Page 1/16 JUMO GmbH & Co. KG Delivery address:Mackenrodtstraße 14, 36039 Fulda, Germany Postal address: 36035 Fulda, Germany Phone: +49 661 6003-0 Fax: +49 661 6003-607 e-mail: [email protected]Internet: www.jumo.net JUMO Instrument Co. Ltd. JUMO House Temple Bank, Riverway Harlow, Essex CM 20 2TT, UK Phone: +44 1279 635533 Fax: +44 1279 635262 e-mail: [email protected]Internet: www.jumo.co.uk JUMO PROCESS CONTROL INC. 885 Fox Chase, Suite 103 Coatesville PA 19320, USA Phone: 610-380-8002 1-800-554-JUMO Fax: 610-380-8009 e-mail: [email protected]Internet: www.JumoUSA.com Data Sheet 90.1000 11.03/00073412 The thermoelectric effect The effect responsible for the action of ther- mocouples is the Seebeck effect. If a term- perature difference exists along a wire, this will cause a displacement of electrical char- ge. The amount of the charge displacement depends on the electrical characteristics of the chosen material. If two wires of different materials are joined at one point and then subjected to a temperature, then a voltage difference will be generated between the open ends of the two wires. This voltage de- pends on the temperature difference along the two wires. In order to be able to measu- re the temperature at the junction, the tem- perature at the open end must be known. If the temperature of the open end is not known, then it must be extended (by a com- pensating cable) into the zone of known temperature (reference junction, usually re- ferred to as the “cold junction”). Fig. 1: Measuring circuit (schematic) The temperature of the reference junction must be known and constant. If no constant reference junction temperature is available, the reference junction has to be arranged as a thermostat, or its temperature has to be determined by means of a second sensor. Thermocouples to EN 60 584 and DIN 43 710 From the variety of possible metal combina- tions, certain ones have been selected (Ta- bles 1 and 2) and their voltage tables and permitted tolerances incorporated in stan- dard specifications (Fig. 2 and Tables 3 and 4). Note that two Fe-Con thermocouples (Type J and L) and two Cu-Con thermocouples Reference junction Termination Measuring junction (Type T and U) have been standardized in both EN 60 584 and DIN 43 710. The “old” thermocouples L and U are now being used less frequently than the ther- mocouples J and T to EN 60 584. The individual thermocouples are not com- patible, because of their differing alloy compositions. If a Fe-Con thermocouple Type L is connected to an instrument lin- earized for Type J, the difference in the thermal voltages leads to errors of up to several °C. The same applies to thermo- couples Type U and T. The maximum temperature represents the limit to which a tolerance is specified. The value under “defined to” is the temper- ature limit to which the thermal voltage is covered by standard specifications. In the thermocouples listed above, the first limb is always the positive one. The color codes apply both to the thermocouple it- self and to the compensating cables. If the thermocouple wires are not color coded, the following differences may help to iden- tify them. Fe-Con: positive limb is magnetic Cu-Con: positive limb is copper colored NiCr-Ni: negative limb is magnetic PtRh-Pt: negative limb is softer These distinctions do not apply to the com- pensating cables. The thermocouples are insulated inside the fittings using ceramic materials. PVC, sili- cone, PTFE or glass fiber are used in the cables. Tolerances EN 60 584 defines three tolerance classes for thermocouples. They normally apply to thermowires between 0.25 to 3mm diame- ter and to the condition as supplied. The standard cannot cover any possible subse- quent ageing, since this largely depends on the conditions of use. The temperature lim- its specified for the tolerance classes are not necessarily the recommended operat- ing temperature limits (see Tables 3 and 4). The larger value applies in each case. Fig. 2: Tolerances Temperature/°C Tolerance/°C Type T Type B, R, S Type E, J, K, N Construction and application of thermocouples Table 1: Thermocouples to EN 60 584 * Continuous temperature in pure air Table 2: Thermocouples to DIN 43 710 Thermocouple Maximum temperature Defined up to Positive limb Negative limb Fe-Con J Cu-Con T NiCr-Ni K NiCr-Con E NiCrSi-NiSi N Pt10Rh-Pt S Pt13Rh-Pt R Pt30Rh-Pt6Rh B 750°C 350°C 1200°C 900°C 1200°C 1600°C 1600°C 1700°C 1200°C 400°C 1370°C 1000°C 1300°C 1540°C 1760°C 1820°C black brown green violet mauve orange orange no data white white white white white white white white Thermocouple Maximum temperature Defined up to Positive limb Negative limb Fe-Con L Cu-Con U 700°C 400°C 900°C 600°C red red blue brown
The thermoelectric effectThe effect responsible for the action of ther-mocouples is the Seebeck effect. If a term-perature difference exists along a wire, thiswill cause a displacement of electrical char-ge. The amount of the charge displacementdepends on the electrical characteristics ofthe chosen material. If two wires of differentmaterials are joined at one point and thensubjected to a temperature, then a voltagedifference will be generated between theopen ends of the two wires. This voltage de-pends on the temperature difference alongthe two wires. In order to be able to measu-re the temperature at the junction, the tem-perature at the open end must be known. Ifthe temperature of the open end is notknown, then it must be extended (by a com-pensating cable) into the zone of knowntemperature (reference junction, usually re-ferred to as the “cold junction”).
Fig. 1: Measuring circuit (schematic)
The temperature of the reference junctionmust be known and constant. If no constantreference junction temperature is available,the reference junction has to be arranged asa thermostat, or its temperature has to bedetermined by means of a second sensor.
Thermocouplesto EN 60584 and DIN 43710From the variety of possible metal combina-tions, certain ones have been selected (Ta-bles 1 and 2) and their voltage tables andpermitted tolerances incorporated in stan-dard specifications (Fig. 2 and Tables 3 and4).Note that two Fe-Con thermocouples (TypeJ and L) and two Cu-Con thermocouples
Reference junction
Termination
Measuring junction
(Type T and U) have been standardized inboth EN 60584 and DIN 43710.The “old” thermocouples L and U are nowbeing used less frequently than the ther-mocouples J and T to EN 60584.
The individual thermocouples are not com-patible, because of their differing alloycompositions. If a Fe-Con thermocoupleType L is connected to an instrument lin-earized for Type J, the difference in thethermal voltages leads to errors of up toseveral °C. The same applies to thermo-couples Type U and T.
The maximum temperature represents thelimit to which a tolerance is specified.The value under “defined to” is the temper-ature limit to which the thermal voltage iscovered by standard specifications.In the thermocouples listed above, the firstlimb is always the positive one. The colorcodes apply both to the thermocouple it-self and to the compensating cables. If thethermocouple wires are not color coded,the following differences may help to iden-tify them.
Fe-Con: positive limb is magneticCu-Con: positive limb is copper coloredNiCr-Ni: negative limb is magneticPtRh-Pt: negative limb is softer
These distinctions do not apply to the com-pensating cables.The thermocouples are insulated inside thefittings using ceramic materials. PVC, sili-cone, PTFE or glass fiber are used in thecables.
TolerancesEN 60584 defines three tolerance classesfor thermocouples. They normally apply tothermowires between 0.25 to 3mm diame-ter and to the condition as supplied. Thestandard cannot cover any possible subse-quent ageing, since this largely depends onthe conditions of use. The temperature lim-its specified for the tolerance classes arenot necessarily the recommended operat-ing temperature limits (see Tables 3 and 4).The larger value applies in each case.
Fig. 2: Tolerances
Temperature/°C
Tole
ranc
e/°
C
Type T
Type B, R, S
Type E, J, K, N
Construction and application of thermocouples
Table 1: Thermocouples to EN 60584
* Continuous temperature in pure airTable 2: Thermocouples to DIN 43710
Thermocouple Maximumtemperature
Definedup to
Positivelimb
Negativelimb
Fe-Con JCu-Con TNiCr-Ni KNiCr-Con ENiCrSi-NiSi NPt10Rh-Pt SPt13Rh-Pt RPt30Rh-Pt6Rh B
Fig. 3: Characteristics of thermocouples to EN 60584
Thermocouple Tolerance classesFe-Con J Class 1
Class 2Class 3
- 40 to + 750°C:- 40 to + 750°C:
±0.004 x t±0.0075 x t
or ±1.5°Cor ±2.5°C
Cu-Con T Class 1Class 2Class 3
- 40 to + 350°C:- 40 to + 350°C:-200 to + 40°C:
±0.004 x t±0.0075 x t±0.0015 x t
or ±0.5°Cor ±1.0°Cor ±1.0°C
Ni-CrNi KandNiCrSi-NiSi N
Class 1Class 2Class 3
- 40 to +1000°C:- 40 to +1200°C:-200 to + 40°C:
±0.004 x t±0.0075 x t±0.015 x t
or ±1.5°Cor ±2.5°Cor ±2.5°C
NiCr-Con E Class 1Class 2Class 3
- 40 to + 800°C:- 40 to + 900°C:-200 to + 40°C:
±0.004 x t±0.0075 x t±0.015 x t
or ±1.5°Cor ±2.5°Cor ±2.5°C
Pt10Rh-Pt SandPt13Rh-Pt R
Class 1Class 2Class 3
0 to +1600°C:- 40 to +1600°C:
±[1+(t-1100) x 0.003]±0.0025 x t
or ±1.0°Cor ±1.5°C
Pt30Rh-Pt6Rh B
Class 1Class 2Class 3
+600 to +1700°C:+600 to +1700°C:
±0.0025 x t±0.005 x t
or ±1.5°Cor ±4.0°C
Thermocouple TolerancesCu-Con U +100 to +400 °C: ±3°C
+400 to +600 °C: ±0.0075 x tFe-Con L +100 to +400 °C: ±3°C
+400 to +900 °C: ±0.0075 x t
Temperature/°C
Ther
mal
vol
tage
/mV
NiCr-Con E
Fe-Con J
NiCr-Ni K
NiCrSi-NiSi N
Pt13Rh-Pt R
Pt10Rh-Pt S
Pt30Rh-Pt6Rh B
LinearityThe voltage produced by a thermocoupleis not linear with temperature and musttherefore be liniearized by the subsequentelectronics. Digital instruments are pro-grammed with linearization tables, or ap-propriate calibration values have to beentered by the user. Analog instrumentsare often provided with non-linear scales.The characteristics of thermocouples (Fig.3) are defined by voltage tables to ensurefull interchangeability.
This means, for example, that a Fe-Conthermocouple Type J can be replaced byany other thermocouple of this type irre-spective of the manufacturer, without re-quiring any recalibration of the instrumentto which it is connected.
Compensating cablesto EN and DINCompensating cables for thermocoupleshave their electric and mechanical proper-ties defined in the EN 60584 or DIN 43714standards. They are made either of thesame material as the thermocouple itself(thermocables, extension cables) or fromspecial materials with the same thermo-electric properties within restricted tem-perature ranges (compensating cablesproper). The use of compensating cablessaves the extra cost in the case of certainnoble metals.Compensating cables consist of twistedcores and are identified by a color codeand code letters as follows:
Letter 1: code letter for thethermocouple
Letter 2: X: same material asthermocoupleC: special material
Letter 3: several types of compensating cable
can be distinguishedby a third letter.
Example:KX: compensating cable for
NiCr-Ni thermocouple Type Kmade from thermocouple material
RCA: compensating cable forPtRh-Pt thermocouple Type R,made from special material Type A
The tolerance classes 1 and 2 are definedfor compensating cables. Class 1 has clos-er tolerances, which can only be met by ex-tension cables made from the samematerial as the thermocouple, i.e. the X-type.
Compensating cables proper are normallysupplied to Class 2. Table 5 shows the tol-erances for the different compensating ca-ble classes.
The operating temperature range in Table 5covers the temperature to which the entirecable may be exposed, including the ther-mocouple terminations, without exceedingthe specified tolerances. Because of thenon-linearity of the thermal voltage, the tol-erances in mV or °C only apply to the mea-sured temperatures specified in the rightcolumn.
This means, for example:
A thermocouple Type J is connected to acompensating cable Type JX, Class 2. Ifthe measured temperature remains con-stant at 500°C and the temperature of theterminals and/or the compensating cablevaries from -25 to +200°C, then the indi-cated temperature varies by not more than±2.5°C.
Color coding ofcompensating cablesThe color coding of compensating cablesis laid down in EN 60584 and DIN 43713(1990). For thermocouples to EN 60584(Table 6) this means:The positive limb has the same color as thesheath, the negative limb is white. The“old” thermocouples Type L and U to DIN43713 (Table 7) are coded differently.
There are no details for the Pt30Rh-Pt6Rhthermocouple Type B. Ordinary copperconnecting cables (plain copper) can beused as compensating cables in this case.
According to DIN 43714, the cable coresare twisted together for electromagneticscreening. Additional screening by foil orbraiding can be provided. The insulationresistance between the cores and betweencores and screening must not be less than107Ω x m-1 at the maximum temperature;the breakdown voltage exceeds 500 VAC.
In addition to these color codes for com-pensating cables, there are also those ac-cording to DIN 43714, 1979 (Table 8). Theydiffer in certain respects from the onesmentioned above.
Where there are no color codes, it is notpossible to identify cables by magnetism,color or hardness. Compensating cablesType KCA and KCB differ from the thermo-cable KX and the thermocouple Type K byhaving a magnetic positive limb.
Table 5: Tolerances for thermocables and compensating cables
Table 6: Color coding for thermocouples to EN 60584
Table 7: Color coding for thermocouples to DIN 43713
Table 8: Color coding for thermocouples to DIN 43714 (1979)
Construction of thermocouplesApart from the virtually unlimited number ofspecial models, there are also those whosecomponents are completely defined bystandard specifications.
Thermocouples with terminal headThese thermocouples are of modular con-struction, consisting of the thermocoupleproper, insert tube, terminal plate, protec-tion tube and the terminal head. A flange ora screw fitting can be provided for mount-ing in position.
Fig. 4: Construction of a thermocouple
The measuring insert is a completely fab-ricated unit consisting of thermocouplesensor and terminal plate, with the thermo-couple contained in an insert tube of 6 or8 mm diameter made from bronze SnBz6to DIN 17681 (up to 300°C) or nickel. It isinserted into the actual protection tube,which is often made from stainless steel.The tip of the insert tube is in full contactwith the inside of the protection tube endplate in order to ensure good heat transfer.The fixing screws of the insert are backedby springs, to maintain good contact evenwith differential expansion between inserttube and protection tube. This arrange-ment ensures that the insert can be readilyreplaced.The thermometers are available in singleand twin versions. Their dimensions arelaid down in DIN 43735. If no measuring in-sert is used, the thermocouple is mounteddirectly in the protection tube using ce-ramic insulation.The choice of the protection tube materialdepends on the thermal, chemical and me-chanical conditions on site.
Terminal head
Terminal plate
Screw fitting
Thermocouple wires
Insert tube
Protection tube
Thermocouple
Metal protection tubes in high-tempera-ture steel, e.g. Material Ref. 1.4749, areused up to 1150°C. The corrosion resis-tance of the protection tube materials isdescribed in DIN 43720.These details are provided for general in-formation only, and the user remains re-sponsible for fully evaluating the protectiontube material for its suitability to the oper-ating conditions on site. The indicated tem-perature refers to the use withoutmechanical loads and (unless otherwisespecified) in clean air.
Ceramic protection tubes are employedwhere local conditions prevent the use ofmetal fittings, either for chemical reasonsor because of high temperatures. Theirmain application is at temperatures be-tween 1000 and 1650°C. They may be indirect contact with the medium, or may beused as a gas-tight inner tube to separatethe thermocouple from the actual protec-tion tube. Even hair cracks may lead to apoisoning and drifting of the thermocouple.The resistance of a ceramic to temperatureshock increases with its thermal conductiv-ity and the tensile strength, and is larger fora lower thermal expansion coefficient. Thewall thickness of the material is also impor-tant; thin-walled tubes are preferable tothose with larger wall thicknesses.
Fig. 5: Thermocouple withceramic protection tube
In the case of noble thermocouples, the ce-ramic has to be of the highest purity.Platinum thermocouples are very sensi-tive to poisoning by foreign chemical ele-ments. These include especially silicon,arsenic, phosphorus, sulfur and boron.Special care must therefore be taken in
Terminal head
Support tube
Stop flange
Protection tube
high-temperature fittings to ensure that in-sulation and protection tube do not containsuch elements, as far as this is possible. Aparticularly damaging material is SiO2. Poi-soning takes place much more rapidly in aneutral or reducing atmosphere and iscaused by the reduction of SiO2 to SiO,which reacts with platinum to form Pt5Si2.As little as 0.2% SiO2 in the insulation ofthe protection tube material is sufficient ina reducing atmosphere to form such brittlesilicides.
Thermocouples with protection tubes thatare permeable to gas can therefore not beused in a reducing atmosphere, such as inannealing furnaces, but are permitted in anoxidizing atmosphere or under a protectivegas blanket. If an inner tube of gas-tight ce-ramic is used, the outer protection tubecan be permeable to gas.In the high-temperature range, the insula-tion properties of the materials become im-portant. Protection tubes in aluminium-oxide (KER 610) and magnesium oxide ex-hibit appreciable conductivity above1000°C. This produces a shunt effectwhich introduces errors into the thermo-couple signal. The insulation of ceramicsdeteriorates with increasing alkali content.Pure aluminium oxide ceramics exhibit thebest characteristics. KER 710 is thereforeused for 4-bore insulators and protectiontubes.Two gas-tight ceramics are described be-low, whose characteristics are defined inDIN 43724:
KER 710 is a pure oxide ceramic consist-ing of more than 99.7% AI2O3, with tracesof MgO, Si2O and Na2O, which is fire resis-tant up to 1900°C and has a melting pointof 2050°C. It is the best ceramic material,with an insulation resistance of 107Ω x cmat 1000°C and good strength under alter-nating temperatures, thanks to its highthermal conductivity and relatively lowthermal expansion. With platinum thermo-couples, both the insulation rod and theprotection tube must be in KER 710.
The material KER 610 has a higher alkalicontent (60% AI2O3, 37% SiO2, 3% alkali)and, therefore, a low insulation resistanceof about 104Ω x cm at 1000°C. Because ofthe high silicon dioxide content, it cannotbe used in a reducing atmosphere. Com-pared with KER 710, it has only one-ninththe thermal conductivity; its mechanicalstability is good.The advantage of KER 610 is its price,which is only about one-fifth that of KER710.
For the terminal heads, DIN 43729 de-fines the two forms A and B, which differ insize and also slightly in style.
Fig. 6: Terminal head to DIN 43729,Form B
The material used is aluminium.
Protection is not covered by a standard; itis usually splash-proof to IP54. The nomi-nal diameter of the bore to take the protec-tion tube is as follows:
Form A: 22, 24 or 32 mm.Form B: 15 mm or
thread M 24 x 1.5.
Thermocouples to DIN 3440Thermocouples for use with temperaturecontrollers or temperature limiters for indi-rect heating systems must meet the re-quirements of DIN 3440 and are subject toadditional TUV approval.
The thermocouples must withstand tem-peratures that are 15% above the uppertemperature limit for at least one hour andhave to meet certain response times in re-
lation to the medium (e.g. air t0.63 =120sec). The thermometers are designedto withstand mechanical loads caused byexternal pressure and the flow velocity ofthe medium at the operating temperature.No modifications to the thermometers arepermitted without obtaining a fresh TUVapproval!
Thermocoupleswith compensating cableThermocouples with an attached compen-sating cable do not have a measuring insertor a terminal head. The thermocouple is di-rectly connected to the thermocable or thecompensating cable and enclosed in theprotection tube. Strain relief is provided bycrimping the protection tube at the entry ofthe compensating cable.The thermocouple is normally insulated; al-ternatively, it can be welded to the protec-tion tube tip for improved thermal contact.The maximum temperature is determinedmainly by the thermal stability of the cablesheath and insulation. Table 9 shows asexamples some insulation materials andtheir upper temperature limit.
Table 9: Temperature limits ofinsulation materials
There are many different thermometer de-signs, and they are often adapted to suitparticular customer requirements.Some characteristic data are given below:
heat-resistant steel or brass- mounting: fixed or sliding flange,
fixed thread or clamp
Material Max. temperature °C
PVC 80
Silicone 180
PTFE 260
Glass fiber 350
Fig. 7: Construction of a thermocouplewith compensating cable
Thermocoupleswith bayonet fittingAnother version incorporates a bayonet fit-ting. The stainless steel pressure spring(Material Ref. 1.4310) also acts as a cableprotector and ensures uniform pressure ofthe protection tube and sensing tip againstthe bottom of the bore.
The fitting length can be varied by rotatingthe bayonet lock. Bayonet fittings andsockets are available in 12, 15 and 16 mmdiameters.
Thermocouples with a bayonet fitting arelargely employed for measuring tempera-tures in solids, on bearings and mouldingtools, e.g. in the plastics industry. Becauseof the special shape of the sensing tip,these thermocouples are suitable for bothflat-bottom and cone-shaped bores.
Mineral-insulated thermocouplesMineral-insulated thermocouples consistof a thin-walled sheath of stainless or high-temperature steel (Inconel 600) in whichthermocouple wires are embedded in com-pressed fire-resistant magnesium oxide.
Fig. 9: Construction of amineral-insulated thermocouple
Excellent heat transfer between sheath andthermocouple enables a fast response (t0.5from 0.1sec) and high accuracy.The shock-resistant construction ensuresa long life.The flexible sheath, minimum bending ra-dius 5 times the external diameter of0.5 – 6mm, permits temperature measure-ment in locations where access is difficult.Thanks to their special features, mineral-in-sulated thermocouples are used in chemi-cal plant, power stations, pipelines, on testbeds and wherever resistance to vibration,flexibility and easy installation are required.
Connection of thermocouplesThe length of the compensating cable is ofminor importance in view of the low internalresistance. With long distances and a smallcross-section, the resistance of the com-pensating cable may, however, becomerelatively large.In order to avoid errors, the resistance ofthe input circuit of the instrument must be
Sheath,flexible
Thermocouple wires
Filling material
Thermocouple
at least 1000 times the resistance of thethermocouple connected.
It is essential to use only compensating ca-bles of the same material as the thermo-couple, or with the same thermoelectriccharacteristics, otherwise an additionalthermocouple is formed at the connectionpoint. The compensating cable has to berun up to the cold junction. The correct po-larity must be observed when connectingup the thermocouple.
Effect on short-circuitand breakA thermocouple produces no voltage if themeasured temperature is equal to the coldjunction temperature.
If a thermocouple or compensating cable isshort-circuited, a new measuring point isproduced at the location of the short-cir-cuit. If it occurs in the terminal head, for ex-ample, the temperature measurementrelates not to the actual measuring point,but to the terminal head. If there is a breakin the measuring circuit, the instrument willshow the cold junction temperature.
Measurement errors arising from the installationA temperature probe can only indicate thetemperature of its temperature-sensitivesensor. This temperature is not necessarilythe same as that for the medium which isintended to be measured. The thermome-ter is not installed purely in the medium, butis also thermally linked to its surroundings.This results in a temperature shift (thermalconduction error). This error depends on anumber of factors. These include: the tem-perature of the medium, ambient tempera-ture, thermal characteristics of themedium, flow velocity and the immersionlength of the thermometer. A lasting reduc-tion of this error requires a suitable choiceof installation site, whereby the immersiondepth of the thermometer in the mediumplays a particularly important role. As arough guide for measurement in liquid me-dia, the immersion depth should be at least15 times the thermometer diameter. Forcritical applications, or to meet require-ments for very high accuracy, the installati-on-induced error should be checked by atest measurement. To do this, the thermo-meter is pulled out of the normal installati-on position by about 10 mm, and thetemperature indication is noted.
Fault findingOne of the most frequent faults is the omis-sion or the incorrect choice of the compen-sating cable. The thermocouple can bereadily checked using a simple continuitytester or ohmmeter. The operation of thethermocouple and its correct polarity canbe tested with a voltmeter (millivolt range),by heating its sensing tip.
Possible connection errors and their ef-fects:
- Indicator shows room temperaturethermocouple or cable open-circuit.
- Indication has correct value butnegative signreversed polarity at the indicator.
- Indication cleary too high or too lowa) incorrect linearization of the indicator.b) incorrect compensating cable or connections reversed.
- Indication too high or too low by a fixed amountincorrect cold junction temperature.
- Indication correct but drifting slowly in spite of constant measured tempera-turecold junction temperature not constant or not evaluated correctly.
- Temperature still indicated with one limb disconnecteda) electromagnetic interference
picked up on the input cable.b)parasitic voltages produced due to
missing or faulty electrical isolatione.g. in furnaces.
- High reading when both thermocouple limbs are disconnecteda) electromagnetic interference
picked up on the input cableb) parasitic galvanic voltages,
e.g. due to damp insulation in thecompensating cable.
Safety notesAll welded joints on thermometers andpockets are monitored through a qualityassurance system to DIN 8563, Part 113.Special regulations apply to certain appli-cations (e.g. pressure vessels) according toSection 24 of the German Trade Regula-tions. Where the user specifies such spe-cial requirements, the weld is monitoredaccording to EN 287 and EN 288.
Pressure loading for temperatureprobesThe pressure resistance of protection fit-tings, such as are used for electric ther-mometers, depends largely on the differentprocess parameters.These include:- temperature- pressure- flow velocity- vibrationIn addition, physical properties, such asmaterial, fitting length, diameter and typeof process connection have to be takeninto account.
The following diagrams are taken from DIN43763 and show the load limit for the dif-ferent basic types in relation to the temper-ature and the fitting length, as well as theflow velocity, temperature and medium
.
Fig. 10: Pressure loadingfor protection tube Form B
stainless steel 1.4571velocity up to 25m/sec in airvelocity up to 3m/sec in water
Fig. 11: Pressure loadingfor protection tube Form G
stainless steel 1.4571velocity up to 40m/sec in airvelocity up to 4m/sec in water
As explained in the standard, the values in-dicated are guide values, which have to beindividually examined for the specific appli-cation. Slight differences in the measure-ment conditions may suffice to destroy theprotection tube.If, when ordering an electric thermometer,the protection fitting needs to be checked,the load type and the limit values must bespecified.
Fig. 12 shows the load limits (guide values)for different tube dimensions on a variety ofadditional thermometer designs. The max-imum pressure loading of cylindrical pro-tection tubes is shown in relation to thewall thickness with different tube diame-ters.The data refer to protection tubes in stain-less steel 1.4571, 100mm fitting length,10m/sec flow velocity in air, or 4m/sec inwater, and a temperature range from-20 to +100°C. A safety factor of 1.8 hasbeen taken into account. For higher tem-peratures or different materials, the maxi-mum pressure loading has to be reducedby the percentage values given in the table.
Material Temperature Reduction
CrNi1.4571
up to +200°C -10%
CrNi1.4571
up to +300°C -20%
CrNi1.4571
up to +400°C -25%
CrNi1.4571
up to +500°C -30%
CuZn2.0401
up to +100°C -15%
CuZn2.0401
up to +175°C -60%
Wall thickness in mm
Pre
ssur
e in
bar
Fig. 12: Load limits on protection tubes for various tube dimensions
The welded protection fittings of JUMOthermometers are subjected to a leakagetest or a pressure test, depending on theconstruction of the protection fitting.
Thermometers which are manufactured toDIN or to application-specific guidelines(chemical or petrochemical plant, pressurevessel regulation, steam boilers) require dif-ferent pressure tests according to the spe-cific application.If the thermometers are to be manufacturedto such standards or guidelines, then the re-quired tests or standards and/or guidelineshave to be specified when ordering.
Scope of testTests can be carried out on each individualprotection fitting and documented in a testreport or acceptance certificate to EN10204 (at extra cost).
Type of testTests can be performed on protection fit-tings up to a fitting length of 1050mm withflange connection DN25 or screw connec-tion up to 1" thread size.
The following tests can be carried out:
Leakage testA vacuum is produced inside the protectiontube. From the outside, helium is applied tothe protection fitting. If there is a leak in theprotection tube, helium will penetrate andwill be recognized through analysis. A leak-age rate is determined by the rise in pres-sure (leakage rate > 1 x 10-6 l/bar).
Pressure test IA positive pressure of nitrogen is applied tothe protection tube from the outside. If thereis a leak in the fitting, a volume flow will beproduced inside the protection tube, whichwill be recognized.
Pressure test IIWater pressure is applied to the protectiontube from the outside. The pressure mustremain constant for a certain length of time.If this is not the case, the protection fittinghas a leak.
Qualified welding processesfor the production ofprotection tubes for thermometers
In addition to using perfect materials, it isthe joining technique which ultimately de-termines the mechanical stability and quali-ty of the protection fittings. This is why thewelding techniques at JUMO comply withthe European Standards EN 287 andEN 288. Manual welding is carried out byqualified welders according to EN 287. Au-tomatic welding processes are qualified bya WPS (welding instruction) to EN 288.
The following table gives an overview ofqualified welding processes:
Based on these experiences, our welderscan also join different materials and dimen-sions.
Laser beam welding is employed for wallthicknesses of less than 0.6mm, which ismonitored by a laser beam specialist ac-cording to guideline DSV 1187.
On customer request, material test certifi-cates can be issued at extra cost. Likewise,special tests and treatments can be carriedout, which are calculated according to theextent of the work, as set out in various ap-plication guidelines. This includes X-ray ex-aminations, crack test (dye penetrationtest), thermal treatment, special cleaningprocesses and markings.
Tolerance classesfor thermocouples (0°C cold junction) to EN 60 584
Thermocouple Class 1Operating range Tolerance (±)1
copper/constantan T - 40 to + 350°C 0.5°C or 0.004 x ltliron/constantan J - 40 to + 750°C 1.5°C or 0.004 x ltlnickel-chrome/constantan E - 40 to + 800°C 0.5°C or 0.004 x ltlnickel-chrome/nickel K - 40 to +1000°C 1.5°C or 0.004 x ltlplatinum-13% rhodium/platinum R 0 to +1600°C 1 °C or [1+(t-1100) x 0.003]°Cplatinum-10% rhodium/platinum S 0 to +1600°C 1 °C or [1+(t-1100) x 0.003]°Cplatinum-30% rhodium/platinum-6% rhodium B - -
Thermocouple Class 2Operating range Tolerance (±)1
copper/constantan T -40 to + 350°C 1 °C or 0.0075 x ltliron/constantan J -40 to + 750°C 2.5°C or 0.0075 x ltlnickel-chrome/constantan E -40 to + 900°C 1 °C or 0.0075 x ltlnickel-chrome/nickel K -40 to +1200°C 2.5°C or 0.0075 x ltlplatinum-13% rhodium/platinum R 0 to +1600°C 1.5°C or 0.0025 x tplatinum-10% rhodium/platinum S 0 to +1600°C 1.5°C or 0.0025 x tplatinum-30% rhodium/platinum-6% rhodium B +600 to +1700°C 1.5°C or 0.0025 x t
Thermocouple Class 32
Operating range Tolerance (±)1
copper/constantan T -200 to +40°C 1 °C or 0.015 x ltliron/constantan J -200 to +40°C 2.5°C or 0.015 x ltlnickel-chrome/constantan E -200 to +40°C 1 °C or 0.015 x ltlnickel-chrome/nickel K -200 to +40°C 2.5°C or 0.015 x ltlplatinum-13% rhodium/platinum R - -platinum-10% rhodium/platinum S - -platinum-30% rhodium/platinum-6% rhodium B +600 to +1700°C 4 °C or 0.005 x t
The standard tolerance for thermocouples corresponds to DIN 43 760 or EN 60 584, Class 2.Restricted tolerance to Class 1 is possible on mineral-insulated thermocouples.
1. The tolerance is the specified value in °C or the percentage based on the actual temperature in °C,whichever is larger.
2. Thermocouples and thermocouple wires are usually supplied conforming to the tolerances according to the table abovefor the temperature range above -40°C.At temperatures below -40°C, the deviations for thermocouples of the same material may exceed the tolerances for Class 3.Where thermocouples according to tolerance classes 1, 2 and/or 3 are required, this has to be specified by the user;specially selected material is then used.
with thermocouples andresistance thermometersMatthias NauElectrical temperature sensors have be-come indispensable in automation and do-mestic engineering, as well as inproduction technology. As a result of therapid expansion of automation in recentyears, they have become firmly establishedin industrial engineering.
Fig. 13: PublicationElectrical temperature measurementwith thermocouplesand resistance thermometers
It is therefore particularly important that theuser can select the product that best fitshis application from the large variety of foravailable products for electrical tempera-ture measuremen.
On 160 pages this publication covers thetheoretical fundamentals of electrical tem-perature measurement, the practical con-struction of temperature sensors, theirstandardization, tolerances and styles.
In addition, it describes in detail the differ-ent fittings for electrical thermometers,their classification to DIN and the great va-riety of applications. The book includes anextensive section with tables for voltageand resistance series to DIN and EN, thusmaking it a valuable guide both for the ex-perienced practical engineer and the new-comer to the field of electrical temperaturemeasurement.You can order a copy under
Sales No. 90/00085081, or download it from www.jumo.netBecause of the high handling costs,schools, institutes and universities areasked to place a bulk order.
Error Analysis of a Temperature MeasurementSystemwith worked examples
Gerd Scheller
This 44-page publication helps in the eva-luation of measurement uncertainty, parti-cularly through the worked examples inChapter 3. Where problems arise, we areglad to discuss specific problems with ourcustomers, and to provide practicaladvice.
Fig. 14: PublicationError analysis of a temperature measu-rement system, with worked examples
In order to be able to make comparablemeasurements, their quality must be esta-blished through details of the measure-ment uncertainty. The ISO/BIPM “Guide tothe Expression of Uncertainty in Measure-ment”, published in 1993 and usually refer-red to as GUM, introduced a standardizedmethod for the determination and definitionof measurement uncertainty. This methodwas adopted by calibration laboratoriesaround the world. However, the applicationrequires a certain level of mathematical un-derstanding.
Further chapters present the topic of mea-surement uncertainty in a simplified andeasily understandable fashion for all usersof temperature measurement systems.Errors in the installation of the temperaturesensors and the connections to the evalua-tion electronics lead to increased errors inmeasurement. To these must be added themeasurement uncertainty components ofthe sensor and of the evaluation electro-nics itself. The explanation of the variouscomponents of measurement uncertaintyis followed by some worked examples.Knowledge of the various measurementuncertainty components and their magni-tudes enable the user to reduce individualcomponents through the selection ofequipment or altered installation conditi-ons. The decisive factor is always, whichlevel of measurement uncertainty is accep-table for a specific measurement task. Forinstance, if a standard specifies tolerancelimits for the deviation of a temperaturefrom a nominal value, then the measure-ment uncertainty of the method used fortemperature measurement should not belarger than 1/3 of the tolerance.
You can order a copy underSales No. 90/00415704 or download from www.jumo.netBecause of the high handling costs,schools, institutes and universities areasked to place a bulk order.
Certification laboratory fortemperatureRaised quality expectations, improvedmeasurement technology and, of course,quality assurance systems, such as ISO9000, make increasing demands on thedocumentation of processes and the mon-itoring of measuring devices.In addition, there are increasing calls fromcustomers for high product quality stan-dards. Particularly stringent demands arisefrom ISO 9000 and EN45000, wherebymeasurements must be traceable to na-tional or international standards. This pro-vides the legal basis for obliging suppliersand manufacturers (of products that aresubject to processes where temperature isrelevant) to check all testing devices, whichcan affect the product quality, before useor at certain intervals. Generally, this isdone by calibrating or adjusting using cer-tified devices. Because of the high demandfor calibrated instruments and the largenumber of instruments to be calibrated, thestate laboratories have insufficient capaci-ty. The industry has therefore establishedand also supports special calibration labo-ratories which are linked to the GermanCalibration Service (DKD) and are subordi-nate to the PTB (Physikalisch-Technische-Bundesanstalt) for all aspects of instru-mentation.
The certification laboratory of the GermanCalibration Service at JUMO has carriedout calibration certification for temperaturesince 1992. This service provides fast andeconomical certification for everyone.DKD calibration certificates can be issuedfor resistance thermometers, thermocou-ples, measurement sets, data loggers andtemperature block calibrators within therange -80 to +1100°C. The traceability ofthe reference standard is the central issuehere. All DKD calibration certificates arerecognized as documents of traceability,without any further specifications. TheDKD calibration laboratory at JUMO hasthe identification DKD-K-09501-04 and isaccredited to DIN EN ISO/IEC 17 025.
