Temperatur Sensors1.pdf

Temperature Sensors 1. 2. 3. 4. 5.

Thermoresistive sensors Thermoelectric sensors PN junction temperature sensors Optical and acoustic temperature sensors Thermo-mechanical sensors and actuators

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A bit of history

• Temperature measurements and thermometers

▫ 1600 - thermometers (water expansion, mercury) ▫ 1650 - first attempts at temperature scales (Boyle) ▫ 1700 - “standard” temperature scales (Magelotti, Renaldini, Newton) - did not catch ▫ 1708 - Farenheit scale (180 div.) ▫ 1742 - Celsius scale ▫ 1848 - Kelvin scale (based on Carnot’s thermodynamic work) ▫ 1927 - IPTS - International Practical Temperature Scale

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More history - sensors

• Temperature sensors are the oldest ▫ 1821 - Seebeck effect (Thomas Johann Seebeck)

 1826 - first sensor - a thermocouple - based on the Seebeck effect (Antoine Cesar Becquerel)

▫ 1834 - Peltier effect (Charles Athanase Peltier).  First peltier cell built in 1960’s  Used for cooling and heating

▫ 1821 - discovery of temperature dependence of conductivity (Sir Humphrey Davey)

 1771 - William Siemens builds the first resistive sensor made of platinum

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Temperature sensors - general

• Temperature sensors are deceptively simple ▫ Thermocouples - any two dissimilar materials, welded together at one end and connected to a microvoltmeter ▫ Peltier cell - any thermocouple connected to a dc source ▫ Resistive sensor - a length of a conductor connected to an ohmmeter • More:  Some temperature sensors can act as actuators as well  Can be used to measure other quantities (electromagnetic radiation, air speed, flow, etc.)

• Some newer sensors are semiconductor based

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Temperature sensors - types • Thermoelectric sensors

▫ Thermocouples and thermopiles ▫ Peltier cells (used as actuators but can be used as sensors)

• Thermoresistive sensors and actuators

▫ Conductor based sensors and actuators (RTDs) ▫ Semiconductor based sensors - thermistors, diodes

• Semiconductor junction sensors • Others

▫ Based on secondary effects (speed of sound, phase of light) ▫ Indirect sensing (infrared thermometers - chapter4) ▫ Expansion of metals, bimetals

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Thermal actuators

• A whole class of thermal actuators ▫ ▫ ▫ ▫

Bimetal actuators Expansion actuators Thermal displays Sometimes sensing and actuation is combined in a single device

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Thermoresistive sensors • Two basic types:

▫ Resistive Temperature Detector (RTD)  Metal wire  Thin film  Silicon based ▫ Thermistors (Thermal Resistor)  NTC (Negative Temperature Coefficient)  PTC (Positive Temperature Coefficient)

Thermoresistive effect • Conductivity depends on temperature • Conductors and semiconductors • Resistance is measured, all other parameters must stay constant.

R= L S

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Thermoresistive effect (cont.) • Resistance of a length of wire • Conductivity is: • Resistance as a function of temperature: • a - Temperature Coefficient of Resistance (TCR) [C]

R= L S =

0 1 + a T  T0

R T = L 1 + a T  T0 0 S

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Thermoresistive effect (cont.)

• T is the temperature [C ] • 0 is the conductivity of the conductor at the reference temperature T0. • T0 is usually given at 20C but may be given at other temperatures as necessary. • a - Temperature Coefficient of Resistance (TCR) [C] given at T0

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Example

• Copper: 0=5.9x107 S/m, a=0.0039 C at T0=20C. Wire of cross-sectional area: 0.1 mm2, length L=1m, • Change in resistance of 6.61x10 /C and a base resistance of 0.017  at 20C • Change of 0.38% per C .

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Example (cont.)

• Conclusions from this example:

▫ Change in resistance is measurable ▫ Base resistance must be large - long and or thin conductors or both ▫ Other materials may be used

Temperature Coefficient of Resistance Material

Conductivity  [S/m]

Temperature Coefficint of Resistance (TCR) C Copper (Cu) 5.7-5.9 x 107 0.0039 5 Carbon (C) 3.0 x10 0.0005 6 Constantan (60%Cu,40%Ni) 2.0 x10 0.00001 6 Cromium (Cr) 5.6 x 10 0.0059 Germanium (Ge) 2.2 0.05 7 Gold (Au) 4.1 x 10 0.0034 7 Iron (Fe) 1.0 x 10 0.0065 Mercury (Hg) 1.0 x 106 0.00089 6 Nichrome (NiCr) 1.0 x 10 0.0004 7 Nickel (Ni) 1.15 x 10 0.0069 6 Platinum (Pl) 9.4 x 10 0.01042 -6 Silicon (Si) (pure) 4.35 x 10 0.07 7 Silver (Ag) 6.1 x 10 0.0016 6 Titanium (Ti) 1.8 x 10 0.042 7 Tungsten (W) 1.8 x 10 0.0056 7 Zinc (Zn) 1.76 x 10 0.0059 7 Aluminum (Al) 3.6 x 10 0.0043 Note: Instead of conductivity  [S/m], some sources list resistivity , measured in ohm.meter  = 1/ [m). 1S/m=1/m 13

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Other considerations

• Tension or strain on the wires affect resistance • Tensioning a conductor, changes its length and cross-sectional area (constant volume)

▫ has exactly the same effect on resistance as a change in temperature. ▫ increase in strain on the conductor increases the resistance of the conductor (strain gauge)

• Resistance should be relatively large (25 and up)

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Construction - wire RTD • A spool of wire (length) ▫ ▫ ▫ ▫

Similar to heating elements Uniform wire Chemically and dimensionally stable in the sensing range Made thin (<0.1mm) for high resistance

• Spool is supported by a glass (pyrex) or mica support

▫ Similar to the way the heating element in a hair drier is supported ▫ Keeps strain at a minimum and allows thermal expansion ▫ Smaller sensors may not have an internal support.

• Enclosed in a glass, ceramic or metal enclosure ▫ Length is from a few cm, to about 50cm

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Glass encapsulated RTDs

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Construction (cont.)

• Materials: • Platinum - used for precision applications ▫ ▫ ▫ ▫ ▫ ▫ ▫ ▫

Chemically stable at high temperatures Resists oxidation Can be made into thin wires of high chemical purity Resists corrosion Can withstand severe environmental conditions. Useful to about 800 C and down to below –250C. Very sensitive to strain Sensitive to chemical contaminants

▫ Wire length needed is long (high conductivity)

Construction (cont.) • Materials: • Nickel and Copper

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▫ Less expensive ▫ Reduced temperature range (copper only works up to about 300C) ▫ Can be made into thin wires of high chemical purity ▫ Wire length needed is long (high conductivity) ▫ Copper is not suitable for corrosive environments (unless properly protected) ▫ At higher temperatures evaporation increases resistance

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Thin Film RTDs

• Thin film sensors: produced by depositing a thin layer of a suitable material (platinum or its alloys) on a thermally stable, electrically non-conducting, thermally conducting ceramic • Etched to form a long strip (in a meander fashion).

• Eq. (3.1) applies but much higher resistance sensors are possible. • Small and relatively inexpensive

• Often the choice in modern sensors especially when the very high precision of Platinum wire sensors is not needed.

Tnin film RTDs - (cont.)

• Two types of thin film RTDs from different manufacturers • Dimensions are typical - some are much larger 20

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Some parameters • • • • • • •

Temperature range: -250 C to 700 C Resistances: typically 100 (higher available) Sizes: from a few mm to a few cm Compatibility: glass, ceramic encapsulation Available in ready made probes Accuracy: ±0.01 C to ±0.05 C Calibration: usually not necessary beyond manufacturing

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Self heat in RTDs • RTDs are subject to errors due to rise in their temperature produced by the heat generated in them by the current used to measure their resistance • Wire wound or thin film

• Power dissipated: Pd=I2R ( I is the current (RMS) and R the resistance of the sensor) • Self heat depends on size and environment

• Given as temperature rise per unit power (C/mW) • Or: power needed to raise temperature (mW/ C)

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Self heat in RTDs (cont.) • Errors are of the order of 0.01C/mW to 10C/mW (100mW/C to 0.1mW/C) • Given in air and in water

▫ In water values are lower (opposite if mW/C used)

• Self heat depends on size and environment

▫ Lower in large elements, higher in small elements ▫ Important to lower the current as much as possible

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Response time in RTDs • Response time • • • • • •

Provided as part of data sheet Given in air or in water or both, moving or stagnant Given as 90%, 50% (or other) of steady state Generally slow Wire RTDs are slower Typical values ▫ 0.5 sec in water to 100 sec in moving air

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Silicon Resistive Sensors

• Conduction in semiconductors • Valence electrons ▫ ▫ ▫ ▫

Bound to atoms in outer layers (most electrons in pure semiconductors) Can be removed through heat (band gap energy) When removed they become conducting electrons (conduction band) A pair is always released - electron and hole

• Conductivity of semiconductors is temperature dependent ▫ Conductivity increases with temperature ▫ Limited to a relatively small temperature range

Silicon Resistive Sensors (cont.) • Pure silicon:

• NTC device - negative temperature coefficient ▫ Resistance decreases with temperature ▫ Resistance in pure silicon is extremely high ▫ Need to add impurities to increase carrier density ▫ N type silicon - add arsenic (As) or antimony (Sb)

• Behavior changes: ▫ Resistance increases up to a given temperature (PTC) ▫ Resistance decreases after that (NTC) ▫ PTC up to about 200 C

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Resistance of silicon resistive sensor

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Resistance of silicon resistive sensor specific device

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Silicon resistive sensors • Silicon resistive sensors are somewhat nonlinear and offer sensitivities of the order 0.5-0.7 %/C. • Can operate in a limited range of temperatures like most semiconductors devices based on silicon • Maximum range is between –55C to +150C.

• Typical range: - 45C to +85C or 0C to +80C • Resistance: typically 1k at 25 C.

• Can be calibrated in any temperature scale

• Made as a small chip with two electrodes and encapsulated in epoxy, metal cans etc.

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Thermistors

• Thermistor: Thermal resistor • Became available: early 1960’s • Based on oxides of semiconductors ▫ High temperature coefficients ▫ NTC ▫ High resistances (typically)

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Thermistors (cont.) • Transfer function: • • • •

R T = a e /T

a [] and  [K] are constants R(T): resistance of the device T: temperature in K Relation is nonlinear but:

▫ Only mildly nonlinear ( is small) ▫ Approximate transfer function

Construction • Beads • Chips • Deposition on substrate

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Epoxy encapsulated bead thermistors

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Thermistors - properties

• Most are NTC devices • Some are PTC devices • PTC are made from special materials

▫ Not as common ▫ Advantageous when runaway temperatures are possible

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Thermistors - properties • Self heating errors as in RTDs but:

▫ Usually lower because resistance is higher ▫ Current very low (R high) ▫ Typical values: 0.01C/mW in water to 1C/mW in air

• Wide range of resistances up to a few M • Can be used in self heating mode ▫ To raise its temperature to a fixed value ▫ As a reference temperature in measuring flow

• Repeatability and accuracy:

▫ 0.1% or 0.25C for good thermistors

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Thermistors - properties • Temperature range:

▫  50 C to about 600 C ▫ Ratings and properties vary along the range

• Linearity

▫ Very linear for narrow range applications ▫ Slightly nonlinear for wide temperature ranges

• Available in a wide range of sizes, shapes and also as probes of various dimensions and shapes • Some inexpensive thermistors have poor repeatability these must be calibrated before use.

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Thermoelectric sensors

• Among the oldest sensors (over 150 years) • Some of the most useful and most common • Passive sensors: they generate electrical emfs (voltages) directly ▫ ▫ ▫ ▫

Measure the voltage directly. Very small voltages - difficult to measure Often must be amplified before interfacing Can be influenced by noise

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Thermoelectric sensors (cont.) • Simple, rugged and inexpensive

• Can operate on almost the entire range of temperature from near absolute zero to about 2700C. • No other sensor technology can match even a fraction of this range. • Can be produced by anyone with minimum skill • Can be made at the sensing site if necessary

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Thermoelectric sensors (cont.)

• Only one fundamental device: the thermocouple • There are variations in construction/materials ▫ ▫ ▫ ▫

Metal thermocouples Thermopiles - multiple thermocouples in series Semiconductor thermocouples and thermopiles Peltier cells - special semiconductor thermopiles used as actuators (to heat or cool)

Thermoelectric effect

• The Seebeck effect (1821)

• Seebeck effect is the sum of two other effects ▫ The Peltier effect ▫ The Thomson effect

• The Peltier effect: heat generated or absorbed at the junction of two dissimilar materials when an emf exists across the junction due to the current produced by this emf in the junction. ▫ ▫ ▫ ▫ ▫

By connecting an external emf across the junction By the emf generated by the junction itself. A current must flow through the junction. Applications in cooling and heating Discovered in 1834

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Thermoelectric effect (cont.)

• The Thomson effect (1892): a current carrying wire if unevenly heated along its length will either absorb or radiate heat depending on the direction of current in the wire (from hot to cold or from cold to hot). ▫ Discovered in 1892 by William Thomson (Lord Kelvin).

Thermoelectric effect (cont.)

• The Seebeck effect: an emf produced across the junction of two dissimilar conducting materials connected together. • The sum of the Peltier and the Thomson effects • The first to be discovered and used (1821) • The basis of all thermoelectric sensors • Peltier effect is used in Thermoelectric Generators (TEG) devices

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The Seebeck effect • If both ends of the two conductors are connected and a temperature difference is maintained between the two junctions, a thermoelectric current will flow through the closed circuit (generation mode)

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The Seebeck effect • If the circuit is opened an emf will appear across the open circuit (sensing mode). It is this emf that is measured in a thermocouple sensor.

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Themocouple - analysis

• Conductors a, b homogeneous • Junctions at temperatures T2 and T1 • On junctions 1 and 2: • Total emf:

emfA = a A T2  T1

emfB = a B T2  T1

emf T = emfA  emfB = a A  a B T2  T1 = a AB T2  T1 45

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Thermocouple - analysis

• aA and aB are the absolute Seebeck coefficients given in V/C and are properties of the materials A, B • aAB=aAaB is the relative Seebeck coefficient of the material combination A and B, given in V/C • The relative Seebeck coefficients are normally used.

Absolute Seebeck coefficients

Table 3.3. AbsoluteSeebeck coefficients for selectedelements(Thermoelectric series) Material [ V/K] p-Silicon 100- 1000 Antimony (Sb) 32 Iron (Fe) 13.4 Gold (Au) 0.1 Copper(Cu) 0 Silver (Ag) 0.2 Aluminum (Al) 3.2 Platinum (Pt) 5.9 Cobalt (Co) 20.1 Nickel (Ni) 20.4 Bismuth (Sb) 72.8 n-Silicon 100to -1000 47

Thermocouples - standard types

Table 3.4. Thermocouples (standard types and others) and some of their properties Materials Sensitivity Standard Temperature Notes [V/C] at Type range [C] designation 25C. Copper/Constantan 40.9 T 270 to 600 Cu/60%Cu40%Ni Iron/Constantan 51.7 J Fe/60%Cu40%Ni 270 to 1000 Chromel/Alumel 40.6 K 90%Ni10%Cr/55%Cu45%Ni 270 to 1300 Chromel/Constantan 60.9 E 90%Ni10%Cr/60%Cu40%Ni 200 to 1000 Platinum(10%)/Rhodium- 6.0 S Pt/90%Pt10%Rh  to 1450 Platinum Platinum(13%)/Rhodium- 6.0 R Pt/87%Pt13%Rh  to 1600 Platinum Silver/Paladium 10 200 to 600 Constantan/Tungsten 42.1 0 to 800 Silicon/Aluminum 446 40 to 150 Carbon/Silicon Carbide 170 0 to 2000 Note: sensitivity is the relative Seebeck coefficient.

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Seebeck coefficients - notes:

• Seebeck coefficients are rather small –

▫ From a few microvolts to a few millivolts per degree Centigrade. ▫ Output can be measured directly ▫ Output is often amplified before interfacing to processors ▫ Induced emfs due to external sources cause noise ▫ Thermocouples can be used as thermometers ▫ More often however the signal will be used to take some action (turn on or off a furnace, detect pilot flame before turning on the gas, etc.)

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Thermoelectric laws: • Three laws govern operation of thermocouples: • Law 1. A thermoelectric current cannot be established in a homogeneous circuit by heat alone.

▫ This law establishes the need for junctions of dissimilar materials since a single conductor is not sufficient.

Thermoelectric laws:

Law 2. The algebraic sum of the thermoelectric forces in a circuit composed of any number and combination of dissimilar materials is zero if all junctions are at uniform temperatures. ▫ Additional materials may be connected in the thermoelectric circuit without affecting the output of the circuit as long as any junctions added to the circuit are kept at the same temperature.

▫ voltages are additive so that multiple junctions may be connected in series to increase the output.

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Thermoelectric laws:

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• Law 3. If two junctions at temperatures T1 and T2 produce Seebeck voltageV2 and temperatures T2 and T3 produce voltage V1, then temperatures T1 and T3 produce V3=V1+V2. ▫ This law establishes methods of calibration of thermocouples.

Thermocouples: connection

• Based on the thermoelectric laws: • Usually connected in pairs

▫ One junction for sensing ▫ One junction for reference ▫ Reference temperature can be lower or higher than sensing temperature

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Thermocouples (cont.)

• Any connection in the circuit between dissimilar materials adds an emf due to that junction. • • Any pair of junctions at identical temperatures may be added without changing the output. ▫ Junctions 3 and 4 are identical (one between material b and c and one between material c and b and their temperature is the same. No net emf due to this pair ▫ Junctions (5) and (6) also produce zero field

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Thermocouples (cont.)

• Each connection adds two junctions. • The strategy in sensing is:  For any junction that is not sensed or is not a reference junction: • Either each pair of junctions between dissimilar materials are held at the same temperature (any temperature) or: • Junctions must be between identical materials. • Also: use unbroken wires leading from the sensor to the reference junction or to the measuring instrument. • If splicing is necessary to extend the length, identical wires must be used to avoid additional emfs.

Connection without reference

• The connection to a voltmeter creates two junctions ▫ ▫ ▫ ▫

Both are kept at temperature T1 Net emf due to these junctions is zero Net emf sensed is that due to junction (2) This is commonly the method used

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Reference junctions

• Reference junctions must be at constant, known temperatures. Examples: • Water-ice bath (0C) • Boiling water (100C) • Any other temperature if measured

▫ A separate, non-thermocouple sensor ▫ The output compensated based on this temperature from Seebeck coefficients

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Thermocouples - practical considerations

• Choice of materials for thermocouples. Materials affect: ▫ The output emf, ▫ Temperature range ▫ Resistance of the thermocouple.

• Selection of materials is done with the aid of three tables: ▫ Thermoelectric series table ▫ Seebeck coefficients of standard types ▫ Thermoelectric reference table

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Thermoelectric series tables

• Each material in the table is thermoelectrically negative with respect to all materials above it and positive with respect to all materials below it. • The farther from each other a pair is, the larger the emf output that will be produced. • Tables are arranged by temperature ranges

Thermoelectric series table

Table 3.5 The thermoelectric series: selected elements and alloys at selected temperatures 100C 500C 900C Antimony Chromel Chromel Chromel Copper Silver Iron Silver Gold Nichrome Gold Iron Copper Iron 90%Pt-10Rh Silver 90%Pt-10Rh Platinum 90%Pt-10Rh Platinum Cobalt Platinum Cobalt Alumel Cobalt Alumel Nickel Alumel Nickel Constantan Nickel Constantan Constantan 60

Seebeck coefficients tables • Seebeck coefficients of materials with reference to Platinum 67 • Given for various thermocouple types • The first material in each type (E, J, K, R, S and T) is positive, the second negative.

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Seebeck coefficient tables

Table 3.6. Seebeck coefficients with respect to Platinum 67 Thermoelement type – Seebeck coefficient [V/C] JN TP TN, EN KP, EP KN Temp. [C] JP 0 5.9 32.9 25.8 13.6 17.9 32.5 100 17.2 37.2 9.4 37.4 30.1 11.2 200 14.6 40.9 11.9 41.3 32.8 7.2 300 11.7 43.7 14.3 43.8 34.1 7.3 400 9.7 45.4 16.3 45.5 34.5 7.7 500 9.6 46.4 46.6 34.3 8.3 600 11.7 46.8 46.9 33.7 8.8 700 15.4 46.9 46.8 33.0 8.8 800 46.3 32.2 8.8 900 45.3 31.4 8.5 1000 44.2 30.8 8.2 62

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Seebeck coefficients tables

• The Seebeck emf with reference to Platinum is given for the base elements of thermocouples with respect to Platinum 67. • Example, J type thermocouples use Iron and Constantan.

▫ Column JP lists the Seebeck emf for Iron with respect to Platinum ▫ Column JN lists the emfs for Constantan.

▫ Adding the two together gives the corresponding value for the J type thermocouple in Table 3.5. JP and JN values at 0C in table 3.3 : 17.9+32.5=50.4 V/C gives the entry in the J column at 0 C in Table 3.5.

Seebeck coefficients by type Table 3.7. Seebeck coefficients for various types of thermocouples Thermocouple type – Seebeck coefficient [V/C] J K R S T Temp. [C] E -200 25.1 21.9 15.3 15.7 -100 45.2 41.1 30.5 28.4 0 58.7 39.5 5.3 5.4 38.7 50.4 100 67.5 54.3 41.4 7.5 7.3 46.8 200 74.0 55.5 40.0 8.8 8.5 53.1 300 77.9 55.4 41.4 9.7 9.1 58.1 400 80.0 55.1 42.2 10.4 9.6 61.8 500 80.9 56.0 42.6 10.9 9.9 600 80.7 58.5 42.5 11.3 10.2 700 79.8 62.2 41.9 11.8 10.5 800 78.4 41.0 12.3 10.9 900 76.7 40.0 12.8 11.2 1000 74.9 38.9 13.2 11.5 64

Thermoelectric reference table

• List the transfer function of each type of thermocouple as an nth order polynomial, in a range of temperatures. • Ensure accurate representation of the thermocouple’s output and can be used by the controller to accurately represent the temperature sensed by the thermocouple. • An example of how these tables represent the transfer function is shown next

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Thermoelectric reference table (cont.)

Table 3.8. Transfer function for type E thermocouples Temperature range Exact reference emf (voltage) E Reference temperature T [mv] [C] [C] Within ± 0.1C 0 to 400 (+5.8695857799 x 10 x T 1.7022525 x 10 x E -2 2 +4.3110945462 x 10 x T -2.2097240 x 10-1 x E2 +5.7220358202 x 10-5 x T3 +5.4809314 x 10-3 x E3 -5.4020668085 x 10-7 x T4 -5.7669892 x 10-5 x E4 +1.5425922111x 10-9 x T5 400 to 1000 -2.4850089136 x 10-12 x T6 2.9347907 x 10 -15 7 +2.3389721459 x 10 x T +1.3385134 x 10 x E -18 8 -1.1946296815 x 10 x T -2.66699218 x 10-2 x E2 +2.5561127497 x 10-22 x T9) +2.3388779 x 10-4 x E3 x10

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Thermoelectric reference table • Table entry for type E thermocouples. • Second column is the exact representation of the output emf (voltage) in V as a 9th order polynomial. • The third column shows the inverse relation and gives the temperature based on the emf of the thermocouple within a specified error – in this case ±0.1C. • The latter can be used by the controller to display temperature or take action

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Standard thermocouples - properties

Table 3.9. Common thermocouple types and some of their properties. Materials Sensitivity Standard Recommended Notes temperature [V/C] at Type designation range [C] 25C. Copper/Constantan 40.9 T Cu/60%Cu40%Ni  to 400 (270 - 400) Iron/Constantan 51,7 J Fe/60%Cu40%Ni  to 760 (210 - 1200) Chromel/Alumel 40.6 K 90%Ni10%Cr/55%Cu45%Ni 200 to 1300 (270 - 1372) Chromel/Constantan 60.9 E 90%Ni10%Cr/60%Cu40%Ni 200 to 900 (270 - 1000) Platinum(10%)/Rhodium- 6.0 S Pt/90%Pt10%Rh  to 1450 Platinum (50 - 1760) Platinum(13%)/Rhodium- 6.0 R Pt/87%Pt13%Rh  to 1600 Platinum (50 - 1760) Silver/Paladium 10 200 to 600 Constantan/Tungsten 42.1 0 to 800 Silicon/Aluminum 446 40 to 150 Carbon/Silicon Carbide 170 0 to 2000 Platinum(30%)/Rhodium- 6.0 B 0 to 1820 Pt/70%Pt30%Rh Platinum Nickel/Cromium-silicon N (270 - 1260) alloy Note: the temperature ranges shown are recommended. Nominal ranges are lower and higher and shown in brackets.

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Thermocouple (exposed junction)

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Thermocouple (flexible, to be cemented to surface)

Thermocouple (protected junction)

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Semiconductor thermocouples

• Semiconductors have highest Seebeck coefficients • Typical values are about 1mV/C

• Junctions between n or p type semiconductors with a metal (aluminum) are most common

• Smaller temperature ranges (usually –55 C to about 150C. • Some materials - up to 225C

• Newer devices - up to about 800C

Semiconductor thermocouples: operation • Pure semiconductor: electrons in valence/covalence bonds • Few electrons are available for conduction • Adding heat moves them across the energy gap into the conduction band • To increase number of electrons - need to dope the material

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Semiconductor thermocouples: operation • Doping

▫ Add impurities - various materials ▫ Increases availability of electrons (n-type) or holes (p-type) ▫ Increases the Seebeck coefficient ▫ Silicon has 4 valent electrons ▫ Add impurity with 5 electrons to create n type silicon ▫ Add impurity with 3 electrons to create p type silicon

Semiconductor thermocouples: operation

• P type silicon junction (on aluminum) ▫ Aluminum is deposited on an intrinsic layer of silicon ▫ The silicon is doped with materials from the IIIrd group in the periodic table ▫ materials such as Boron (B), Aluminum (Al), Galium (Ga), Indium (In) and Thalium (Tl)

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• N type silicon junction (on aluminum) ▫ The silicon is doped with materials from the Vth group in the periodic table ▫ materials such as Phosphorus (P), Arsenic (As), Antimony (Sb) and Bismuth (Bi)

Periodic table - semiconductors

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Thermopile

• n thermocouples in series electrically • In parallel thermally • Output is n times the output of a single thermocouple

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Thermopiles (cont.) • Used to increase output • Sometimes done with metal thermocouples • Example: pilot flame detector: 750 mV at temperature difference of about 120C. about 100 metal thermocouples.

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Semiconductor thermopiles

• Each thermocouple has higher output than metal based devices • A few thermocouples in series can produce relatively high voltage • Used to produce thermoelectric generators. • Outputs upwards of 15V are available • Known as Peltier cells

Peltier cells

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• Made of crystalline semiconductor materials such as bismuth telluride (Bi2Te3) (n-p junctions)

• Peltier Cells are often used for cooling and heating in dual purpose refrigerators, • Can also be used as sensors and can have output voltages of a few volts (any voltage can be achieved)

• Also used as power generators for small remote installations

Peltier cells (cont.)

• Junctions are sandwiched between two ceramic plates • Standard sizes are 15, 31, 63, 127 and 255 junctions

• May be connected in series or parallel, electrically and/or thermally. • Maximum temperature difference of about 100C

• Maximum operating temperatures of about 225C

• Also used as power generators for small remote installations

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Some thermopiles (Peltier TEGs)

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Details of the TEG construction

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P-N Junction temperature sensors

• A junction between a p and an n-doped semiconductor • Usually silicon arsenide, etc.)

(also

• This is a simple diode • Forward biased

germanium,

galium-

P-N junction sensor (cont.) • Construction of the sensor

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P-N junction sensor (cont.)

• Forward current is temperature dependent • Any semiconductor diode will work • Usually the voltage across the diode is sensed

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P-N junction sensor (cont.) • Forward current through diode • Voltage across diode ▫ ▫ ▫ ▫ ▫

I0 - saturation current Eg - band gap energy q - charge of electron k - Boltzman’s constant C - a temp. independent constant ▫ T - temperature (K)

I = I0 e qV/2 kT Eg 2kT C Vf = q  q ln I

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P-N junction sensor (cont.) • If C and I are constant, Vf is linear with temperature • Diode is an NTC device • Sensitivity: 1-10mV/C (current dependent)

88

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P-N junction - operation parameters • • • • • •

Forward biased with a current source 10-100A typically (low currents - higher sensitivity) Maximum range (silicon) –55 to 150C Accuracy: ±0.1 C typical Self heating error: 0.5 mW/C Packaging: as a diode or as a transistor (with additional circuitry)

90

The LM35 sensor

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Other temperature sensors • • • •

Optical Acoustical Thermomechanical sensors Thermomecahnical actuators

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Optical temperature sensors • • • •

Noncontact Conversion of optical radiation into heat Most useful in infrared temperature sensing Relies on quantum effects - discussed in the following chapter • Other sensors rely on phase difference in propagation ▫ Light propagates through a silicon optical fiber ▫ Index of refraction is temperature sensitive ▫ Phase of detected light is a measure of temperature

Acoustical temperature sensor • Speed of sound is temperature dependent vs = 331.5

T 273.15

m s

• Measure the time it takes to travel through the heated medium • Most sensors use ultrasonic sensors for this purpose. 93

Acoustical temperature sensor

94

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Acoustical temperature sensor

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Thermo-mechanical sensors

• Changes of physical properties due to temperature • • • • •

▫ Length ▫ Volume ▫ Pressure, etc.

Expansion of gasses and fluids (thermometers) Expansion of conductors (thermometers, thermostats) Many have a direct reading (graduation, dials) Some activate switches directly (thermostats) Examples:

Gas expansion temperature sensor

• Rise in temperature expands the gas • Diaphragm pushes on a “sensor” (strain gauge, potentiometer) or even a switch • The sensor’s output is graduated in temperature

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Thermo-pneumatic sensor • • • •

Called a Golay cell Gas expands in a flexible cell Motion moves a mirror and deflects light Extremely sensitive device

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Thermal expansion of metals • • • •

All metals expand with temperature Volume stays constant - length changes Each metal has a coefficient of linear expansion a. a is usually given at T1, temperatures in C.

l2 = l1 1 + a T2  T1

m

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Coefficients of linear expansion

Table 3.10. Coefficients of linear expansion for some. Coefficients are given per C. Material Coefficient of expansion a, x10 Aluminum 25.0 Chromium 30.0 Copper 16.6 Gold 14.2 Iron 12.0 Nickel 11.8 Platinum 9.0 Phosphor-bronze 9.3 Silver 19.0 Titanium 6.5 Tungsten 4.5 Zink 35 100

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Thermal expansion of metals • Coefficients of linear expansion are small • They are however measurable • Can be used to directly operate a lever to indicate temperature • Can be used to rotate a shaft • In most cases the bimetal configuration is used • Serve as sensors and as actuators

Example: direct dial indication • • • •

Metal bar expands as temperature increases Dial arrow moves to the left as temperature rises Very small motion The dial can be replaced to a pressure sensor or a strain gauge

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Bimetal sensors

• Two metal strips welded together • Each metal strip has different coefficient of expansion • As they expand, the two strips bend. This motion can then be used to: ▫ ▫ ▫ ▫

move a dial actuate a sensor (pressure sensor for example) rotate a potentiometer close a switch

Bimetal sensors (cont.)

• To extend motion, the bimetal strip is bent into a coil. The dial rotates as the coil expands/contracts

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Bimetal sensors (cont.)

• Displacement for the bar bimetal:

▫ r - radius of curvature ▫ T2 - sensed temperature ▫ T1 - reference temperature (horizontal position) ▫ t - thickness of bimetal bar

d = r 1  cos 180L r r=

m

2t

3 au  al T 2  T 1

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Bimetal switch (example)

• Typical uses: flashers in cars, thermostats) • Operation ▫ Left side is fixed ▫ Right side moves down when heated ▫ Cooling reverses the operation

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Bimetal coil thermometer

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Bimetal switch (car flasher)