microsoft word - section 3

7/27/2019 Microsoft Word - Section 3

1/14

Level I Course Manual Section 3Publ No 1 560 009 C

1

3. Heat and TemperatureWhat will you learn

What is heat and how is it measured

Contact temperature measruement v non contact

How radiation is converted to temperature

An introduction to heat transfer

Temperature

We are all familiar with thermometers. In a book about temperaturemeasurement, even if it is devoted to non-contact temperature measurement, itis necessary to start by defining the existing temperature scale. Most of theworld is using the Celsius temperature scale, therefore this literature isexclusively metric.

The Celsius temperature scale goes back to Anders Celsius, a Swedishphysicist, who set the water freezing point to 0 C and the water boiling point to100C. Today it is defined in no less than 17 well reproducible melting orfreezing points for various materials. Some of these points are shown in thefigure below.

The Kelvin scale, which is the international SI scale, is displaced in relation tothe Celsius scale with 273.15 C. 0 K = -273.15 C.


2/14


2

0 K absoluter Nullpunkt13.81 K Wasserstoff Tripelpunkt17.04 K Wasserstoff* Siedepunkt20.28 K Wasserstoff Siedepunkt27.10 K Neon Siedepunkt54.36 K Sauerstoff Tripelpunkt90.19 K Sauerstoff Siedepunkt

273.16 K Wasser Tripelpunkt

373.15 K Wasser Siedepunkt

692.73 K zinc - melting point

1235.08 K silver - melting point

1337.58 K Gold Erstarrungspunkt

961.93 C

1064.43 C

419.58 C

100.00 C

0.01 C

-182.96 C-218.79 C-246.05 C-252.87 C-256.11 C-259.34 C-273.15 C

* bei Normaldruck 1013.25 hPa

gold melting point

water boiling point

water freezing point

oxygen boiling

oxygen - freezing

neon boiling

hydrogen boiling

hydrogen freezing

hydrogen triple point

Absolute zero


3/14


3

Temperature measurement

Temperatures are in everyday life usually measured by means of thermometersof various kinds. The thermometer is one type of tool forcontact temperaturemeasurement. That denomination comes from the fact that the thermometer istouching the object when the measurement takes place. Other types of contactthermometers use a bimetal plate or an electric coil, where the temperaturecauses the electrical resistance to change, to sense the temperature.

The non-contacttemperature measurement is carried out by measuring theheat radiation from the object. This technique is called radiometryorpyrometry .

Contact temperature measurement devicesThere are seven basic types of temperature sensors:

ThermocouplesResistive temperature devices/thermistorsBimetallic devicesLiquid expansion devices/thermometersMolecular change of state devicesSilicon diodes

ThermocouplesA thermocouple indicates the temperature by measuring the electromotive force(EMF). The EMF increases with temperature but it is not linear to it.

A simple termocouple consists of two alloys joined to a hot junction, which hasan open and one closed end. The voltage is measured at the open end. Thevoltage is a function of the temperature at the closed end. It needs calibration tomeasure temperatures.

Many different metal alloys are used in combination for the hot, thermoelectric

junction. They are adapted to many different measurement situations andobjects.

Resistive temperature device (RTD) and thermistors

It is in principle the electrical resistance, which changes with temperature that ismeasured. The RTD is stable and linear. The resistance increases withtemperature. They can measure temperatures up to some 850C.


4/14


4

The thermistor is designed with a semiconductive device in which resistance

decreases as temperature increases. Thermistors are the most accurate of thedevices mentioned so far. These devices cannot measure too hightemperatures, i.e. a couple of hundred C is possible.

The advantage with RTDs and thermistors is their accuracy. However, theysend a small current through the measuring point, which might jeopardise themeasurement result. The thermocouple does not do that.

Bimetallic devicesIf you bond two different metals together they will expand with different speedsif heated up and so the form of such a bimetallic device will change. This

movement can be used for measuring temperature, but it is also used as acircuit breaker etc.

Liquid expansion devices/thermometersWe all know the normal thermometer with its liquid in a thin, even capillary,glass tube. Mercury is used. It freezes at 39C and can thus not be used at thevery low temperatures. Instead some other liquid may be used e.g. colouredethanol.

Molecular change of state devicesAn example of such a device is the paint which changes colour if it surpasses a

certain temperature. The change of colour is irreversible and it is thus a witnessof the fact that the temperature has at least for a while surpassed some limit.Another example is the liquid crystal. It changes colour with temperature.Normally a couple of different crystals are used together, which gives areasonable idea of the temperature over an area. It is, though, hardly possibleto use it for measurement.

Silicon diode sensorsThese devices are used for measurement of very low, cryogenic, temperatures.It is a linear device. They are most linear at temperatures below 100 K (-173C).Depending on the type of silicon sensor they are typically applicable in thetemperature range from 75 K down to 1.5 K and they have high precision.

Silicon diodes also, like RTDs and thermistors, need an excitation current. Thismight in specific applications be a problem.


5/14


5

What is thermography?

From object to thermogram

Shown here on the left is a thermal image with its corresponding normalphotograph on the right. It is also an example of how infrared can be used.

The heat picture or the thermogram is closely related to the normal photographand it is easy to see analogies with normal photography when we want to explainhow thermograms are created. We live in a world where most of the time we aresurrounded by light of various types. We might not always remember, however, that

this light, almost without exception, comes from a source that is very hot. The mostusual source is the sun. However, heat sources can come in the form ofincandescent lamps, light arcs, sparks and indeed any object, which emits radiation.Consider for a moment a lamp.

In fact this is a special, high-power type of lamp, used in airports etc. Thetemperatures on the shell of this lamp are extremely high. However, switch it off andit goes dark. With our hands we can feel, without touching the lamp, that the lamp iswarm long after it has been switched off. A lamp may be switched off, but it radiatesheatfor a long time after.

In fact all objects with a temperature above absolute zero, i.e. 273 C, radiate heat.Thermography is a technique of obtaining imaging objects by means of their ownheat radiation. This radiation has properties, which to a large extent are the same orsimilar to the properties of visible light.

Thermography allows us humans to visualise and understand what the thermalcamera sees, i.e. like an image with light or dark parts, or with differentcolours. Not only does it help us to see thermal variations but it is also atechnique for quantifying these differences.

30

35


6/14


6

500

The temperature of this hand is around 32 C whereas the table beneath isabout 21 C. If we take a closer look at the table we can see some reflectionsfrom the hand. These reflections seem to be of a slightly higher temperaturethan the rest of the table. Obviously the thermal camera perceives not onlydirect radiation from an object but also radiation, which has been reflected from

another object before being registered by the camera.

As the camera stands at some distance from the object, the radiation passesthrough the atmosphere before hitting the camera and as it is transmittedthrough the atmosphere it is subjected to a degree of influence.

Finally, the radiation hits the thermal camera, which has an objective. It focusesthe heat radiation onto a sensor for heat radiation. That element is called the

infrared detector.

15 mm

25

30

3

22


7/14


7

This detector transforms the incident radiation into an electrical signal, which isthen processed into a visible image, i.e. the thermogram.

The whole process can be illustrated in the following way:

What is heat radiation?Heat radiation is closely related to visible light, and belongs to what is calledelectromagnetic radiation. We are familiar with many types of electromagneticradiation, like radio, radar, visible light, X-ray etc. They are all of the samenature and propagate with the same speed, 300 000 km/sec, which is known asthe speed of light.

From radiation to temperature

So far we have been discussing only radiation. The operator is, however,generally interested in the temperature. As the relation between radiation andtemperature is a physical law, hence the camera measures the radiation andthen it calculates the temperature. The camera is able to do this calculationprovided it has been calibrated. Every camera is calibrated before delivery.

ObjectAtmosphericinfluence

Thermal

camera

Thermogram


8/14


8

By having the camera look at very exact blackbody radiators we can establishthis relation for the camera, store that relation in the memory and use it for the

calculation of the temperature of the object. That relation has a mathematicalform, which is derived from Plancks law. The figure below gives an idea of therelation between radiation and temperatu

As we can see it is a non-linear relation between the temperature t and the radiationW. The calculation of radiation into temperature is carried out by the camera. But togo from the received radiation to a true, trustworthy temperature read-out, thecamera requires some further input. Let us consider a normal measurementsituation.

Temperature

Radiation

t

W


9/14


9

IR Imaging and Heat Transfer

Source: Microsoft Encarta

Multimedia Encyclopedia

TemperatureThe sensation of warmth or coldness of a substance on contact is determinedby the property known as temperature. Although it is easy to compare therelative temperatures of two substances by the sense of touch, it is impossibleto evaluate the absolute magnitude of the temperatures by subjective reactions.Adding heat to a substance, however, not only raises its temperature, causing itto impart a more acute sensation of warmth, but also produces alterations inseveral physical properties, which may be measured with precision. As thetemperature varies, a substance expands or contracts, its electrical resistivitychanges, and in the gaseous form, it exerts varying pressure. The variation in astandard property usually serves as a basis for an accurate numericaltemperature scale (see below).

Temperature depends on the average kinetic energy of the molecules of asubstance, and according to kinetic theory energy may exist in rotational,vibrational, and translational motions of the particles of a substance.

Temperature, however, depends only on the translational molecular motion.Theoretically, the molecules of a substance would exhibit no activity at thetemperature termed absolute zero.

Temperature, in physics, is the property of systems that determines whetherthey are in thermal equilibrium. The concept of temperature stems from the ideaof measuring relative hotness and coldness and from the observation that theaddition of heat to a body leads to an increase in temperature as long as nomelting or boiling occurs. In the case of two bodies at different temperatures,heat will flow from the hotter to the colder until their temperatures are identicaland thermal equilibrium is reached. Thus, temperatures and heat, although

interrelated, refer to different concepts, temperature being a property of a bodyand heat being an energy flow to or from a body by virtue of a temperaturedifference.

Temperature changes have to be measured in terms of other property changesof a substance. Thus, the conventional mercury thermometer measures theexpansion of a mercury column in a glass capillary, the change in length of thecolumn being related to the temperature change. If heat is added to an idealgas contained in a constant-volume vessel, the pressure increases, and thetemperature change can be determined from the pressure change by Gay-Lussac's law, provided the temperature is expressed on the absolute scale.


10/14


10

Temperature ScalesFive different temperature scales are in use today: the Celsius scale, knownalso as the centigrade scale, the Fahrenheit scale, the Kelvin scale, theRankine scale, and the international thermodynamic temperature scale. The

centigrade scale, with a freezing point of 0C and a boiling point of 100C, iswidely used throughout the world, particularly for scientific work, although it wassuperseded officially in 1950 by the international temperature scale. In theFahrenheit scale, used in English-speaking countries for purposes other thanscientific work and based on the mercury thermometer, the freezing point ofwater is defined as 32F and the boiling point as 212F. In the Kelvin scale, themost commonly used thermodynamic temperature scale, zero is defined as theabsolute zero of temperature, that is, -273.15C, or -459.67F. Another scaleemploying absolute zero as its lowest point is the Rankine scale, in which eachdegree of temperature is equivalent to one degree on the Fahrenheit scale. Thefreezing point of water on the Rankine scale is 492R, and the boiling point is672R.

Heat Flow between Two GasesTwo identical gases at different temperatures are kept apart by a barrier. Thearrows in the boxes represent the speed of the molecules. The gas at the highertemperature is composed of molecules which move at a higher average speed.When the barrier is removed, the gases mix and the individual gas moleculescollide with each other. The molecules in the higher temperature gas slow downand its temperature decreases. The molecules in the lower temperature gasspeed up and its temperature increases. The final temperature of the gas iscalled the equilibrium temperature.


11/14


11

In 1933 scientists of 31 nations adopted a new international temperature scale

with additional fixed temperature points, based on the Kelvin scale andthermodynamic principles. The international scale is based on the property ofelectrical resistivity, with platinum wire as the standard for temperature between-190 and 660C. Above 660C, to the melting point of gold, 1063C, a standardthermocouple, which is a device that measures temperature by the amount ofvoltage produced between two wires of different metals, is used; beyond thispoint temperatures are measured by the so-called optical pyrometer, whichuses the intensity of light of a wavelength emitted by a hot body for the purpose.

In 1954 the triple point of waterthat is, the point at which the three phases ofwater (vapor, liquid, and ice) are in equilibriumwas adopted by international

agreement as 273.16 K. The triple point can be determined with greaterprecision than the freezing point and thus provides a more satisfactory fixedpoint for the absolute thermodynamic scale. In cryogenics, or low-temperatureresearch, temperatures as low as 0.003 K have been produced by thedemagnetization of paramagnetic materials. Momentary high temperaturesestimated to be greater than 100,000,000 K have been achieved by nuclearexplosions.

One of the earliest temperature scales was that devised by the Germanphysicist Gabriel Daniel Fahrenheit. According to this scale, at standardatmospheric pressure, the freezing point (and melting point of ice) is 32F, andthe boiling point is 212F. The centigrade, or Celsius scale, invented by theSwedish astronomer Anders Celsius, and used throughout most of the world,assigns a value of 0C to the freezing point and 100C to the boiling point. Inscientific work, the absolute or Kelvin scale, invented by the Britishmathematician and physicist William Thomas, 1st Baron Kelvin, is most widelyused. In this scale, absolute zero is at -273.16C, which is zero K, and thedegree intervals are identical to those measured on the centigrade scale. Thecorresponding absolute Fahrenheit or Rankine scale, devised by the Britishengineer and physicist William J. M. Rankine (1820-72), places absolute zero at-459.69F, which is 0R, and the freezing point at 491.69R. A more consistent

scientific temperature scale, based on the Kelvin scale, was adopted in 1933.

Effects of TemperatureTemperature plays an important part in determining the conditions in whichliving matter can exist. Thus, birds and mammals demand a very narrow rangeof body temperatures for survival and must be protected against extreme heator cold. Aquatic species can exist only within a narrow temperature range of thewater, which differs for various species. Thus, for example, the increase intemperature of river water by only a few degrees as a result of heat dischargedfrom power plants may kill most of the native fish.


12/14


12

The properties of all materials are also markedly affected by temperaturechanges. At arctic temperatures, for example, steel becomes very brittle and

breaks easily, and liquids either solidify or become very viscous, offering highfrictional resistance to flow. At temperatures near absolute zero, many materialsexhibit strikingly different characteristics. At high temperatures, solid materialsliquefy or become gaseous; chemical compounds may break up into theirconstituents.

The temperature of the atmosphere is greatly influenced by both the land andthe sea areas. In January, for example, the great landmasses of the northernhemisphere are much colder than the oceans at the same latitude, and in Julythe situation is reversed. At low elevations the air temperature is alsodetermined largely by the surface temperature of the earth. The periodic

temperature changes are due mainly to the sun's radiant heating of the landareas of the earth, which in turn convect heat to the overlying air. As a result ofthis phenomenon, the temperature decreases with altitude, from a standardreference value of 15.5C (60F) at sea level (in temperate latitudes), to about -55C (about -67F) at about 11,000 m (about 36,000 ft). Above this altitude, thetemperature remains nearly constant up to about 33,500 m (about 110,000 ft).

ThermometerA thermometer is an instrument used to measure temperature. The mostcommonly used thermometer is the mercury-in-glass type, which consists of auniform-diameter glass capillary that opens into a mercury-filled bulb at oneend. The assembly is sealed to preserve a partial vacuum in the capillary. If thetemperature increases, the mercury expands and rises in the capillary. Thetemperature may then be read on an adjacent scale. Mercury is widely used formeasuring ordinary temperatures; alcohol, ether, and other liquids are alsoemployed for this purpose.

120

100

80

60

40

20

50

40

30

20

10

0

10


13/14


13

Outdoor ThermometerA red-dyed alcohol thermometer measures an outside air temperature of about6C (about 43F). In a thermometer, an expanding fluid such as alcohol or

mercury is trapped within a closed glass rod. As the fluid expands or contracts,it is measured by marks calibrated for given temperatures. The scale may bemarked for either the Celsius or Fahrenheit temperature scales or both.

The invention of the thermometer is attributed to Galileo, although the sealedthermometer did not come into existence until about 1650. The modern alcoholand mercury thermometers were invented by the German physicist GabrielFahrenheit, who also proposed the first widely adopted temperature scale,named after him, in which 32F is the freezing point of water and 212F is itsboiling point at standard atmospheric pressure. Various temperature scaleshave been proposed since his time; in the centigrade, or Celsius, scale, devisedby the Swedish astronomer Anders Celsius and used in most of the world, thefreezing point is 0, the boiling point is 100.

Types of ThermometersA wide variety of devices are employed as thermometers. The primaryrequirement is that one easily measured property, such as the length of themercury column, should change markedly and predictably with changes intemperature. The variation of that property should also remain fairly linear withvariations in temperature. In other words, a unit change in temperature should

lead to a unit change in the property to be measured at all points of the scale.

The electrical resistance of conductors and semiconductors increases with anincrease in temperature. This phenomenon is the basis of the resistancethermometer in which a constant voltage, or electric potential, is applied acrossthe thermistor, or sensing element. For a thermistor of a given composition, themeasurement of a specific temperature will induce a specific resistance acrossthe thermistor. This resistance can be measured by a galvanometer andbecomes a measure of the temperature.

Various thermistors made of oxides of nickel, manganese, or cobalt are used to

sense temperatures between -46 and 150C (between -50 and 300F).Similarly, thermistors employing other metals or alloys are designed for use athigher temperatures; platinum, for example, can be used up to 930C (1700F).With proper circuitry, the current reading can be converted to a direct digitaldisplay of the temperature.


14/14


14

Very accurate temperature measurements can be made with thermocouples, inwhich a small voltage difference (measured in millivolts) arises when two wires

of dissimilar metals are joined to form a loop, and the two junctions havedifferent temperatures. To increase the voltage signal, several thermocouplesmay be connected in series to form a thermopile. Since the voltage depends onthe difference of the junction temperatures, one junction must be maintained ata known temperature; otherwise an electronic compensation circuit must bebuilt into the device to measure the actual temperature of the sensor.

Thermistors and thermocouples often have sensing units less than 1/4 cm (lessthan 1/10 in) in length, which permits them to respond rapidly to temperaturechanges and also makes them ideal for many biological and engineering uses.

The optical pyrometer is used to measure temperatures of solid objects attemperatures above 700C (about 1300F), where most other thermometerswould melt. At such high temperatures, solid objects radiate sufficient energy inthe visual range to permit optical measurement by exploiting the so-called glowcolor phenomenon. The color at which hot objects glow changes from dull redthrough yellow to nearly white at about 1300C (about 2400F). The pyrometercontains a light bulb type of filament controlled by a rheostat (dimmer switch)that is calibrated so that the colors at which the filament glows correspond tospecific temperatures. The temperature of a glowing object can be measured byviewing the object through the pyrometer and adjusting the rheostat until thefilament blends into the image of the object. At this point the temperatures of thefilament and the object are equal and can be read from the calibrated rheostat.Another temperature-measuring device, used mainly in thermostats, relies onthe differential thermal expansion between two strips or disks made of differentmetals and either joined at the ends or bonded together.

Special-Purpose ThermometersThermometers may also be designed to register the maximum or minimumtemperature attained. A mercury-in-glass clinical thermometer, for example, is amaximum-reading instrument in which a trap in the capillary tube between thebulb and the bottom of the capillary permits the mercury to expand with

increasing temperature, but prevents it from flowing back unless it is forcedback by vigorous shaking. Maximum temperatures reached during the operationof tools and machines may also be estimated by special paint patches thatchange color when certain temperatures are reached.

microsoft word - section 3

Documents