[BS 1041-7-1988] -- Temperature measurement. Guide to selection and use of temperaturetime indicators.pdf

7/25/2019 [BS 1041-7-1988] -- Temperature measurement. Guide to selection and use of temperaturetime indicators.pdf

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BRITISH STANDARD BS 1041-7:1988

Temperaturemeasurement

Part 7: Guide to selection and use oftemperature/time indicators

UDC 536.5:536.49:531.761:536.522.3


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BS 1041-7:1988

This British Standard, havingbeen prepared under thedirection of the Industrial-process

Measurement and ControlStandards Committee, waspublished under the authorityof the Board of BSI andcomes into effect on30 September 1988

BSI 12-1999

First published June 1943

First revision September 1964

Second revision September 1988

The following BSI referencesrelate to the work on this

standard:Committee reference PCL/1

Draft for comment 86/21686 DC

ISBN 0 580 16670 8

Committees responsible for thisBritish Standard

The preparation of this British Standard was entrusted by the

Industrial-process Measurement and Control Standards Committee(PCL/-) to Technical Committee PCL/1, upon whichthe following bodies were represented:

British Coal

British Gas plc

British Pressure Gauge Manufacturers Association

Department of Energy (Gas and Oil Measurement Branch)

Department of Trade and Industry (National Weights and MeasuresLaboratory)

Energy Industries Council

Engineering Equipment and Materials Users AssociationGAMBICA (BEAMA Ltd.)

Health and Safety Executive

Institution of Gas Engineers

Coopted members

Amendments issued since publication

Amd. No. Date of issue Comments


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Contents

Page

Committees responsible Inside front cover

Foreword iiSection 1. General

1 Scope 1

2 Definitions 1

Section 2. Indicators that change shape or size

3 General 2

4 Pyrometric cones 2

5 Thermoscope bars 4

6 Bullers rings 5

Section 3. Colour-change indicators

7 General 8

8 Temperature indicating paints 89 Reversible colour-change indicators and paints 9

10 Temperature indicating labels 12

Figure 1 Form of pyrometric cones 2

Figure 2 Thermoscope bars on stand before and after firing 5

Figure 3 Bullers ring and gauge 6

Figure 4 Examples of trigger temperature-time relationshipsfor some single-change paints 11

Figure 5 Examples of trigger temperature-time relationshipsfor a multi-change paint 12

Table 1 Approximate touch-down temperatures of pyrometric cones 3

Table 2 Approximate bending temperatures of thermoscope bars 4Table 3 Approximate temperature chart for Bullers rings 7

Table 4 Examples of single-change paints 10

Table 5 Examples of multi-change paints 10

Publications referred to Inside back cover


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Foreword

This Part of BS 1041 has been prepared under the direction of theIndustrial-process Measurement and Control Standards Committee. It isa revision of BS 1041-7:1964 which is withdrawn.

It is Part 7 of the standard and it differs somewhat from the other four specialistparts which deal with specific types of temperature measuring equipment, in thatthe devices considered herein are used primarily to indicate the effect oftemperature and time by purely comparative means, as opposed to instrumentswhich give a direct reading or recording of the temperature conditions. In theceramic industry the effect of temperature-time treatment is generally referredto as heat work.

A British Standard does not purport to include all the necessary provisions of acontract. Users of British Standards are responsible for their correct application.

Compliance with a British Standard does not of itself confer immunityfrom legal obligations.

Summary of pages

This document comprises a front cover, an inside front cover, pages i and ii,pages 1 to 12, an inside back cover and a back cover.

This standard has been updated (see copyright date) and may have hadamendments incorporated. This will be indicated in the amendment table on theinside front cover.


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Section 1. General

1 Scope

This Part of BS 1041 provides guidance on changeof state devices used to indicate the amount of heat

work which the subject being considered hasundergone. The two groups of change of statedevices which are included are those which changeshape or size and those which change colour.

Included in the former are the following:

a) pyrometric cones;

b) thermoscope bars;

c) Bullers rings.

Of the preceding, a) and b) change their shapeduring heating and c) change size.

The change of colour devices are:

1) temperature indicating paints;

2) reversible colour change indicators and paints;

3) temperature indicating labels.

NOTE The titles of the publications referred to in this standardare listed on the inside back cover.

2 Definitions

For the purposes of this Part of BS 1041 thefollowing definitions apply.

2.1heat work

a qualitative indication of the effect of heattreatment, increasing with temperature and timespent at temperature

NOTE It is used especially in the firing of ceramics.

2.2pyrometric cone

a slender truncated trihedral pyramid of pressedceramic material that deforms under particulartemperature/time treatment

2.3touch-down temperature (end-point)

the temperature at which a pyrometric cone will

bend so that the tip touches the stand or plaquewhen heated at a specified rate of temperature rise

2.4signal cone

in a series of pyrometric cones, the cone thatdeforms after the least heat work

2.5indicator cone

in a series of pyrometric cones, the cone whose tiptouches the stand when the desired heat work hasbeen applied

2.6witness cone

in a series of pyrometric cones, the cone that beginsto deform next after the indicator cone

2.7thermoscope bar

a pressed ceramic bar that, when supported at each

end in a horizontal position, deforms under its ownweight under specified temperature/time treatment

2.8bullers ring

an annular ring of pressed ceramic powder thatcontracts with heat work

2.9temperature indicating paint

paint that undergoes a change in colour afterexposure to a specified temperature for a specifiedtime

2.10single-change paint

a temperature indicating paint that undergoes onlyone principal colour change under temperature/timeexposure

2.11multi-change paint

a temperature indicating paint that undergoesseveral colour changes after various specifiedtemperature/time exposures

2.12signal colour

the colour which a temperature indicating paintturns after the specified temperature/time exposure

2.13trigger temperature

the temperature at which a temperature indicatingpaint changes colour at a particular exposure time

2.14cut-off temperature

the temperature below which a temperatureindicating paint exhibits no colour change, nomatter how long the exposure

2.15reversible colour-change indicators

indicators that revert to the original colour oncooling below the colour-change temperature

2.16temperature indicating label

an adhesive label containing a patch whose colourchanges when the surface to which it is attachedreaches a specified temperature

NOTE One label may contain several patches to indicate theattainment of a range of temperatures.


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Section 2. Indicators that change shape or size

3 General

The temperature/time indicators that depend on thechange of shape or size are largely used in the firing

of ceramic wares such as grinding wheels, bricks,refractories, electrical porcelain, china,earthenware, sanitary ware, or tiles. Theseindicators can only be said to measuretemperature/time treatment if they are used underthe same conditions which existed when indicatorsof the same specification were compared with atemperature measuring instrument.

Such an indicator does not replace a temperaturemeasuring instrument but is used in conjunctionwith it; the instrument will indicate thetemperature-time treatment at a particular spot ina kiln during a firing cycle, whereas a number ofsuitably positioned indicators can give a measure ofthe variation in thermal conditions within a kilnsetting.

Pyrometric cones and thermoscope bars areprepared from complex mixes containing variousproportions of frits, fluxes, clays, calcium andmagnesium compounds, silica, etc., designed to meltor soften within certain temperature ranges thatdepend on the rates of heating used. Cones deformand slump fairly sharply at a certain stage in theheating cycle. Bars, which are supported at theirends, bend in the middle.

Bullers rings are prepared from clays, fluxes, andinert materials, and do not melt or deform butshrink progressively during a heating cycle; theeffect of temperature and time (heat work) isassessed by measuring the shrinkage of theindicator.

4 Pyrometric cones

4.1 General

Cones are manufactured in two sizes, the normal orstandard cone with a height of about 60 mm, andthe laboratory cone with a height of about 30 mm.

Both can be used to monitor and control industrialkilns, but small cones are specified for determiningthe refractoriness of raw materials in accordancewith BS 1902-5.2, ISO 528, and DIN 51063-1 andDIN 51063-2.

Cones from different sources vary in shape, mannerof usage, and performance.

4.2 Characteristics

4.2.1 Construction. Pyrometric cones (see Figure 1)are typically slender truncated trihedral pyramidsabout 30 mm or 60 mm in height.

4.2.2 Cone numbers. Pyrometric cones aretraditionally numbered from 022 to 20; this methodof numbering has a historical basis, followingoriginal Seger practice, which is broadly accepted byother manufacturers. Temperature steps of 15 Cup to 30C exist between each cone number, and 42cones cover the temperature range 600Cto 1 535 C (see Table 1).

4.2.3 Touch-down temperature. This is thetemperature at which a cone set up in the correctmanner (see 4.3.1) will bend and touch the standwhen heated in a kiln at a specified rate of rise oftemperature.

A heating rate of 60 C/h was used in preparingTable 1. Faster rates of heating will raise thetouch-down temperature and vice-versa.

Laboratory cones, because of their lower mass, andbecause they are subjected to faster heating rates,usually fall at higher temperatures than normal

cones of the same number and composition.NOTE Though the temperatures in Table 1 may correspondapproximately to those in tables provided by overseas conemanufacturers, cones of the same number from differentproducers should not be regarded as interchangeable.

Figure 1 Form of pyrometric cones


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Table 1 Approximate touch-downtemperatures of pyrometric cones

4.3 Method of operation

4.3.1 General. The method described in 4.3.2to 4.3.5can be generally applied under most firingconditions for which cones are suitable.

4.3.2 Setting up. While single cones are sometimesused it is recommended that three or fourconsecutively numbered cones are inserted intospecially made unfired plaques which hold the conesat the correct angle to the vertical. These plaqueshave tapered holes and protrusions on one or moreinternal faces so that the cone is firmly held; duringfiring, plaque and cone contract similarly so that the

cone is held firmly throughout the firing. A conemay be set up in other ways, such as inserting itsbase into refractory clay, but the importance ofmaintaining a correct angle and a firm hold at alltimes cannot be over-stressed. Failure in theserespects will cause the cone to bend unpredictably,and to give an incorrect assessment of the heattreatment.

NOTE Cones from different sources differ in shape, size, andthe manner in which they are set up before firing. Eachproducers instructions should therefore be studied carefully andstrictly adhered to if satisfactory performance during firing is tobe obtained. Some manufacturers make cones with thickenedbases which do not require stands.

4.3.3 Choice of cones. The range of the three or fourcones chosen should extend above and below that ofthe cone expected to indicate the desired heattreatment.

4.3.4 Usage in kilns. Since the temperaturethroughout a kiln is rarely uniform several plaques

of cones can be placed in different parts of the kiln,so that an estimate of the variation in heat workfrom place to place can be obtained.

4.3.5Behaviour during firing. As the firing cycleprogresses and the heat work approaches that atwhich the signal cone is affected, the cone will beginto soften and bend, and will continue to bend untilthe tip just touches the plaque on which the cone hasbeen placed. This is considered to be the end-point ofthe cone. With further prolongation of the firingcycle, with soaking or increase in temperature, thecone will melt completely to form a blob on theplaque. The next cone of the series will begin to

deform some time after the first started to bend andwill continue to bend until its end-point is reached.The appropriate level of heat work will have beenreached when the indicator cone has just reached itsend-point. Under these firing conditions the witnesscone will be only partially bent over.

4.4 Operating features of cones

4.4.1 General. Cones containing frits givingrelatively low viscosity glasses are appropriate forlow temperatures (e.g. less than 1 200C) whereasmixtures of minerals reacting and melting toproduce more viscous glasses are used for higher

temperatures. Since cones consist partly of glass orglass-formers plus more refractory clays and silica itfollows that cones do not have definite meltingpoints; they deform only when the decreasingviscosity of the liquid phase permits movementunder the influence of gravity. The performance ofcones is largely controlled by temperature, but time(e.g. rate of heating, soaking at a giventemperature) and kiln atmosphere are alsosignificant.

Enamels and glazes fired at the lower temperaturescontain proportions of frit as do the cones. Manyceramic products are based on mixtures of silicate

minerals as are the cones for higher temperatures.Because temperature and time affect ware andcones similarly, cones are useful for controlling theprogress of firing of many ceramic products. Thetouch-down of a cone can indicate the point at whichware will mature under a controlled rate of heatingand a particular set of atmospheric conditions,which should then be maintained. If such conditionsdo not alter, the cone will always touch-down at thesame temperature.

4.4.2Burnout of binder. As cones contain an organicbinder, heating rates in the low temperature regionshould be slow enough to ensure that the bindingmaterial is completely burned out, as residualbinder can prevent satisfactory behaviour of thecone later, and even cause swelling and instability.

Coneno.

Temperature

Coneno.

Temperature

Coneno.

Temperature

C C C

022021020019018017016015014013

01201101009

600615630665700730760790810830

860880900925

0807060504030201

12

3456

950975

1 0001 0301 0601 0851 1051 1201 1351 150

1 1651 1801 1951 210

789

10111213141516

17181920

1 2301 2501 2701 2901 3101 3301 3501 3801 4101 435

1 4601 4851 5101 535

NOTE 1 Each temperature given in the table is that at whichthe tip of a cone will bend sufficiently to touch the base, in anelectric kiln with a heating rate of 60 C/h.

NOTE 2 The touch-down temperature depends on the rate ofheating; reports on firing behaviour should quote the conenumber, not the temperature taken from the above table.

NOTE 3 Intermediate degrees of bending can be referred to thehands of a clock, e.g. 3 oclock would represent a cone benthalfway to the stand.


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4.4.3 Rate of heating. The viscosity of the moltenglass decreases sharply with temperature, andreaction rates between glass-producing mineralsapproximately double with every increase of 10 C;

the touch-down temperature of a cone is thereforedependent on the rate at which the temperature hasincreased. The rate of heating is less important inthe earlier stages of firing as long as glasses havenot melted or liquid-forming reactions commenced.However as soon as these processes begin to operatethe rate of heating becomes significant. In generalthe slower the rate of heating the lower will be thetemperature at which deformation proceeds.

With cones heated at 20 C/h and 150 C/h,differences in end-points of as much as 60 C havebeen recorded. The trend to very fast firing ofcertain ceramic products raises the end-point of a

cone still further.

NOTE Some manufacturers of cones provide tables showing thetouch-down temperatures of cones at various rates of heating.

4.4.4Kiln atmosphere. In practice cones should beprotected from direct contact with flames and fromdraughts of cold air. The effect of atmospheredepends to some extent on the composition of thecone, e.g. those containing iron oxide (red cones) canbe reduced. British pyrometric cones do not containadded iron oxide and are relatively insensitive toreducing conditions.

Direct exposure to flames or to dissociated

hydrocarbons in the early stages of firing maydeposit graphitic carbon in surface pores or leave ahard crust of carbon: this fails to melt so that theouter portion of the cone remains as an erect shell,even after the centre has softened and flowed out atthe base.

Sulphurous gases can also create shells in bad cases;these gases can also raise the viscosity of moltensilicate glasses, e.g. the end-point of some coneswhen used in an atmosphere containing 0.35 %sulphur dioxide can be raised as much as 35 C.

5 Thermoscope bars

5.1 Construction

Thermoscope bars contain a range of frits, clays,feldspars, silica, and other minerals, similar tothose used in pyrometric cones. Compositions, aswith cones, are varied to produce bars which bendat different temperatures. The bars are made bydust-pressing, with an organic binder, and aretypically about 57 mm 8 mm 6 mm. Thoughnormally used in the as-pressed state, bars can behardened in manufacture, by pre-firing at a suitabletemperature below that at which bending shouldoccur.

5.2 Bar numbers

Table 2 lists the approximate bending temperaturesfor a range of 42 bars, giving an operating rangefrom 590 C to 1 525 C. Intervals between bars

vary from 15 C to 30C.

Table 2 Approximate bending temperaturesof thermoscope bars

5.3 Operation

Up to four consecutively numbered bars are placedhorizontally on a refractory stand (preferablystepped), so that they are about 12 mm apart andsupported only at their ends (see Figure 2).

When subjected to an increasing temperature,eventually the lowest numbered bar will begin tobend; the degree of bending continuously increaseswith rising temperature until the bar contacts thebase of the stand and finally fuses. As the heat workapproaches that which the selected bar is toindicate, this bar will have just begun to bend, whilethe higher numbered bars remain unaffected.

The use of an alumina-china clay wash on therefractory stand will help release of the fired bars,so that the stand can be reused.

5.4 Operating features

As cones and bars are manufactured from verysimilar materials all the effects of heating rate,soak, and atmosphere described for cones areapplicable to thermoscope bars.

Barno.

Temperature

Barno.

Temperature

Barno.

Temperature

C C C

123456

789

1011121314

590610625650685715

745775800820845870890915

151617181920

2122232425262728

940965990

1 0151 0451 075

1 0951 1151 1301 1451 1601 1751 1901 205

293031323334

3536373839404142

1 2201 2401 2601 2801 3001 320

1 3401 3651 3951 4251 4501 4751 5001 525

NOTE 1 Each temperature given in the table is that at whichthe bar starts to bend in an electric kiln with a heating rateof 60 C/h.

NOTE 2 The bending temperature depends on the rate ofheating; reports on firing behaviour should quote the bar

number, not the temperature taken from the table.NOTE 3 The bar can be expected to bend sufficiently to touchthe stand at a temperature of 10C to 30C higher than thevalues given in the table, depending on the composition of thebar.


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6 Bullers rings

6.1 GeneralIn contrast to cones and bars, Bullers rings give noimmediate visible indication that a certain level ofheat work has been reached. Instead, thecontraction of the ring, which occurs on firing andwhich indicates the heat work, is measured on agauge.

6.2 Characteristics

6.2.1 Construction. A Bullers ring (see Figure 3)consists of an annular ring of rectangular crosssection made of pressed, unfired ceramic material.It is approximately 63 mm diameter and 8 mm

thick with a centre hole of 22 mm diameter. Thering is made from material which contracts fairlyuniformly with increase of temperature throughoutthe operating range.

6.2.2 Operating range. The temperature rangecovered by Bullers rings is approximately 960 Cto 1 400 C as shown in Table 3, this overall rangebeing covered by materials of four differentcompositions each suitable for different groups ofproducts as follows.

a) Number 55 rings (coloured brown). Theserings, whose range is 960 C to 1 100 C, have arapid shrinkage above 1 000C and are used inthe firing of glost ware and common buildingbricks where the finishing temperature is ratherlow.

b) Number 27/84 rings (coloured green).Recommended over the temperature range

of 960 C to 1 250 C when firing earthenwareand tiles, building bricks, engineering bricks andfire bricks.

c) Number 75/84 rings (coloured natural).Recommended for firing temperaturesfrom 960 C to 1 320 C. These rings can besatisfactorily used wherever the number 27/84rings are suitable but have an increased firingrange which permits their use at rather highertemperatures. They are chiefly used in the firingof electrical porcelain, china biscuit, grindingwheels, fire bricks and refractories.

d) Number 73 rings (coloured yellow).Recommended for temperatures from 1 280Cto 1 440 C, especially in slow firing conditions asis customary with higher grade fire bricks andheavy refractories.

6.3 Method of operation

6.3.1 Several rings of the same composition areplaced vertically in a prefired stand; several standsare located in the kiln in such a manner thatindividual rings can easily be withdrawn.Alternatively rings can be placed on any horizontalsurface and not removed until the firing has been

completed. In continuous car tunnel kilns, ringsmay be placed where convenient and withdrawn atsuitable points along the tunnel up to and includingthe firing zone.

Figure 2 Thermoscope bars on stand before and after firing


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6.3.2At various times, as firing progresses, a ring iswithdrawn from each stand, cooled and measured(see 6.3.3) so as to determine:

a) the uniformity of heating throughout the kiln;

b) whether the firing is progressing favourably;

c) when the kiln has been fired sufficiently.

Firing is complete when rings of certaincontractions have been obtained in various parts ofthe kiln, as determined (for the particular ware) by

previous experience.6.3.3 Measurement of the contraction, and thereforethe heat work indicated by the rings, is carried outon a gauge consisting of a brass base plate on whichare mounted a radial arm with pointer moving overa scale and two steel dowel pins against which thering is pressed by the movable arm whenmeasurements of pointer number are being taken(see Figure 3). A contraction of the ring gives rise toan amplified movement of the pointer over the scale.The scale is engraved on the brass plate and is sonumbered that the higher fired rings, which havecontracted more, give a higher reading on the gauge.

The divisions on the scale are numbered 0 to 60above and 0 to 5 below the zero mark, the latterreadings indicating expansion and the formercontraction.

NOTE On being heated the rings first expand slightly thenreturn to their initial size when a temperature ofapproximately 960C is attained and subsequently contractgradually to an extent which depends on the heat treatmentreceived.

Rings should be measured across several diameters,

by turning them round in the gauge, and the meanvalue given to the nearest half pointer number.

A necessary precaution with the gauge is to see thatthe bearing on which the pointer works and the twopins and side of the pointer against which the ringrests when being measured, are not worn. In thecourse of time these are liable to wear and an errorof one or two divisions on the scale can be obtained.Allowance should be made for this by checking fromtime to time against a control ring which may be ofmetal. When the wear is pronounced the gaugeshould be replaced or repaired.

6.4 Operating featuresSince the rings themselves, throughout theiroperating range, consist largely of non-fusibleceramic material they form the most trulycomparative means of heat work measurement forceramic ware. Variations in the rate of heatingwould generally affect the ware in the same way asthe rings.

6.5 The effect of firing rate and soak on thebehaviour of Bullers rings

Users of Bullers rings are always advised that thering values given in the temperature chart should

be used with caution because they are dependent onthe firing cycle to which the rings are subjected.

With the introduction of rapid firing for certaintypes of ceramics this cautionary advice has becomemore significant. However, in some branches of theindustry temperatures may be held for long periodsto ensure that thick sections are correctly fired. Ringvalues can be altered very considerably at these twoextremes of firing and some measure of thesedifferences has been determined experimentally.

It is probable that the rate of heating up to 950Chas a negligible effect on ring behaviour. In

laboratory trials rings were heated to a giventemperature and rings were withdrawnat 0, 1, 2, 4, 8, 16, 32, 64 h intervals and theirvalues recorded. When plotted it was seen that mostof the contraction of the rings occurred in the earlypart of the soak period, and that rings reachednear-stable values with a prolonged soak.

Figure 3 Bullers ring and gauge


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Table 3 Approximate temperature chart for Bullers rings

Temperature

Ring no. 55

(brown)

Ring no. 27/84

(green)

Ring no. 75/84

(natural)

Ring no. 73

(yellow)Gauge reading

C

960970980

9901 0001 010

1 0201 0301 040

1 0501 0601 070

1 0801 0901 100

1 1101 1201 130

1 1401 1501 160

1 1701 1801 190

1 2001 2101 220

1 2301 2401 250

1 2601 2701 280

1 2901 3001 320

1 3401 3601 380

1 4001 4201 440

37

11

151821

242730

323436

373839

012

457

81011

131415

171820

212324

262728

303133

343637

384041

012

345

678

101112

141517

182021

222324

262728

293031

323334

363840

424446

29

303134

374044

485154

NOTE These values should be used with caution because they are dependent on the firing cycle to which the rings are subjected.


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Section 3. Colour-change indicators

7 General

Colour-change indicators comprise paints or labels

which undergo colour changes at specifictemperatures, either reversibly or irreversibly. Forsome substances several distinct colour changesmay occur. These changes provide a visualindication or record of temperature exposure, andsince in most cases the time of exposure is animportant parameter, they fall naturally into thisPart of BS 1041.

The measurement of the temperatures of surfacesusing thermocouples is often problematical becausegood contact has to be established, without at thesame time altering the temperature of the surface to

be measured. Radiation pyrometers operate withoutcontact and can possess good sensitivity even atmoderate temperatures, but the accuracy of themeasurement depends substantially on aknowledge of the emissivity of the target surface. Inthese circumstances the application of a paint or alabel to a surface to obtain an indication oftemperature exposure can be extremely convenient.It offers the advantage of simplicity and flexibility,and can also achieve useful reliability and accuracy.

8 Temperature indicating paints

8.1 General

Temperature indicating paints incorporatethermally sensitive pigments which change colourwhen exposed to heat. The colour change inindicators is the result of chemical change and isirreversible. The paint provides a clear visual recordthat a certain temperature exposure has beenexceeded. Colours before and after heating are quitedistinct and the change is easily recognizable. Asthe paint is in intimate contact with the surfaceunder examination, temperature variations overareas of any dimension can be recorded. The paintscan be directly applied at room temperature, by

brush or spray, to practically any surface. In orderto obtain the most accurate and clear indication oftransition temperature, the paint should be appliedso as to give as complete and even a coverage as ispossible.

8.2 Composition and characteristics

Temperature indicating paints are basicallyacrylic lacquers containing finely dispersedtemperature-sensitive inorganic pigments. Thepigments used in each type of paint are specific forthe transition temperature required. Thesepigments, once heated above a particular level, willundergo chemical change which is seen as a changein colour.

The fact that heat causes a gradual transition in themolecular structure of the sensitive particles means

that the change is less rapid than that which occurswith a melting-point material. There is therefore atime factor to be considered when assessing thetemperature of a colour transition. However, whenthe temperature is below a certain level, known asthe cut-off temperature, this level being different foreach paint, then no colour change takes place, nomatter how long the paint is heated.

Paints can be divided into two groups.Single-change paints undergo one reliable colourchange in the range 60 C to 610 C. Multi-changepaints change through a spectrum of colours atvarious points in the range 150 C to 1 100 C.

Up to 235 C single-change paints are available ina two-pack form. The pigment is dispersed inan hydroxy-functional acrylated resin, and justprior to application a second resin, a cyanidecopolymer, is added. The chemical reaction of thepolymerization between the resins provides a toughand weather-resistant finish.

Manufacturers prepare batch calibration curvesshowing the temperature/time relationships which,for appropriately formulated paints, are reliableand repeatable. Experienced operators using themanufacturers calibration curves should obtain

results accurate within 5 % of the Celsiustemperature. The most accurate measurement willbe made if the conditions of use exactly correspondto the calibration conditions so far as heating andcooling times, temperatures and atmosphericconditions are concerned.

8.3 Applications

When a body undergoes a rise in temperature it isusual for a temperature gradient to appear acrossthe surface. This lack of uniformity can be efficientlydemonstrated by the use of temperature indicatingpaints. When any point on the surface reaches atemperature in excess of the transition value for thepaint in use, a colour change will take place. Thiswill leave part of the area in the original colour andpart in the new colour, the two hues being, as itwere, bounded by an isothermal line, i.e. all pointsalong this line were, at the time of transition, at thesame temperature. When a multi-change paint isused, a number of bands of colour will appearsuccessively as the temperature rises. The colourchange of temperature indicating paints is causedby physical or chemical alterations in the pigments.In most cases molecules of water, carbon dioxide,etc., are lost, resulting in a change of hue, and it isthese colour changes which enable a permanentthermal record to be made of the temperaturegradient across a surface.


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The many industrial uses of temperature-sensitivepaints fall into two categories. First they may be

used purely to observe heat patterns, so enablinghigh or low temperature points to be detected. Thisis particularly important at very high temperatureswhen the effect of cooling gases may be upset byrivet heads, etc. This would cause gas to be locallydiverted from the surface, and so give rise to anabnormally high surface temperature. Both theefficiency of insulation and the uniformity of heatdispersion can be studied using paints.

The second way in which thermal paints are used isin the measurement of temperature. This can bedone on any surface no matter if it is inaccessible orrevolving at high velocity during the thermal

examination. To make such measurements it isessential to have calibration graphs to relate thecolour to the time/temperature cycle.

The composition of multi-change paints allows themto perform in hostile gases. Some paints have goodsurface adhesion at high temperatures, and so arean invaluable asset in gas turbine and jet enginediagnostics.

For the selection of the most suitable paint for aparticular application, reference should be made tocalibration graphs, the estimated surfacetemperature, and the trigger temperature required.

8.4 Assessment of paint characteristicsThermal paints can be conveniently assessedon a low-voltage resistive test rig, using abutterfly-shaped metal (nimonic or similar) plate asthe test surface. The construction of the butterflyoffers a non-uniform resistance per unit length, thusproducing temperature gradients during thepassage of an electric current. The heating currentis regulated using thermocouples tack-welded to thetest plate near the constriction. Thermocouples arealso attached to the ends of the plate to measure thesoak temperature.

When a paint is to be checked, a uniform coat isapplied and allowed to dry. The control and soakthermocouples are then attached, and the rigcontrol set to the desired temperature for a selectedheating period, typically 10 min. After this time thesoak temperature is measured, and the rig isswitched off.

A new thermocouple is now attached to theisothermal line at the boundary of the colour change

of the paint. The rig is then reheated foranother 10 min period, with the control unit settingunaltered from the first run. The soakthermocouples are then read and the temperatureat the colour change thermocouple is recorded.Having thus established the temperature at whichthe paint changes colour, the rig is switched off andthe control unit is connected to the thermocoupleswhich were used to measure the isothermal line.The plate is then repainted and the rig runfor 10 min with the control unit set at the measuredchange temperature. If the reading is correct, thenew isothermal line of colour change crosses this

thermocouple after 10 min heating.Using this system all time/temperature readingscan be measured and checked. Time/temperaturegraphs of up to 50 h duration are customarilyproduced for each paint.

Table 4 and Table 5 show the transitiontemperatures for a number of single- andmulti-change paints when heated at a constanttemperature for a period of 10 min. Figure 4 andFigure 5 show that the transition temperaturesinitially decrease rapidly with increasing exposuretime, but that the curves level off at longer heatingperiods. After 5 h or so any further decrease is smallin most cases.

9 Reversible colour-change indicatorsand paints

9.1 General

Reversible colour-change indicators are invaluablewhen it is necessary to detect undesirabletemperature excursions, correct faults and revert tonormal conditions. They are widely used in theelectrical industry, especially on busbars, liveconductors and connectors in high current switches,and in electronic fault-finding. They are also used aswarning and indicating devices in both the domesticand industrial environments, for example in place ofneon indicators in domestic appliances.

9.2 Characteristics

Reversible temperature indicators are available aspaints and in label form. Paints are available thatgive strong colour changes at various temperaturesup to 170 C. At the lower temperatures the thermalpigments are mercury-based complexes whichcannot be applied directly to metal surfaces as thiscauses decomposition. They also tend to decomposeafter long exposure to heat, but the decomposition

can be retarded by using a clear over-lacquer. Thesepigments find their most successful applicationwhen encapsulated into labels.


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9.3 Repeatability and testing

For temperatures up to 70 C under test conditions

a repeatability of indication of 1 C can beachieved. For temperatures up to 150 C thisincreases to 2 C and from 150C to 170 C itincreases to 3 C. Ageing effects are small.

Paints and labels of this type have been shown to bedurable and to withstand sunlight and adverseweather conditions for a projected life of 2 years.

9.4 Liquid crystal indicators

A liquid crystal, as the name implies, is a chemicalwhich is both liquid and crystalline, possessingsome of the mechanical properties of a liquid and theoptical qualities of a crystal. Several different types

of liquid crystal have been discovered, some of whichare widely used in electronic displays.

Certain liquid crystals are thermochromic; that is,they change colour with temperature. The structureof these crystals is such as to selectively reflect lightof a narrow waveband whilst transmitting lightoutside this band. If the transmitted light is finallyabsorbed into a dark background, only the reflectedwavelengths are seen, as a pure iridescent colour.Typically, as the temperature increases thewavelength of the reflected light decreases. A liquidcrystal on warming will first show a red colour, thengo through the spectrum to green, blue and violet,ending up in the ultraviolet, at which point only theblack background is seen. Thus the colour is anindication of temperature, and the span over whichthe changes occur can be 0.5C to 20C wide.

Among materials used, cholesteric esters operate inthe range from 2 C to + 44 C, typically with aresolution of 1 C. These compounds are not verystable, however. Other materials, such as chiralnematics, which are more stable, offer greaterresistance to ultraviolet light, and are usable withinthe approximate range 30 C to + 150 C.

In the form of adhesive labels, they can be used for

a variety of applications where continuous display isrequired. Non-adhesive labels are especiallyadapted to clinical use, in the measurement of skintemperature. However, the relatively highexpansivity of the crystals limits the long-term useof such labels, at least when large temperatureexcursions are involved.

Table 4 Examples of single-change paints

Table 5 Examples of multi-change paints

Original

colour

Signal colourInitial trigger

temperaturea

Cut-off

temperatureC C

pinkpinkmauve pinkblueyellowbluemauve redmauvegreengreenorange

red

bluebluebluedark greenredfawngreywhitesalmon pinkwhiteyellow

white

48135148155235275350386447458555

630

30110120

46180150220290312312482

450a Colour-change temperature for 10 min heating.

Originalcolour

Signalcolour

Initialtrigger

temperaturea

Cut-offtemperature

C C

light tan

bronze green

bronzegreenpale indianred

160

230

150

210

reddishorangedark grey

medium grey

dark grey

mediumgreydirty white

242

255

338

193

211

228

purplepinkfawn

pinkfawnblue

395500580

355386408

reddusty greyyelloworangegreenbrown

dusty greyyelloworangegreenbrowngreen/grey

420555610690820

1 050

310328450535621945

a Colour-change temperature for 10 min heating.


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Figure 4 Examples of trigger temperature-time relationships for somesingle-change paints


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10 Temperature indicating labels

10.1 General

Most temperature indicating labels, unlike thosediscussed in clause 9, rely on the melting of a whitechemical coating to reveal a black paper backing. Asthe melted chemical is absorbed by the paper, thecolour change is irreversible.

The chemically coated paper is protected byencapsulation under a polyester film. The labels areself-adhesive and, once applied to a clean surface,

are water and oil resistant. Such indicators areproduced in various forms, depending on theapplication. Often several labels are assembled in astrip to give the appearance of a thermometer,although the indication changes only at successivetrigger points.

10.2 Characteristics

Many labels are presently available, giving colourchanges within the range from 30 C to 300 C. Thechemicals are stable on heating up to the triggermelt-point, and unlike the paints described inclause 8, no change takes place until the triggertemperature has been reached. Also they are notappreciably time sensitive.

Thermal papers are stable and effective and can bemade to give indications under test conditions witha tolerance of 1 C up to 100C and 1 % at

higher temperatures.

Figure 5 Examples of trigger temperature-time relationships for a multi-change paint


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Publications referred to

BS 1902, Methods of testing refractory materials.

BS 1902-5.2,Determination of pyrometric cone equivalent (refractoriness) (method 1902-502).

ISO 528, Refractory products Determination of pyrometric cone equivalent (refractoriness).DIN 51063-1, Testing of ceramic raw and finished materials: Pyrometric cone of Seger (SK): Determinationof bending point (pyrometric cone equivalent).

DIN 51063-2, Testing of ceramic materials: Pyrometric reference of Seger (SK): Testing of Seger cones.


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[BS 1041-7-1988] -- Temperature measurement. Guide to selection and use of temperaturetime indicators.pdf