Thermocouple

Nimish Shah

ThermocouplesMost frequently used method to measure temperatures with an electrical output signal.

OutlineWhat Is A Thermocouple Sensor??Basic Working PrinciplePractical Thermocouple Construction Thermocouple MaterialsStandard Thermocouple TypesThermocouple Color CodesCharacteristics Major SpecificationsCapabilities and LimitationsSelecting A Temperature Sensor Comparisons

What Is A Thermocouple Sensor??

Thermocouple comprises of at least two metals joint together to form two junctions: Hot junction & Cold junction hot or measuring junction : connected to the body

What Is A Thermocouple Sensor??

cold or reference junction: connected to the body of known temperature.Thus the thermocouple enables measuring the unknown temperature of the body with reference to the known temperature of the other body.

Working principle: thermocouplePrinciple: A circuit made by connecting two dissimilar metals produces a measurable voltage (emf-electromotive force) when a temperature gradient is imposed between one end and the other.

ThermocouplesTwo alloys joined at one end and free at the other.At the open end, the emf is a function of temperature T1 at closed endAs T rises, emfincreasesT2 must be kept at a standard temp (0°C)

ThermocouplesJunction composition does not affect thermocouple actionT1 must be kept constant throughout the junctionJunction material must be electrically conductive

Basic Working Principle

Seeback effectThomson effectPeltier effect

ThermocouplesBackground

Seebeck – 1822 – Current flow is proportional to temperature differencePeltier – 1834 – If battery inserted in the circuit, flowing current will cause heat to be absorbed & liberatedThomson – analyzed effect thermodynamically

Seeback Effect

In, 1821 T. J. Seebeck observed the existence of an electromotive force (EMF) at the junction formed between two dissimilar metals (Seebeckeffect).

Seebeck effect is actually the combined result of two other phenomena, Thomsonand Peltier effects.

Thermocouples Principle of OperationThomson observed the existence of an EMF due to the contact of two dissimilar metals at the junction temperature.Peltier discovered that temperature gradients along conductors in a circuit generate an EMF.The Thomson effect is normally much smaller than the Peltier effect.

Practical Thermocouple Construction

Practical Thermocouple ConstructionA thermocouple construction consist of two

conductors, welded together at the measuring point and insulated from each other long the length.

It will usually have an outer protection sheath.

Practical Thermocouple Construction Practical Thermocouple Construction

Thermocouple Materials

Which materials should be used?

Depends on requirements:Temperature rangeRequired accuracyChemical resistance issuesAbrasion or vibration resistanceInstallation requirements (size of wire)Thermal conduction requirements

Thermocouple Material Vs EMF

Thermocouple MaterialsMost commonly used three materials:

Iron-Constantan (Type J), Copper-Constantan (Type T), and Chromel-Alumel (Type K).1-) The first named element of the pair is the positive element. 2-) The negative wire is color coded red.

Standard Thermocouple Types

Standard Thermocouple Types

Iron-Constantan: (Type J) color code: white and red generates about 50 µV/°C (28 µV/°F). The Iron wire is magnetic. Junctions made by welding or soldering

with commonly available solders and fluxes. can generate a galvanic EMF between the

two wires and should not be used in applications where they might get wet.

Major Specifications

Type J: Iron / Constantan:Useful range of temperature is -300F to 1200F. Maximum temperature 1600F. Possible problems: Oxidizes rapidly due to the iron wire. preferred over the beaded bare wire style of thermocouple.

Standard Thermocouple TypesChromel Alumel: (Type K) color code: yellow and red generates about 40 µV/°C (22 µV/°F). The Alumel wire is magnetic. Junctions made by welding or soldering,

but high temperature silver-solders and special fluxes must be used.

generate electrical signals, while the wires are being bent, and

should not be used on vibrating systems, unless strain relief loops provided.

Major Specifications

Type K: Chromel / AlumelUseful range of temperature is -300F to 1800F. Maximum temperature 2300F.

Type E: Chromel / ConstantanUseful range of temperature is -300F to 1800F. Maximum temperature 1000F.

Standard Thermocouple TypesCopper-Constantan: (Type T) color coded blue and red generates about 40 µV/°C (22 µV/°F).

Neither wire is magnetic. Junctions made by welding or soldering

with commonly available solders and fluxes. Are very susceptible to conduction error due to the high thermal conductivity of

the copper, and should not be used unless long runs of wire (100 to 200 wire diameters) can be laid along an isotherm.

Major Specifications

Type T: Copper / ConstantanUseful range of temperature is -300F to 700F. Maximum temperature 700F.

Thermocouple Color Codes

Thermocouple Color CodesThermocouple wiring is color coded by thermocouple types.

Different countries utilize different color coding.

Jacket coloring is sometimes a colored stripe instead of a solid color as shown.

Characteristics Thermocouples have non-linear

characteristics given by an approximating polynomial.

For example for type J (range 1, -210 to 760°C) the characteristic is given by

where V is in and T in °C

The output signal from a thermocouple is up to 50mV.

Capabilities and LimitationsCapabilities:

- Wide Range

- Fast Response

- Passive

- Inexpensive

Limitations:

- Non-Linear

Advantages & DisadvantagesThermocouple can be used over great distances since emf is a function of T1onlyThese long leads can cause problemsThere could be a break in the circuit or a reversal of leadsInduced voltage from nearby A/C magnetic field

Selecting A Temperature Sensor

Comparisons

Stability (Drift)Reasonable for limited lifetime Good Good Good

Repeatability Reasonable Good Good GoodHysteresis Excellent Good Good Good

Vibration Very Resistant Less Resistant Good Tolerant

Measurement Area Single Point Whole RT Element

Whole bead

(Small) Varies

DiameterSmall Sizes (to

0.25mm) Larger (3.0mm min)

Small (0.5mm

min) Varies

Linearity Not Linear Reasonably Linear Not LinearReasonably

Linear

Reference Junction Required Not RequiredNot

Required Not Required

Lead Wire Resistance No Problem Must be Considered No Problem Not RequiredContact Required Yes Yes Yes NoResponse Fast Slower Medium Fast

InfraredThermocoupleResistance

Thermometer (RTD) Thermistor