temperature measurement with thermocouples

8/11/2019 Temperature Measurement With Thermocouples

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December, 2013 Page 1 of 13

Temperature Measurement with Thermocouples

Application Note

Introduction

Temperature measurement generally can bedivided into two main categoriescontactthermometry and radiation thermometry.Contact thermometry consists of a thermo-couple which always remains in contact withthe device under test, while radiationthermometry measures the radiation of thedevice under test without contact, by meansof an infrared sensor.In order to guarantee a long lifetime forLEDs, the junction temperature must not beexceeded. The maximum junctiontemperature is specified in the data sheet forthe LED.

This application note provides informationabout measurement procedures, the thermo-couples used and their systematic errors aswell as the ways in which thermocouples aremounted.

Explanation of underlyingcircumstances

Since the LED junction temperature cannotbe measured directly, it is necessary to takemeasurements at another defined point. Anappropriate location is the solder point, sincethe thermal resistance RthJSbetween thesolder point and the junction is fixed by thepackage design and can be obtained from

the corresponding data sheet. For aparticular solder point temperature TS, thejunction temperature TJcan be calculated,allowing the junction temperature, drivecurrent and thermal characteristics forvarying ambient temperatures to beobtained.The calculation of the LED junctiontemperature for a given solder point

temperature is described in the applicationnote Thermal Management of SMT LED.

Functionality of a thermocouple

Thermocouples are the most commonlyused temperature sensors; accuratetemperature measurements can be madewith a typical low-level voltmeter. Theequipment required is relatively inexpensive.

A thermocouple simply consists of twodifferent metal wires (for example, copperand constantan) which are welded togetherat one end, and then separated from eachother with insulated leads. With the influenceof heat at the welded junction, a DC voltage(thermocouple voltage) is produced betweenthe two metals which can be measured andused to provide information about theprevailing temperature. The voltagegenerated by the thermocouple is largely

proportional to the difference between thetemperature of the device under test and thereference temperature.

Figure1: Principle test arrangement for the

temperature range of -200C to +600C

Thermocouple selection

Various thermocouples are available whichare differentiated according to type andconstruction:

V

Copper

Constanan

ConnectionPointUThermal

Measurement

Device


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Types of thermocouples, materials andcolor codes

Thermal Pair Types, Materials, Color Codes

Element

Type Standard

Material

Combination

Color

Code

Type T EN 60 584 Cu - CuNi

Type E EN 60 584 NiCr - CuNi

Type J EN 60 584 Fe - CuNi

Type K EN 60 584 NiCr - Ni

Type S EN 60 584 Pt10%Rh - Pt

Type R EN 60 584 Pt13%Rh - Pt

Type B EN 60 584 Pt30%Rh - Pt

Table 1 Color coding according to IEC 304

The insulation for the negative leadis whitefor all thermocouples.

The insulation for the positive leadhas a

color according to the above table.

Note:Certain manufacturers use a different colorcoding or adhere to country-specificstandards

According to IEC 60584, thermocoupletypes are subdivided into three toleranceclasses:

Class 1

Tolerance

()Type T

0.5C or 0.004*|t|-40C ... +350C

Tolerance

()Type EType JType K

1.5C or 0.004*|t|-40C ... +800C-40C ... +750C

-40C ... +1000C

Tolerance

()Type R and S

Type B

1C or 1+(t-300)*0.003C0C ... +1600C

Class 2Tolerance

()Type T

1.0C or 0.0075*|t|-40C ... +350C

Tolerance

()Type EType JType K

2.5C or 0.0075*|t|-40C ... 0+900C-40C ... 0+750C-40C ... +1200C

Tolerance

()Type R and S

Type B

1.5C or 0.0025*|t|0000C ... +1600C+600C ... +1700C

Class 3

Tolerance ()Type T 1.0C or 0.015*|t|

-200C ... +40C

Tolerance ()Type EType JType K

2.5C or 0.015*|t|-200C ... +40C

-200C ... +40C

Tolerance ()Type R and S

Type B

4C or 0.005*|t|

+600C ... +1700C

Table 2 Tolerance classes according to IEC60584


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Spec.

Resist.

2mm

m

Copper Cu: 100% (pure copper) 0.017

Constantan CuNi:55% Cu, 45% Ni or 55%Cu, 44% Ni, 1% Mn 0.495

Iron Fe: 100% (pure iron) 0.11

Nichrome NiCr: 90% Ni, 10% Cr 0.72

Nickel Ni: 95% Ni, rest Mn, Al, Si 0.27

Platinum Rhodium: 90% Pt, 10% Rh 0.193

Platinum: 100% (pure platinum) 0.107

Table 3 Material Composition

Since these measurement proceduresinvolve contact thermometry, a systematic

error is introduced. When attaching athermocouple, energy is dissipated.Therefore, it is important to know which levelof accuracy is required.

In order to measure the LED solder pointtemperature TS, a thermocouple of type K isrecommended, since the thermalconductivity for this type is lowest andtherefore less energy is dissipated than withother types. To minimize the occurrence ofsystematic errors, the dimensions of thethermocouple should be as small aspossible.

Transparent cladding

Color codedinsulation 0.25mm

NiCr wire 0.12mm

Ni wire 0.12mm

Weld point 0.32mm

Figure 2: Thermocouple of type K

Mounting the thermocouple to thedevice under test

Several mounting methods are possible;however, only two methods will be

presented:

1. Solder Method

Soldering guarantees that the thermocouplemaintains good thermal contact with thedevice under test, and permits exacttemperature measurement. In addition, themounting point can be exactly determined.This good thermal coupling allows quicklyvarying temperatures to be accurately

measured. However, this method hasessentially three significant disadvantages:

An electrical connection is present betweenthe thermocouple and device under test (novoltage isolation).

Thermal noise can arise during solderingwhich can create noticeable fluctuations inthe thermocouple voltage.

In addition, EMI disturbances can arise,

which influence the measurement sequence(for example, a cell phone at themeasurement site).

2. Adhesive Method

Using an adhesive is an alternative tosoldering. The primary advantage of anadhesive is the voltage isolation betweenthe thermocouple and the device under test.Furthermore, the thermal resistance

between the measurement object and thesensor is increased. This means, however,that less energy is dissipated from thethermocouple which inevitably causes themeasurement to become sluggish and lessprecise.Consideration should be given regarding theadhesive type and the secondary influenceswhich arise at the mounting location.


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A thermal adhesive such as Arctic SilverAdhesive, Arctic Alumina Adhesive or similartwo-component thermal adhesive isrecommended. These thermal adhesives areelectrically nonconductive and have athermal conductivity in the range of7.5 W/mK. The adhesive is easy toadminister and handle. Mix components Aand B in a ratio of 1:1 on a glass surface.Apply the adhesive to the measurement site,and attach the thermocouple to the preparedlocation and then secure the componentfirmly in place. This can be accomplishedwith a rubber band or hot glue.Make sure that there is not too muchpressure on the thermocouple and that noundesired metal contact is present between

the thermocouple and the LED. To be sure,the circuitry should be checked with anohmmeter. In case of electrical contact, theprocedure must be redone. After about 40minutes, the adhesive is suitable formeasurement purposes. A soldering ironcan be used to remove the thermocouple,since most adhesives become fluid at thistemperature.

Power TOPLED LA E67B

Thermal Adhesiveon Cathode

Thermocouple

Figure 3: Thermocouple attached to a

Power TOPLED

SmartLED

Thermal Adhesive

Thermocouple

2 mm

Anode

Figure 4:Thermocouple attached to a

SmartLED

Error estimation

The following error estimation is expresslyfor thermocouples of type K from OMEGAwith the specific construction shown inFigure 2.

The error estimation is based on a series ofexperimental comparison measurements.Comparison measurements carried out withan infrared camera and thermocoupleshowed no significant error for the thermo-couples used.

The following comparison measurementsclarify that the error term arising from theenergy transfer of the thermocouple is lessthan the tolerance bandwidth of the infraredcamera (2C) and the thermocouple(2.2C).

The specially built measurement devicepictured in Figure 5 serves only forcomparison measurements, permittingsimultaneous measurements by infraredcamera and thermocouple.


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Aluminum Tube35 mCopper Foil

Thermocouple

Figure 5: Measurement device

The measurement device is heated at thenarrow end. The thick-walled aluminumcylinder serves to homogeneously distributethe heat to the contact surface. A specially

etched copper foil (thickness 35m) withthermally conductive paste, a fastening ringand four screws are mounted on the contact

surface. A black foil with =0.94 is used asthe emission converter for the camera. Thethickness of the copper foil corresponds tothe thickness of a solder pad which isnormally used with LEDs. This permits theeffect of the thermocouple to be visualizedwith the aid of an infrared camera.

Figure 6: Uncalibrated IR camera image

In the above IR image, the stripes of the35m thick and 1mm wide copper foil can berecognized (Figure 6).The brighter area(white) shows the temperature distribution ofthe copper foil the noticeable dark part(yellow stripes) indicate the symmetricaletched area on the foil. The yellow stripes

results from missing any thermal conductivematerial. No objective conclusion can bereached from this image, however; it onlyserves to provide a better idea.

Copper Foil

Thermocouplemounted on backside

SP1: 102.5 C

SP2: 101.5 CSP3: 102.6 C

Figure 7: Calibrated IR camera image

In contrast, in this image (Figure 7), it isclear that the temperature at the thermo-couple is lower that of the two neighboringsymmetrical strips. Specifically, thetemperature deviation (error) in this imageamounts to 1.0C for a device temperatureof around 100C and an ambienttemperature of 27C.

Note:The error increases with increasingtemperature and decreases with decreasing

temperature. In the operating temperaturerange of an LED (-40C...+100C) the errorremains nearly linear.

Conclusion:Since the error originating from the thermo-couple of 1C lies within the tolerance rangeof the IR camera and the range specified forthe thermocouple itself, no further correctionis necessary for the thermocouple with thedimensions and data shown in Figure 2.

Verification 1: SmartLEDSolder point temperature measurement for a

SmartLEDwith IR camera and thermo-couple.


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25

30

35

45

40

50

Temperature[C]

0 5 1510

SmartLED

IR Imaging SystemAmbient Temperature: 26 CForw ard Current: IF= 30 mA

Time [s]

Figure 8a: Solder point temperature

measurement of a SmartLEDwith IRcamera

25

30

35

40

45

50

55

0 5 10 15

Time [min]

Temperature[C]

Ambient Temperature

Solder Point Temperature

Figure 8b: Solder point temperature

measurement of a SmartLEDwiththermocouple

Verification 2: PowerTOPLEDSolder point temperature measurement of a

PowerTOPLEDwith IR camera andthermocouple.

25

30

35

45

40

Tempera

ture[C]

Time [min]0 5 10 15

PowerTOPLED

IR Imaging SystemAmbient Temperature: 25 CForw ard Current: IF= 50 mA

Figure 9a: Solder point temperature

measurement of a PowerTOPLEDwith IRcamera

25

30

35

40

45

50

55

0 5 10 15

Time [min]

Temperature[C]

Ambient Temperature

Solder Point Temperature

Figure 9b: Solder point temperature

measurement of a PowerTOPLEDwiththermocouple

Both verifications show very little deviation of

the solder point temperature whenmeasured with the thermocouple andinfrared camera. This evidence confirms theabove comparison for two different LEDs.No correction factor needs to be calculated,however, since the error for the twocomparison measurements clearly lies withinthe deviation range of the instruments.


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Important Information

1. Adhesives

In principle, one must ensure that theadhesive possesses a high thermalconductivity. Most thermal adhesives have athermal conductivity of > 7.5 W/mK, which isquite sufficient for this purpose.

Caution is advised when usingcyanoacrylate-based adhesives (superglue): the thermal transmission is notparticularly good and the adhesive isrelatively brittle and unstable. Furthermore,an exothermic reaction occurs duringhardening which causes a noticeable

increase in temperature during the first tenminutes.

Polymer adhesives offer an alternativemethod of bonding. However, they are notall-purpose adhesives, and a UV lamp isadditionally required. Furthermore, removalof the thermocouple is extremely difficult.Epoxy adhesives have a relatively longhardening time (ca. 5h), which requires thatthe thermocouple be fixed securely in place,and are therefore less appropriate in

practice. Removal of the thermocouple is

relatively difficult, since this adhesive has ahigh mechanical cohesiveness.

In general, the bonding surface should be assmall as possible, exhibit no electricalcontact, and allow for removal of thethermocouple.

2. Power supplies for the device undertest and measurement equipment

A stable power supply must exclusively beused for the device under test (e.g.: circuitboard with LEDs) which is electricallyisolated from the supply voltage (e.g.: aconventional power supply with atransformer). It should be noted that many

switching power supplies do not have anisolating transformer, which can lead tounwanted voltage swings during themeasurement process (Diagram 1). Thesevoltage swings can also be observed froman attached thermocouple which iselectrically isolated from the device undertest (Diagram 1 and 2).Faulty electrical isolation of the power supplyfor the device under test can be amplified tobecome an error and lead to feedback in themeasurement equipment.

Diagram 1: Switching power supply without electrical isolation from power line

-100

-50

0

50

100

150

200

0 50 100 150 200 250 300 350 400

Time [s]

Temperature[C]

Ambient Temperature Soldered Thermocouple Glued Thermocouple


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0

10

20

30

40

50

60

70

80

0 100 200 300 400

Time [s]

Temperature[C]


Diagram 2: Conventional power supply with electrical isolation from power line

In order to achieve a higher level of certainty and precision, it often makes sense to use a leadbattery for the device under test.

0

10

20

30

40

50

60

70

80

90

0 100 200 300 400

Time [s]

Temperature[C]


Diagram 3: Lead battery (lowest noise)


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For thermocouples which are soldered inplace, it is especially important to payattention to which type of measurementinstrument will be used and what type ofpower supply it has. Since the majority ofthermocouples exhibit a thermocouplevoltage of around 5mV at 100C, the lowvoltage levels are susceptible to EMI. Withbattery powered measurement instruments(e.g.: VOLTKRAFT 502), electrical isolationof the device under test is less important,since no connection to the power line ispresent and therefore no feedback canarise.

Measuring Equipment

When selecting a measuring device, it isimportant to know whether only one discretevalue or several discrete values will bemeasured over time.For measuring a single value, a smallhandheld battery driven temperaturemeasuring device with two connections forexternal thermocouples is recommended.The second connection is important formeasuring the ambient temperature.For recording several discrete values over

time, a more elaborate instrument isrequired. At best, a multichannel instrumentwith computer interface (e.g.: CambridgeAccuSense TCM- 24) should be used, sincethe data is easier to manage, and iscompatible with standard software(spreadsheet calculations).This instrument is particularly precise, has ahigh sensitivity, an error correction unit, andoffers the possibility to measure up to 24thermocouples simultaneously. Because ofthe unit's high sensitivity and external power

supply, the above indications should beobserved.

Extending thermocouple leads

When thermocouple leads are extended, ithas the effect of creating additionalthermocouples at the connection points

which under certain circumstances cansignificantly influence the outcome of themeasurement.In case an extension is inevitable, the use ofspecial clamps is highly recommendedwhich serve to compensate for the errorswhich arise. In contrast to typically availableserial connectors, thermocouple voltageconnectors should be used, since theirspecial construction expressly provides forpair wise connections. The contact barsconsist of various metals and are individuallymatched to the material in the thermocouple.This permits the combination of metals in thetemperature measuring element to beextended without interruption.

MTKD-NiCr/NiTyp K

thermocouple leads

Nickel Ni

Nichrome NiCr

Nickel Ni

Nichrome NiCr

Nickel Ni

Nichrome NiCr

thermocouple voltage connector

thermocouple extension wire

measuring point

Figure 10a: Principle arrangement ofthermal clamps

Figure 10b: Thermocouple voltageconnectors connected with a thermocoupleextension cable. Suitable for thetemperature range of 0C to +1200C


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Error Sources:

Thermocouple Polarity

Reversing the polarity of the thermo-couple results in incorrect measurementdata.

Thermocouple Type

The type of thermocouple used mustalso be set in the measurementinstrument.

Extension of Thermocouple Leads

Appropriate thermocouple voltage

connectors must be used, correspondingto the type of thermocouple.

Mounting the Thermocouple

Use an exact dosage of adhesive,limited to a small bonding area

Correct Location

The correct measurement location is thesolder point for the LED. However, it

should be noted which terminal (anodeor cathode) of the LED is thermallyactive. The correct terminal of the LEDcan be found in the OSRAM data sheet.

Correct Measurement

Calibrate the measurement equipment;Create the appropriate test environment;Use or modify the original housing;Avoid forced convection, if not desired;Avoid direct sunlight;No metal objects should be used as aplatform, mounting assembly, etc;No contact should occur outside of theweld point of the thermocouple;

EMI

Cell phones, powerful transmitters andphase control devices can have anegative effect on the measurementsequence.

External Power Supply

Devices with external power supplies,PC interfaces and the device under testshould be electrically isolated from thepower line. Warning: many switchingpower supplies have no electricalisolation.

Voltage Isolation

Strong fluctuations of the measurementdata can occur when the thermocoupleis soldered in place. This effect rarelyoccurs for thermocouples which aremounted with adhesives. If this doesoccur, the mounting procedure should berepeated.


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Brief Instructions: Mounting the thermocouple to the device under test

Determine thermallyactive terminal from

OSRAM data sheet

Clean contactlocation withalcohol

Mountthermocouple

Prepare thermaladhesive

Dip thermocouplein thermal adhesive

Apply thermal adhesiveto thermally activeterminal of the LED

Potential-free connectionbetween thermocoupleand LED?

Positionthermocouple

Allow adhesiveto harden for 45min.Yes

Removethermocouple andadhesive

No


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Summary

Temperature measurement by means ofthermocouples is a multilaterally applicablemethod. In many cases, it is not possible touse infrared cameras, pyrometers and othertemperature sensors for temperature meas-urement. Thermocouples prove to be wellsuited for these applications.The advantages and disadvantages of thismethod are based on a series of numerousmeasurements. These measurements wererepeated several times and verified withvarious LEDs. In addition, some measure-ments were performed with boundary valuesand constraints.By comparison measurements and other

measurement techniques, an error estimatecan be made. The analysis of thesemeasurement procedures shows that onecan achieve satisfactory results regarding

the physical principles with relativelyinexpensive equipment. Bonding thethermocouple with "Artic Silver ThermalAdhesive" has proven to be a reliablemounting method.

Sources:

Thermocouples & measuring equipment

www.omega.com

General sources for thermal management

www.electronics-cooling.comwww.coolingzone.com

Author: Rainer Huber

ABOUT OSRAM OPTO SEMICONDUCTORSOSRAM, with its headquarters in Munich, is one of the two leading lighting manufacturers in theworld. Its subsidiary, OSRAM Opto Semiconductors GmbH in Regensburg (Germany), offers itscustomers solutions based on semiconductor technology for lighting, sensor and visualizationapplications. OSRAM Opto Semiconductors has production sites in Regensburg (Germany) andPenang (Malaysia). Its headquarters for North America is in Sunnyvale (USA). Its headquarters forthe Asia region is in Hong Kong. OSRAM Opto Semiconductors also has sales offices throughoutthe world. For more information go towww.osram-os.com.

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temperature measurement with thermocouples

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