocouple Thermometers

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Maximum Thermometer

MinimumThermometer

The maximum thermometer records the highesttemperature and minimum thermometer the lowestfor the day (or a period).

Maximum and Minimum ThermometerMaximum and Minimum Thermometer

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This thermometerrecords the highestand the lowesttemperature forthe day. The metal index can be moved up or down by a bar magnet.

Maximum and Minimum ThermometerMaximum and Minimum Thermometer

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Pyrometer

• A pyrometer is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry. This device can be used to determine the temperature of an object's surface.

• The word pyrometer comes from the Greek word for fire, "πυρ" (pyro), and meter, meaning to measure. Pyrometer was originally coined to denote a device capable of measuring temperatures of objects above incandescence (i.e. objects bright to the human eye).

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Principles of thermal radiation

The relationship between the energy's wavelength and the intensity of the radiation (amount of radiated energy) atvarious temperatures is shown in figure

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Principles of thermal radiation contd.

The intensity is greater at higher temperatures, and for any one temperature, it increases to a maximum value and then decreases• The graph illustrates the shift of maximum wavelength intensityas temperature increases

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• The total radiation emitted per unit surface area of the black body is according Stefan-Boltzmann law:

H0 = σ T4 ; σ = 5,67X10-3 [W m-2 K-4]

• The thermal energy radiated by a perfect radiating material is known as black-body radiation

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Black Body

• A black body is a surface that absorbs all the radiant energy that falls on it and radiates the maximum thermal energy possible for a particular temperature

• The concept of a black body is an idealization, since real surface do not absorb all the radiant energy that falls on them; they reflect radiation to some extent, and do not radiate all thermal energy theoretically possible.

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IR-thermometers

• There are several types of IR-thermometers. The simplest device is wideband or broadband pyrometer.

• It can measure radiation of all wavelengths in both the visible and infrared regions.

• The wideband pyrometer measures temperature according to Stefan-Boltzmann law:

H = εσT4

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Components of IR-pyrometer

IR-pyrometer has four basic parts:

• optical system

• radiation detector

• electrical system

• readout display

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• IR-detectors are classified as:

• thermal detectors, which produce an output because their temperature changes with the absorption of thermal energy

• photon detectors, which produce an output because the radiation releases electric charge in the detector

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Criterion for the selection of a thermometer

• Range:- The thermometer selected should be such so as to suit the range of temperature to be measured say to measure human body temperature clinical thermometer is used. For measuring temperature at a point, thermocouple is used; similarly for measuring high temperature, pyrometer is used.

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• Sensitiveness: A sensitive thermometer is one which responds to a small change in temperature. The gas thermometers are most sensitive. Other thermometers are calibrated with gas thermometers.

• Accuracy: accuracy desired should be kept in mind. Different thermometers have different accuracy.

• Time of Response: The time taken by a thermometer to indicate the temperature. The time of response should be minimum i.e. indicate temperature quickly.

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• Minimum Absorption of Heat: For measurement, thermometer is put in hot substance and reading is taken when thermal equilibrium between the thermometer and the hot body is established. As the thermometer takes up some heat, the temperature, measured is slightly than true temperature. So, for accurate measurements, heat absorbed by thermometer should be minimum.

• Accessibility: Sometimes the body is not accessible. So the thermometer should be such so that it can easily be put into place whose temperature is to be measured.

• Cost: The cost is also taken into consideration. Very accurate thermometers are costly. A cheaper thermometer may serve the purpose well.