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PHYS 352

Temperature Transducers – Part II

  Seebeck effect: two dissimilar metals in contact generate a potential difference between 'cold' and 'hot' junctions held at different temperatures

  size of effect for some combinations and the reproducibility (i.e. metal purity) lead to established pairs of metals

Thermocouples

+V −V

unknown high T known cold reference T

V

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Closer Look at the Seebeck Effect

hot end cold end

electrons diffuse this way (heat conduction by electrons this way)

leave behind +ve

accumulate −ve charge

+ −

- if wire attached, does current flow? - does it if the wire is the same metal? - can you measure the voltage?

Seebeck coefficient for the metal [V/K]

Table of Seebeck Coefficients   typical value: few µV/K

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Re-Draw Thermocouple Circuit V

all at same cold T

Thermocouple Circuit   Sa – Seebeck coefficient for

copper [V/K]   Sb – Seebeck for constantan   Tc – “cold” terminal block   Sc – another wire

  could be equal to Sa

  ΔT = Tx-Tc

  voltmeter meter measures   Sa(ΔT)−Sb(ΔT)

- if Sa = Sb (same metal), then no voltage - since Sc = Sc, don’t have to worry about Tc and voltmeter temperature making another thermocouple, distorting reading

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Standard Thermocouple Pairs   many work at very high

temperature   no self-heating   mV output but

reasonable sensitivity   ΔV/V per ΔT is large

Thermocouple Practicalities   thermocouple “laws” exist [from Wikipedia]

  Law of homogeneous material - A thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the application of heat alone, regardless of how it might vary in cross section. In other words, temperature changes in the wiring between the input and output do not affect the output voltage, provided all wires are made of the same materials as the thermocouple.

  Law of intermediate materials - The algebraic sum of the thermoelectric forces in a circuit composed of any number of dissimilar materials is zero if all of the junctions are at a uniform temperature. So If a third metal is inserted in either wire and if the two new junctions are at the same temperature, there will be no net voltage generated by the new metal.

  Law of successive or intermediate temperatures - If two dissimilar homogeneous materials produce thermal emf1 when the junctions are at T1 and T2 and produce thermal emf2 when the junctions are at T2 and T3 , the emf generated when the junctions are at T1 and T3 will be emf1 + emf2.

  this permits:   lead wires to be exposed to unknown and/or varying T   voltage measuring device is “third metal” and does not affect   soldered (which adds a different metal!) junctions okay

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Thermocouples as Transducers   they are a most popular device (cheap, rugged,

historical)   bare metal packed in ceramic withstands high T, high

vibration, mechanical and thermal shocks   standardized and replaceable

  for accuracy must use “thermocouple wire” (special grade of homogeneity otherwise wiring inhomogeneity gives addition thermal emfs)

  require reference junction compensation   electronic sensors can do this

  thermocouple sensing junction can be small for fast temperature response time

  ok linearity over a very wide range   ...but you sacrifice a little accuracy

  fabrication not as reproducible as Pt RTD

IC Temperature Sensors   based upon “ΔVBE bandgap” of silicon junction transistors

having a known temperature dependence   a simple description of the operating principle starts with

the bipolar junction transistor collector current equation:

  two identical transistors are operated with a constant ratio of collector currents (in saturation):

  the difference in VBE between the two will be directly proportional to T

IC = K(eqVBE /kT −1)

ΔVBE =kTqln

IC1IC2

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⎝⎜

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⎠⎟

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  Q8 and Q11 are the two transistors that produce the ΔVBE proportional to T

  R5 and R6 are laser-trimmed on the wafer to calibrate the sensor at 25°C (298.2 µA output)

sensitivity set to 1 µA/K

AD590 IC Temperature Sensor

IC Temperature Sensor Advantages   good linearity at low cost   does not require other external resistance measuring

circuitry or external linearization circuitry or precision voltage amplifiers

  does not need cold junction compensation that a thermocouple does

  can be built right onto a chip substrate (e.g. to measure CPU temperature)   can add memory to the chip   can add communications protocol

  drawbacks include limited temperature range   −55°C to +150°C

  requires DC power supply

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AD590 Specs

External Trim Circuitry   even if the AD590 is laser trimmed on the wafer (i.e.

calibrated at the factory) the accuracy might not be as good as you need

  add an external trim circuit

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  for higher temperatures, when you can't use probes   wavelength of

maximum emission changes with T

  hence measure the colour of the emission to determine temperature

  no need to equilibrate temperature probe (fast response time)

Optical Pyrometry blackbody formula

  overall intensity of thermal radiation changes with T   thermometry becomes photometry

  use an infrared-sensitive semiconductor detector (or other detector) to measure the intensity and infer the temperature   more on photometry in upcoming lectures

  measure intensity of radiation

Stefan-Boltzmann constant:   ε emissivity – between 0 and 1, defines how close to a

perfect “blackbody”   surface changes result in changes in emissivity   ε not necessarily constant with wavelength “grey body

approximation”   accurate measurement of ε is difficult and likely to change

  infrared photometry: limited accuracy because of limited knowledge of ε

Infrared Photometry

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Ratio Pyrometry   blackbody intensity:

  ratio at two wavelengths:

  b = 1.44 × 104 µm·K; λ ~ 1 µm, T < 6000K

from S. Salvatori, J. Vac. Sci. Technol. B 19(1), 219-223 (2001)

•  ratio has simple T dependence •  if the emissivity is the same at both wavelengths, it cancels •  only the wavelength dependence of the emissivity matters and not knowledge of the absolute value

for b/λT >> 1

Cryogenic Thermometry   special thermistors and special resistance devices

(e.g. Au-doped Ge or RuO2) can be used   self-heating effect is even more pronounced (even

heat load imposed upon cryogenic system could be a concern)

  temperature is an equilibrium concept; at ultra-low temperatures often non-equilibrium conditions are of concern   e.g. detecting ballistic phonons

  often, physical processes exploited for cooling below 1K are related to temperature measurement (e.g. paramagnetism at low temperatures, 3He-4He mixture phase properties)