Temperature transducers: Embarrassment of riches Most material properties change as a function of...

Temperature transducers:

Embarrassment of riches • Most material properties change as a function of temperaturelength, density, resistivity, Young’s modulus What about specific heat Hs? • Nitinol demo: shape memory alloy• What we most passionately want: a sensor providing voltage output• Diode current• Platinum strip• Thermistors• Thermocouples

http://www.nitinol.com/

“Shape setting refers to the process used to form Nitinol. Whether the Nitinol is superelastic or shape memory, in the cold work or straightened condition, it is often necessary to form the material into a new “memory” shape. This is done by firmly constraining the material into its new shape in a fixture or on a mandrel and then performing a heat treatment. The heating method can be an air or vacuum furnace, salt bath, sand bath, heated die, or other heating method. The temperature should be in the range of 500-550C with higher temperatures resulting in lower tensile strengths. Cooling should be rapid to avoid aging effects, a water quench is recommended. The heat treatment time should be such that the material reaches the desired temperature throughout its cross-section.”

Fever strips: color change in cholesteric liquid crystalsMeasure change in reflected light

rose chafer beetle www2.cnrs.fr

eformsnews.blogspot.com/2007/10/how-e-paper-works.html

Specs: accuracy / range speed of response spatial localization

• What we really want is not temperature-sensitive shape, or temperature-sensitive color, but temperature-sensitive voltage, for direct input into electronic instrumentation… • We will see that desire for voltage in pure form, with thermocouples, but for now…

•How does increasing temperature affect resistivity? of metals? of semiconductors?

•The case of the diode:

“What man most passionately wants is his living wholeness and his living unison, not his own isolate salvation of his ‘soul’. Man wants his physical fulfillment first and foremost, since now, once and once only, he is in the flesh and potent. For man, the vast marvel is to be alive. For man, as for flower and beast and bird, the supreme triumph is to be most vividly, most perfectly alive. Whatever the unborn and the dead may know, they cannot know the beauty, the marvel of being alive in the flesh.” DH Lawrence

but

Interlude: sensing heat by capturing photons: E = hn

Demo: IR sensing (camera)

http://www.flir.com/US/

Interlude 2: Bimetallic strip

www.diracdelta.co.uk/science/source/b/i/bimetallic strip/source.html

howstuffworks.com

Bimetallic strip: thermal expansion coefficients α from S.E. Derenzo, Interfacing, Prentice-Hall (1990):

Platinum temperature standard: From Derenzo, page 157: "Platinum is a noble metal that can withstand high temperature and harsh chemicals with good stability."

The problem: measuring resistance normally involves passing current through the “resistor”, thereby heating up the component…Joule heat: I^2*R

Thermistors (Lab 5) sintered semiconductor material

TE is a property of the semiconductor material used to make the thermistor. Boctor and Ryff suggest a nominal value of 4000 deg K

Power-in will equal power-out at asymptotic temperature equilibrium:

While the self-heat temperature is increasing, mass is storing energy:

The Leaky Integrator again!

∆T = Ttherm - Tamb

Comes then our humble servant

the Lab 5 FTQ

Thermoelectric effects and thermocouples

• Seebeck effect (passive diffusion)

• Thomson effect (heat differential EMF)

• Joule effect (I2R heating)

• Peltier effect (cooling by pumping electrons)

“Electrons speed up or slow down under the influence of contact potential difference. In the first case the kinetic energy of the electrons increases, and … turns into heat. In the second case the kinetic energy decreases and the joint temperature falls down.” ixbtlabs.com/articles/peltiercoolers/

Attaching N and P doped elements in series

If electrons are forced from the low mobility to the high mobility side, the effect is like an expanding gas in a refrigerator, and the junction will be cooled.

More elements in series

Mammals sensing temperature

• Peripheral vs central locations of sensing• In the skin, two types:

*sensors for warmer*sensors for cooler

• Warm range: 30-45˚C• Cold range 10-35˚C• Once skin is taken below 10˚C, cold receptors stop firing

and cold becomes a good local anesthesia• Temperature-sensitive proteins in sensor cell

membranes open channels for Na+ or K+

Mammalian temperature sensors• Firing rates of skin sensors obey power laws

as a function of Δ temperature• “When the body becomes too warm, blood vessels in the skin dilate,

allowing heat to escape through the surface of the skin. Special glands called sweat glands produce a salty secretion called perspiration that evaporates off the surface of the skin, taking heat with it. When the body becomes too cold, the opposite processes occur. Sweat glands are shut down, and blood vessels in the skin constrict, keeping the blood away from the surface of the body, where heat could be lost. In addition, the muscles begin to contract rapidly and shiver, which generates significant heat.”www.sparknotes.com/testprep/books/sat2/biology/chapter9section1.rhtml

• If hypothalamic neurons sense temperature less than normal, then metabolism, and muscle activity (including shivering) will be increased.

• If hypothalamic neurons sense core temperature greater than normal, they trigger responses like cooling by evaporation of sweat.

Fever

• Increased core temperature may be due to athletic activity, or fever, or high ambient temperature.

• Fever is due to toxins released by infecting bacteria or virus.

• The toxins affect membrane proteins in membranes of temperature-sensitive neurons. An abnormally higher response threshold is set.

A mammal’s reaction to high or low ambient temperature…

• How can a mammal cool itself when the ambient temperature is 110°?

• How can a mammal warm itself when the ambient temperature is 20°?