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CERAMIC SODIUM GRAPHITE FREEZING METALA9,Au,Cu TUBE HEAT PIPE CRUSIBLE THERMOCOUPLE WATER COOLING BODY CAVITY RADIATION SHIELD Standard resistance thermometers are used for interpolation

Calibration and Testing of

Temperature Measuring

Instruments

22.1 Definitions and Terminology

The following main terms are used in the calibration and testing of temperature measuringinstruments, necessary for the maintenance and dissemination of ITS-90

" Calibration of a thermometer is the sum of activities concerned with the determination

of its thermometric characteristics These characteristics define the function correlatingthe chosen property of the thermometer with the temperature If a thermometer directlyindicates the measured temperature, its calibration depends on correlating certainnumerical values with the scale graduation For example, this concerns liquid-in-glassthermometers

" Testing a thermometer is the sum of activities concerned with verifying that thethermometer complies with the relevant regulations

" Primary standards are thermometers used for reproduction of ITS-90, as well as forinternational comparisons

" Transfer standards are thermometers used for the transfer of temperature units to otherthermometers, which thus have lower accuracy than these standards They comprisesecondary, tertiary and other standards, which occupy important transfer levels in what

is called the chain of traceability of standards

" Working standards are thermometers destined for the calibration of other workingstandards, situated lower in the traceability hierarchy They are also used in thecalibration of industrial thermometers

" Industrial thermometers are thermometers used in the day-to-day practice of temperaturemeasurement

" Laboratory thermometers are thermometers used in laboratories

Calibration and testing procedures for thermometers comprise a general scheme whichdefines the hierarchy of thermometers They also determine the methods for and accuracy

Temperature Measurement Second Edition

L Michalski, K Eckersdorf, J Kucharski, J McGhee

Copyright © 2001 John Wiley & Sons Ltd ISBNs: 0-471-86779-9 (Hardback); 0-470-84613-5 (Electronic)

of, transferring the temperature unit from primary standards to industrial thermometers(Richardson, 1962; Gray and Chandon, 1972; Gray and Finch, 1972).

Most industrialised countries have established national standard laboratories which areequipped for reproducing ITS-90 through the calibration and testing of standardthermometers

The following are among the more well known Laboratories:

" Institut National de M6trologie (INM), France,

" Institute of Metrology "Mendeleiew", Russia,

" Istituto di Metrologia "G Collonetti" (IMGC), Italy,

" National Institute of Standards and Technology (NIST), USA,

" National Institute of Metrology (NIM), China,

" National Physical Laboratory (NPL), UK,

" National Research Council of Canada (NTC), Canada,

" Physikalisch-Technische Bundesanstalt (PTB), Germany

Industrial thermometers are tested in regional and industrial laboratories

22.2 Fixed Points of ITS-90

22.2.1 General information

Fixed points which define ITS-90 are given in Chapter 1

Cryogenic fixed points in the range from 13.8033 K (-259 3467 °C) to 83 8058 K(-189.3442 °C) are used for calibration of capsule-type platinum resistance sensors Theyconsist of two boiling points of H2 and the four triple points of H2, Ne, OZ and Ar.Although the triple point of Ar is often used in the calibration of long-stem platinumresistance sensors, the triple point ofOZ is very rarely used Cryogenic fixed points, belowthe Ar triple point, are not dealt with, as this book does not cover extremely lowtemperatures

Fixed points in the rangefrom 234.3156 K (- 38 8344 °C) to 1234.93 K (961 78 °C) areused for calibration of long-stem and high temperature platinum resistance sensors In thisrange there are two triple points, of Hg and H2O, one melting point, of Ga, and five freezingpoints, of In, Sn, Zn, Al and Ag

Fixed points in the rangefrom 961.78 °C to 1084.62 °C, which are used for calibration

of pyrometers, consist ofthe three freezing points ofAg, Au and Cu

Construction of a fixed point has to be adapted to the kind of calibrated sensor Whencalibrating resistance thermometer sensors, it is extremely important to provide as good aheat transfer as possible between the sensor and the medium applied in the fixed point Inthe calibration of pyrometers, the fixed points used should have all the properties

of a black body

FIXED POINTS OF ITS-90 421

22.2.2 Realisation of fixed points

Some of the more important and commonly used fixed points of Table 1.1 will now be described

The triple point of argon 83 8058 K (-189 3442 °C), for the calibration of long-stem platinum sensors is shown in Figure 22.1 (Bonnier, 1987) Argon of 99.9999 % purity is kept in a sealed cell of stainless steel, immersed in liquid nitrogen This cell should be able

to withstand pressures up to a maximum of 10 MPa, which occurs at maximum ambient temperature The triple point of argon is attained in the melting process from the solid state After supplying each pulse of energy to the cell with frozen argon, the cell temperature is measured Until the melting process commences, only smaller and smaller steps of energy are supplied Pavese et al (1984) point out that the temperature of the triple point of argon can then be determined by observing the temperature as a function of time during the melting process The protecting tube, in which the calibrated sensor is placed, is in direct contact with the liquid and solid phase of argon.

The triple point of mercury, 234.3156 K (-38 8344 °C), is realised in a glass cell as shown in Figure 22.2 (Preston-Thomas et al., 1990) A cell with mercury is thermally insulated and placed in a vacuum stainless steel sheath The degree of vacuum between the cell and the sheath is controlled to achieve the desired insulation between the cell and the surroundings For calibration the RTD sensor is placed in the protecting tube, which is filled with ethyl alcohol to enhance the thermal contact between both of them Very high purity mercury (1 to 108 ratio) allows the triple point of mercury to be obtained during either melting or solidifying The realisation of the Hg triple point, during solidification, proceeds

as follows The steel sheath is cooled down by a mixture of solidified COZ and ethyl alcohol At the moment when the ampule temperature attains the mercury solidifying point, the air contained between the cell and its sheath is pumped off A rod, cooled in liquid

HELIUM GAS INLET LONG-STEM PLATINUMRESISTANCE SENSOR

VACUUM SYSTEM VALVE

RING TUBE SEAL SENSOR TUBE ETHYL ALCOHOL INDIUM SEAL

PAPER INSULATION STAINLESS STEEL JACKET CONNECTION FOR CLEANING AND FILLING PAPER INSULATION

Cu FOIL CYLINDER BOROSILICATE GLASS CELL MERCURY

QUARTZ WOOL STAND

AL-SILICATE INSULATION

50mm

Figure 22.2 Triple point of mercury (Preston-Thomas et al., 1990)

nitrogen, is then introduced in place of the sensor This creates a layer of solidified mercury

in the ampule In the final stage of this procedure, the calibrated sensor, which has been cooled down beforehand in a mixture of solidified C02 and ethyl alcohol, is then introduced

in place of the rod The triple point of mercury in solidification is reproduced with a precision better than f0.1 mK Different ways of realising the Hg triple point as well as their precision are given by Hermier and Bonnier (1992) and by Furukawa(1992)

The triple point of water, 273 16 K (0.01 °C) , which is shown in Figure 22.3, is realised in a sealed glass cell filled with distilled water under vacuum After cooling the water down to about 0 °C a layer of some millimetres of ice is formed around the inside of the tube by means of the powdered solid C02 With the solid C02 removed, the inner tube

is filled with water at about +20 °C for a short time, until a thin layer of ice is melted and replaced by a thin layer of water The inner tube is then filled by water at 0 °C to enhance the heat transfer to the calibrated thermometer Keeping the cell in the ice-water mixture maintains the triple point temperature with an accuracy better than t0.1 mK to f0.3 mK for

FIXED POINTS OF ITS-90 423

at least 24 hours A more detailed description is given by Stimson (1956), Hall and Barber(1964) and Furukawa and Bigge (1982)

The melting point of gallium 29.7646 °C, is realised in the apparatus built in NPL(Chattle and Pokhodun, 1987) shown in Figure 22.4 Owing to an increase in the volume of

Ga on solidification, it is placed in an elastic teflon container Standardised thermometersare introduced into a nylon tube with a lining of Al The sealed cell which is immersed inmelting Ga, is filled with an atmosphere of pure argon Each cell is supplied withrecommendations from the manufacturer specifying the necessary immersion depth in thebath An accuracy in the reproduction of the melting point of gallium of about f0.4 mK can

be attained (Chattle and Pokhodun, 1987) A commercially produced gallium melting pointcell, which can be removed for freezing and then returned to the apparatus, holds themelting temperature for many hours, ensuring a precision of about fl mK

The freezing point of indiuM 156.5985 °C, consists of high purity indium (better than99.999 %) in either a graphite crucible (Chattle and Pokhodun, 1987) or in a teflon cell(Mangum, 1989) The indium container is placed in a special furnace After the ingot ismelted, the furnace temperature is stabilised about a degree below the freezing point Whenthe temperature indicated by a resistance sensor in a protecting sheath, placed in the cell,has fallen close to the freezing point, the sensor is withdrawn and allowed to cool for up to

1 min before being put into the cell again The loss of heat is sufficient to form a thin layer

of solid indium around the sensor well The plateau corresponding to the freezing pointtemperature is then quickly reached The freezing point of 99.999 % pure indium isreproducible to about f0.1 mK (Mangum, 1989; Hanafy et al., 1982)

The freezing point of tiny 231 928 °C, when based on tin of very high, 99.9999 %,purity, is reproducible to about 0.1 mK (Preston-Thomas, 1990) In realising this point, thephenomenon of large supercooling at the beginning of freezing, should be taken into

PUMPING TUBE NYLON CAP

- - TEFLON CONTAINER

OUTER NYLON CASE GALLIUM

consideration (McLaren and Murdock, 1960) The construction of this freezing point, which

is described by Marcarino (1992), is similar to the freezing point of zinc, shown in Figure22.5 It is important to achieve a high degree of supercooling (over 4 K) to attain the plateautemperatures by means at outside slow freezing To attain this supercooling the ingot should

be kept in an inert atmosphere and not topped with graphite powder

The freezing point of zing 419.527 °C, is realised with a reproducibility of about 2 mK

if zinc of very high purity (99 9999 %) is used (Preston-Thomas et al., 1990; McLaren,

1958, Ma and Lawlor, 1992; Furukawa et al., 1981 ; Marcarino, 1992) The apparatus used

to produce the fixed point should ensure the necessary zinc purity as well as a uniformtemperature distribution during its solidification, with equality of the temperatures of thesensor and the metal A tubular furnace with a copper block, which is shown in Figure 22.5,can ensure this uniformity of temperature Inside the block there is a graphite crucible with alid, through which a sheath for carrying calibrated sensors is inserted At the beginning ofmetal solidification, the observable under-cooling by some hundredths of a degree may beeliminated by withdrawing the calibrated sensor for a short time After reaching ambienttemperature the sensor is reinserted The temperature which then quickly rises to thefreezing point, stays constant at the freezing point temperature for a long period of time.Zinc purity should be periodically tested

The freezing point of aluminiuiA 660.323 °C, is produces in a similar way as for zinc.Graphite fibre insulation and a graphite crucible are used combined with an atmosphere ofargon gas to prevent oxidation The reproducibility of this point is about 1 mK with thetemperature stability at the beginning of freezing of 0.2 mK (Ancsin,1992; McAllan andAmmar,1972; Furukawa, 1974) No contact of molten aluminium with moisture, oxygenand silicon ceramic can exist

The freezing point of silver, 961 78 °C, is realised in a similar apparatus to that shown

in Figure 22.5, but with a higher temperature furnace To get high temperature uniformity inthe middle part of furnace, where the crucible with the silver is placed, it is advisable toinstall a heat pipe screen around the crucible Even though silver does not oxidise easily, itshould still be protected from air contact, since it absorbs oxygen in the molten condition,

SENSOR TUBE A1203 -POWDER ELECTRIC FURNACE LID

GRAPHITE CRUCIBLE MOLTEN ZINC COPPER BLOCK

CERAMICS THERMAL INSULATION

t

Figure 22.5 The freezing point ofzinc

PRIMARY STANDARDS 425resulting in depression ofthe freezing point As oxygen absorption starts at a temperature ofabout 30 °C above the melting point, any unnecessary overheating of the metal should beavoided If the ingot is kept in the molten state in an inert gas, the surrounding graphite willeffect the complete removal of the oxygen within a few hours The constant temperaturelevel, corresponding to the freezing point of silver, is reached in a time period, covering 20

to 60 % of the total solidifying time, with an accuracy of 1 mK (Preston-Thomas et al.,1990)

The freezing point of silver, 961 78 °C, gold, 1064.18 °C, and copper, 1084.62 °Cwhich are intended for the calibration of pyrometers, are each realised in water cooledelectric resistance furnaces with a graphite chamber inside as shown in Figure 22.6(Ohtsuka and Bedford, 1982) The chamber has all the properties of a black body Thegraphite black body and crucible containing the metal are made ofgraphite ofhighest purity

so that an attainable emissivity of about 0.99999 is achieved (Lee, 1966) The crucible isplaced in a sodium heat-pipe liner, giving a uniform temperature of the crucible walls.Additional blocks and rings of graphite keep oxygen from reaching the crucible Thethermal insulation of the furnace is made of quartz wool and inconel heat shields Duringoperation nitrogen or argon flows slowly (0.1 litre/min) along the furnace length to inhibitgraphite oxidation The temperature is measured by a type S thermocouple The calibratedpyrometer is aimed at the bottom of the graphite chamber

CERAMIC SODIUM GRAPHITE FREEZING METAL(A9,Au,Cu) TUBE HEAT PIPE CRUSIBLE THERMOCOUPLE

WATER COOLING BODY CAVITY RADIATION SHIELD

Standard resistance thermometers are used for interpolation from -259.3467 °C to

961 78 °C, which is the freezing point of silver Following ITS-90 the thermometer resistormust be strain free, annealed, pure platinum and wound from 0.05 to 0 5 mm Pt wire It isadvisable to use resistors of 25 S2, at 0 °C In the upper temperature range, 0.1 to 2.5 S2resistors are recommended The resistor should be enclosed in a hermetic sheath filled by

dry, neutral gas with an addition of oxygen At the lower temperature range, up to 13 K, itshould be helium filled The resistor should be annealed at a temperature higher than thehighest expected working temperature, but in any case never below 450 °C (except forcryosensors) The quality of a sensor, its design and annealing are verified duringcalibration, determining the constants from interpolation equations and checking thestability of the resistance (Curtis, 1972; Foster, 1972).

Depending on the working temperature range, there are three types of resistancetemperature sensors:

" -260 °C to 0 °C - low temperature capsule-type sensors,

" -190 °C to 600 °C - normal long-stem sensors,

" +600 °C to 960 °C - high temperature sensors

Capsule-type resistance sensors, which are beyond the scope ofthis book, are described byCurtis (1972), Hust (1970) and Sparks and Powell (1972)

Long-stem-type resistance sensors, which are used as interpolation standards of ITS-90from -190 °C to 600 °C, have undergone many modifications to increase their accuracy andstability, to reduce their size and to intensify the heat transfer between the resistor andsheath and between the sheath and environment

A typical contemporary design is presented in Figure 22 7 Platinum wire, wound in aspiral of about 1 mm diameter is placed in a thin-walled Pyrex tube matching the spiraldiameter and shaped as shown in Figure 22.7 Platinum terminals, which are soldered toboth spiral ends, are sealed in glass in such a way that the spiral is totally strain free It isalso important for the spiral to remain strain free during its subsequent working life Theseterminals are extended by low resistance gold wires in ceramic insulation

The whole assembly, which is encapsulated in a glass sheath, is hermetically sealed aftercareful drying at about 400 °C The resistance of the sensor is about 25 0 at 0 °C As anexample, the standard Pt resistance sensor produced by Rosemount Inc (USA) (Berry,1982) is hermetically sealed in a metal sheath containing a helium-oxygen atmosphere Itsstability within the specified temperature range of -200 °C to +650 °C is better than 0.01 °Cper year The self-heating increase in temperature is less than 0.002 °C with an insulationresistance from the resistor to the outside sheath greater than 5000 MQ at 100 V dc, whileits nominal resistance is about 25 S2 at 0 °C

High temperature resistance sensors, operating from 600 °C to 960 °C replace theS-type thermocouple as a primary standard ofITS-90 Design ofhigh temperature resistancesensors is the subject of many publications (Arai, 1997; Anderson, 1972, Evans and Burns,

Au-WIRES Pt-WIRES GLASS SHEATH d=6mm

CERAMIC INSULATOR Pt-SPIRAL PYREX TUBE

Figure 22.7 Standard long-stem-type resistance sensor

PRIMARY STANDARDS 4271962; Chattle, 1972; Curtis, 1972; Evans, 1972; Furukawa et al., 1981; Strouse et al.,1992) One of the designs proposed by Nubbemeyer (1992) is shown in Figure 22.8.The resistor is composed of a bipolar spiral winding of 0.4 mm diameter platinum wire,supported by a notched quartz blade Two Pt wires of the same diameter, which are welded

to both ends of the spiral, are extended by two 75 cm long, 0.35 mm diameter Pt wires.These wires are insulated by quartz tubes passing through 9 quartz disks placed along thesensor After the sensor has been annealed at 700 °C, the external 7 mm diameter quartzsheath is hermetically sealed and the tube filled with a gas mixture of 90 % Ar and 10 % Oz.The sensor resistance is 0.25 SZ at 0 °C

Resistance sensors for interpolation in ITS-90, whose resistance is measured by an acbridge of highest precision in a four-wire circuit, are calibrated at relevant fixed pointswithin the sensor application range An example of the bridge is the F 18 bridge produced byAutomatic Systems Laboratories Ltd (1999) having the following technical data:

" accuracy better than + 0.25 mK,

" resolution: 0.75 pK,

" measuring range: 0-390 fl (for Ra = 2.5-100 S2 in the temperature range : 13 K-960 °C),

" frequencies: 25/75 Hz or 30/90 Hz,

" possibility of measuring temperature difference,

" automatic or manual balancing,

TWO PARTS

OF WINDING

Figure 22.8 Standard high temperature resistance sensor (Nubbemeyer, 1992)

Standard pyrometers and their calibration methods have undergone many modifications in recent years The photoelectric spectropyrometer, which was the early type, is based on the principle of the disappearing filament pyrometer, where a photoelectric detector replaces the human eye, as described by Hahn et al (1992), Kandyba and Kowalewski (1956), Lee (1966), Lee et al (1972) and Nutter (1972) This early type has now been replaced by a narrow-band photoelectric pyrometer with either a photomultiplier detector or by the increasingly popular Si detector, which is characterised by high sensitivity, high stability and good linearity (Coslovi and Righini, 1980; Jung 1979)

Standard photoelectric pyrometers, which have an optical system like that shown in Figure 22.9 (Rosso and Righini, 1985), also have an accuracy better than 0.1 K in the temperature range of 800 to 1400 K and about 1 K at a level of 2000 K similar to others (Zhao et al., 1990, 1992) The silicon detector of the pyrometer, which is placed in a thermally stabilised housing, gives an equivalent stability of 0.1 K per month and some tenths of kelvin per year Methods of pyrometer calibration at the fixed points are considered by Bussolino et al (1987).

Although pyrometers are mainly calibrated for measuring temperature, some pyrometers may also be used as instruments for comparing heat fluxes (Preston-Thomas et al., 1990 ; Zhao et al., 1992).

Standard tungsten strip lamps are used for interpolation in the temperature range from 1337

to 2600 K Vacuum lamps can be used up to about 2000 K, whereas above this temperature the use of gas-filled lamps, shown in Figure 22.10, is advised The strip length must be big enough to prevent any substantial influence of ambient temperature on the strip temperature

A `place' is also marked on the strip where the measurements should be made The sighting angle of the pyrometer is given by two points One point is on the sighting window and another is on the strip These two points should coincide during measurements To prevent any reflection of the radiation, which might be a source of errors, both lamp windows are situated at an angle of 5° to the lamp axis The dependence of the temperature of the lamp's strip upon the lamp current is called the thermometric characteristic of the lamp To achieve high stability of this characteristic the lamps are degassed many times during the production process before being finally glued and annealed (Quinn and Lee, 1972) Calibration of tungsten strip lamps is made by a comparison method based on the readings of a photoelectric spectropyrometer and simultaneous measurement of the lamp current at different strip temperatures.

OBJECTIVE SHUTTER IN TEM E RA RLENS MIRROR APERTURE PETURE

"' DIAPHRAGM STOP CONTROLLEDENCLOSURETARGET

INTERFERENCE EYE PIECE MIRROR

Ir

MIRROR Figure 22.9 Standard photoelectric pyrometer - optical arrangement (Rosso and Righini, 1985)

Figure 22.10Standard tungsten strip lamp

Tungsten strip lamps must be fed by a direct current, maintaining the correct polarityduring the measurements, because the temperature distribution along the strip depends uponthe current direction through the Thomson effect:

22.4 Working Standards

The primary standards described above are used both for the realisation of ITS-90 as well

as for international comparisons Working standards are usedfor calibration and testing ofother thermometers

Resistance sensors are used in the temperature range from -190 to +960 °C for thecalibration and testing of resistance sensors of lower accuracy Below 0 °C they are used forthe calibration and testing of type T thermocouples and semiconductor thermometers Thenominal value ofthe reference resistance, Ro (at 0 °C), ofthese sensors is about 10 0, 25 0

or 100 S2 These values do not apply to first order sensors (transfer standards) from 0 to

650 °C, for which only 10 S2 and 25 0 are permitted This is also true for high-temperaturesensors of lower resistance General outlines of the design of working standard resistors arealso applicable to the design of standard resistance thermometers Calibration of resistancesensors, can be made at fixed points In many laboratories the ice-point is used instead ofthe triple point of water The comparison method in testing baths is also used, comparingthe readings of the thermometer to be calibrated with those of a thermometer of higheraccuracy Calibration thermostats to be used are described in Section 22 6.2 To avoiderrors arising from conduction losses along the sheath and leads, the recommendedminimum depth of immersion of sensors is about 20 cm All sensors should be annealedprior to calibration at 500 °C for about 30 min Self-heating errors are usually less than0.004 °C, provided the sensor resistance for which Ro (at 0 °C) is about 25 S2 are generallymeasured at a current of 1 mA dc

Mercury-in-glass thermometers (-30 to +630°C) and mercury-thallium-in-glassthermometers (-55 to +30°C) are used as transfer standards for calibration and testing ofother glass thermometers, as well as that of manometric, resistance and thermocouplethermometers Different liquid-in-glass thermometers are used Their permissible errors are

±0 01 °C to ±3 °C To increase the accuracy of measurement by mercury-in-glassthermometers, errors due to zero changes, external pressure variations and variations in

temperature of the mercury column should be eliminated Detailed information on thedesign of precision mercury-in-glass thermometers, including consideration ofthe glass andquartz used, as well as measuring technique and errors is given by Hall and Barber (1964).Calibration of glass thermometers is conducted at fixed points or by the comparisonmethod in baths.

Thermocouple sensors used as working standards are types S, R and B and some otherthermocouples of non-rare metals They are calibrated against a resistance thermometer and

at higher temperatures against standard optical pyrometers Types S and R working standardthermocouples are used in the temperature range from 300 to 1300 °C for the calibrationand testing of the same types of thermocouples of lower accuracy as well as otherthermocouples They have to be annealed carefully, in most cases by direct current flow.They are mounted in twin-hole insulation of pure A1z03 The thermocouple conductorsrange in diameter from 0.35 mm to 0.65 mm

The accuracy of different working standard thermocouples, such as those produced byIsothermal Technology Ltd (1999a), are:

" rare metal types:

Tungsten strip lamps are used in the temperature range from about 850 to 2800 °C forthe calibration and testing of similar lamps of lower accuracy as well as for calibration andtesting of pyrometers These working standard lamps which are constructed in a similar way

to standard tungsten strip lamps, are calibrated by standard photoelectric pyrometers.Disappearing fllament pyrometers are used in the temperature range from about 800

to 2600 °C for the calibration of working standard tungsten strip lamps and for testing ofsimilar pyrometers of lower accuracy Grey filters are used for expanding their measuringrange These pyrometers are calibrated against standard tungsten strip lamps, based on theirthermometric characteristics Now they have been almost entirely replaced by the Sidetector pyrometer

Photoelectric silicon detector pyrometers, operating in a similar way to the standardpyrometers shown in Figure 22.9, are used for temperatures over 960 °C Recently a newpyrometer with a detector of InGaAs as well as a number ofnew black-body radiators havebeen described All of them are to be used for calibration in temperature range of 275-

960 °C (McEvoy et al., 1997; Fischer and Gutschwager, 1997)

TESTING OF INDUSTRIAL THERMOMETERS 43122.5 Testing of Industrial Thermometers

22.5.1 Introduction

During normal operation industrial thermometers are subject to various factors, such as high temperatures, mechanical and chemical influence and so on Under the impact of such factors, their thermometric characteristics can significantly vary Thus they have to be checked periodically and recalibrated or repaired if needed The frequency of such periodical checking, which depends upon the working conditions, should be determined by the maintenance service.

Industrial thermometer testing is achieved by the fixed points method or by the comparison method

Fixed points testing uses some fixed points of ITS-90, described in Section 22.2.1, but realised with lower accuracy These are combined with some additional fixed points listed

in Auxiliary Table XXI, as well as freezing points of some chemical compounds The most popular instruments used in this method are described in Section 22.6.1 Testing at fixed points, especially at the freezing points of metals, is precise and free from the subjective judgement of operators However, because of the expensive equipment required and time consuming nature of the measurements, it is not so popular for industrial thermometers Comparison methods for testing industrial thermometers is based on a comparison of the readings of tested and reference thermometers, while ensuring that the sensitive sensor parts are always kept at exactly the same temperature This condition is difficult to satisfy especially at higher temperatures The tested and reference thermometers are placed in a testing bath, fluidised bed thermostat or a testing furnace, depending on the necessary temperature range The testing accuracy depends upon the accuracy of the standard thermometer, on uniformity and constancy of temperature and, to a great extent, on the ability and experience of the operator.

Industrial pyrometer testing by the comparison method requires that both the standard and testing pyrometers are directed at the same radiation source In Section 22.6.4 some black body radiation sources are described.

22.5.2 Variable volume thermometers

Testing of liquid-in-glass thermometers comprises visual examination and testing of thermometer accuracy.

Visual examination covers control of the scale and of its position relative to the capillary In mercury-in-glass thermometers the mercury column should neither be broken nor contain observable impurities If dirt is observed or if moisture traces exist in the capillary of any thermometer it must be rejected Before testing, breaks in the mercury column must be rejoined.

Thermometer accuracy is tested by the comparison method using a stirred-liquid bath and two standard liquid-in-glass thermometers described in Section 22.6.2 Readings are taken while the temperature is slowly rising, completing two measuring cycles During each cycle

the routine of measurements should be : standard thermometer No 1, tested thermometers,standard thermometer No 2, standard thermometer No 2 again, tested thermometers inreverse succession and standard thermometer No l Calculated mean values of the fourreadings for each tested thermometer correspond to the mean temperature valuesdetermined by the standard thermometers.

In the range -200 to +90 °C, the thermometers must be tested while totally immersed.Over +90 °C they are tested, while partially immersed, with the average value of thetemperature of the emergent liquid column also being measured

Testing of manometric thermometers consists of visual examination, testing of thethermometer accuracy, testing ofthe hysteresis of the indications and testing of the variationofthe indications

Visual examination includes verifying the correctness of the markings The general state

of the thermometer, including the capillary, pointer, scale and elastic element, should also

be assessed

Testing ofaccuracy and hysteresis is by the comparison method in a stirred-liquid bath.Because of large time lags in the indications of manometric thermometers during theirtesting, it is important to keep the sensor in the bath for as long as necessary until thereadings are stabilised Meanwhile the bath temperature should be kept constant Foraccuracy tests, the mean values of readings at each temperature are considered Forhysteresis testing, measurements are taken at several temperatures, starting at the lowestone, going up to the highest and then reversing the procedure on the way back down.The difference in the indications between increasing and decreasing temperature, is ameasure ofhysteresis

Indication variations are determined by measuring the same temperature several timesunder constant measuring conditions and observing differences in the readings Theaccuracy and variations of the indications should be evaluated from sensor and read-outinstrument, obtained at the same level

22.5.3 Resistance thermometer sensor

Testing of resistance thermometer sensors requires visual examination, testing thethermometric characteristic of the resistor, checking the stability of the resistance, testingfor self- heating errors and testing of both the break- down strength of the sensor insulationand insulation resistance

During visual examination of the sensor, its general state is checked and defectsobserved Correct marking of the working range, type of resistor, sheath material and so on,are to be verified

Testing ofthe thermometric characteristic has to verify that it conforms to the standard

EN 60751 with permissible tolerances Usually the testing is performed by the comparisonmethod in liquid baths and in metal block calibrators

Resistance stability checking (only for new sensor construction), which is performed ifthe resistor is used over 300 °C is made by measuring the resistanceRoat 0 °C before andafter 250 hours of heating at the lowest and the highest working temperature of the sensorand before and after ten cycles of heating in both these temperatures The changes ofresistance, R, should not exceed one-fourth of the permissible resistance tolerance, asdefined by EN 60751

TESTING OF INDUSTRIAL THERMOMETERS 433

Testing of the self-heating error (only for new sensor construction) is described in detail

in standard EN 60751

Testing ofbreak-down strength and of resistance ofthe sensor insulation has to be done

to ensure conformity with relevant national standards

To measure the resistance ofthermometer detector calibrators (Section 22.7), bridge or voltage comparison systems equipped with digital read-out instruments as well as computerised data acquisition and processing systems, as described in Chapter 13, are increasingly popular.

22.5.4 Thermocouples

Testing of thermocouples (Roeser and Wensel, 1941; Standard EN 60584) involves visual examination, testing of emf versus temperature characteristics and testing of sensor insulation resistance

Visual examination is carried out after removing the thermocouple from its sheath and removing the ceramic wire insulators Thermocouples exhibiting stains and scale are rejected.

Before testing the emf versus temperature characteristic, rare metal thermocouples are chemically cleaned by 50 % hydrochloric acid, carefully washed in distilled water and annealed by direct current flow Type S and R thermocouples are annealed at 1150±50 °C, which corresponds to a current of about 1 l A for a diameter of 0.3 mm Type B thermocouples B are annealed at 1400±50 °C, which corresponds to about 13.5 A for 0.5 mm diameter wires.

The emf versus temperature characteristic, has to be compared with the standard

EN 60584 Testing methods used are the comparison method, the differential comparison method, the measurement ofemfatfixed points and the measurement ofemfatfixed points

by the wire method.

The comparison method is used for all of the following standardised industrial thermocouples in the manner described below.

Type S, R and B thermocouples in ceramic insulators are placed, together with the standard thermocouple in a tubular electrical furnace, having a normal working temperature of at least 1200 °C The measuring junction of the thermocouples to be tested, should be placed simultaneously in the middle of the furnace length described in Section 22.6.3 and should not exceed five in number To ensure that the temperature of the measuring junctions of standard and tested thermocouples is equal, they should be bound by platinum wire or welded together.

Type K thermocouples are put in ceramic insulators, together with the standard Type K thermocouples, and placed in a metal block in a tubular furnace described in Section 22.6.3,

so that the measuring junctions are in direct contact with the metal block The working temperature ofthe furnace should be at least 1000 °C If a Type S thermocouple is used as a standard thermocouple, it is also placed in one of the block holes In that case the thermocouple is not bare but is placed in a gas-tight glass or ceramic sheath.

To ensure better equalisation of the temperature of tested and standard thermocouples,

in more precise measurements, the measuring junction of the standard thermocouple is placed in a hole, drilled in the junction ofthe tested thermocouple, which is always larger

Contamination is prevented by protecting the wires of the standard thermocouple withrefractory cement Inhomogeneity in the junction itself does not influence the results as long

as no temperature differences occur in it When testing a number of thermocouples, they can

be welded together to form one common measuring junction

Type J, T and E thermocouples are tested in baths, up to 300'C as described inSection 22.6 2, using a mercury-in-glass thermometer as the standard From 300'C to

700 °C, Type J thermocouples are tested in the same way as Type K units The number ofsimultaneously tested thermocouples should not exceed six

In all thermocouple testing, their reference junction temperature should be atthe ice point

Formeasuring the thermoelectric force, the specialised digital voltmeters or calibratorsdescribed in Section 22.7, are used Computer programs also exist, enabling fullautomisation of measurements (Isothermal Technology Ltd, 1997b) Thermocouple testing

by the comparison method is more and more frequently conducted using electric furnaceswith programmed temperature control (Automatic Systems Laboratories Ltd, 1999; Jonesand Egan, 1975; Kirby, 1982; Techne (Cambridge) Ltd, 1999a; TMS Europe Ltd, 1997) The differential comparison methodis only used for rare-metal thermocouples

with the same type of thermocouple used both as the standard and as the testedthermocouples Conductors with the same polarity for both the standard and testedthermocouples, up to four at a time, are bound together by a platinum wire as near themeasuring junctions as possible to ensure as good a thermal and electrical contact aspossible The same type of tubular furnace is used as in the comparison method.Measurements of the differential emf values between conductors of the same polarity of thestandard and the tested thermocouples are taken from the first to the last thermocouple andthen in reverse succession At the beginning and at the end of each cycle, the true-furnacetemperature is measured by the standard thermocouple The relevant electric circuit for thethermocouples is shown in Figure 22.11

Following the American National Standard ASTM E220-80, the differential comparisonmethod has the following advantages, relative to the comparison method:

" Measured differential emfs are small relative to the relevant thermocouple emf at thegiven temperature and thus do not need to be measured very precisely

" During testing, much higher rates of temperature increase can be applied, because thedifferences of the thermoelectric characteristics of the tested and the standardthermocouples vary insignificantly as a function of temperature Also, the furnacetemperature does not need to be precisely measured

90%Pt10%Rh = const.

b

MEASURING INSTRUMENT V/-

TESTED THERMOCOUPLES

Figure 22.11 Testingof Stype thermocouples by differential comparison method

TESTING OF INDUSTRIAL THERMOMETERS 435

Emf measurement at fixed points is very popular, especially in USA (Richardson,

1962 ; Trabold, 1962) The tested thermocouple is immersed consecutively in crucibles with metals and salts of different freezing temperatures The tested thermocouples should be immersed to an adequate depth to prevent heat flow from the measuring junction along the thermocouple conductors Any small changes in immersion depth should not affect the measured emf values provided this depth is adequate In recent times, new types of miniature slim cells, with the fixed points described in Section 22.6 1, have been developed These are mainly intended for testing rare-metal thermocouples The emf values, corresponding to relevant fixed point temperatures, are recorded as a function of time with the horizontal part ofthe emf versus temperature curve determining the sought emf value Emf measurement at fixed points by the wire method (Hall and Barber, 1964; Roberts, 1980; Trabold, 1962) is mainly used for rare-metal thermocouples The measuring junction of the tested thermocouple is cut in two with a short pure metal wire then soldered

to both thermocouple conductors as in Figure 22 12(a) After being prepared in this way, the thermocouple is slowly heated up in a tubular furnace, till the inserted wire melts The recorded emf versus time value clearly indicates the constant temperature part of the curve, till the circuit is broken Instead of wire an Au, Pd or Pt plate can also be used as shown in Figure 22.12(b) and (c).

In-situ testing of thermocouples is becoming increasingly more important Thermocouples should not be tested in a laboratory environment after a longer working life

or after being contaminated.

The emf of a non-homogeneous thermocouple depends upon the temperature distribution along the thermocouple and thus the emf values measured by the laboratory method do not correspond to in-situ readings Testing is made by the comparison method in which the standard thermocouple is placed alongside the tested one so that their measuring junctions are at the same temperature At a constant measured temperature, both thermocouples can be placed alternately at the same place If many thermocouples are used

in one installation, an additional empty sheath is introduced to hold the standard thermocouple during testing Such an empty ceramic sheath is sometimes placed together with two other sheaths in one common outer ceramic tube This third sheath which is kept empty

is only to house the standard thermocouple temporarily during testing (K6rtvelyessy, 1987) Testing should be carried out at some temperatures distributed over the working range Although the method described above is not as precise as the laboratory method, it is very useful in many cases If a temperature sensor is always connected to the same measuring instrument then the whole installation is tested instead of only testing the thermocouple Readings of the whole installation are compared with those of a standard thermometer

(a) WELDED OR IN WELDED (c) LOCKED

Figure 22.12 Testing ofthermocouples by the wire method