Transcript Welcome to Bioinstrumentation Design

Temperature transducers:
Embarrassment of riches
• Most material properties change as a
function of temperature
length, 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 crosssection.”
Fever strips: color change in cholesteric liquid crystals
Measure change in reflected light
eformsnews.blogspot.com/2007/10/how-e-paper-works.html
rose chafer beetle www2.cnrs.fr
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:
but
“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
Interlude: sensing heat by capturing photons: E = hn
Demo: IR sensing (camera)
http://www.flir.com/US/
Interlude 2: Bimetallic strip
howstuffworks.com
www.diracdelta.co.uk/science/source/b/i/bimetallic strip/source.html
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:
∆T = Ttherm - Tamb
While the self-heat temperature is increasing, mass is storing energy:
The Leaky Integrator again!
Comes then our humble servant
the Lab 5 FTQ
Thermoelectric effects and thermocouples
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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°?