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1. 06:21 AM - Re: EFIS Erratic Temp Readings (Robert L. Nuckolls, III)
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| Subject: | Re: EFIS Erratic Temp Readings |
>> What is this phenomenon called? Can you point
>> me to literature that explains it?
>
>It is called the Seebeck effect. I previously included a link that
>explains the effect: tinyurl.com/3njog7qv Also refer to the links in
>that article for more information.
>
>For practical purposes you can certainly continue to say that the
>voltage is generated at each junction and that nothing happens in
>between. All I am doing is explaining what actually happens.
I had to dig out references I've not consulted
for oh . . . say . . . 40 years? Even my
revered CRC Handbook of Chemistry and Physics
doesn't dig this deep into the phenomenon. But
I did find some recitations on the phenomenon
Dick brought to the conversation.
Thermocouples are but one instance where the
characteristics of materials push electrons
around. These are studies in electro-motive
physics.
There are many instances where 'differences'
in two materials encourage motion of electrons
under certain conditions.
MECHANICALLY induced electro-motion:
Shuffle your rubber soled shoes on a carpet
in dry weather and you build a potential on
your body that can produce visible sparks
when discharged to some metal object or perhaps
another person. Leather soled shoes, yeah but
not so much.
Mechanically modulated magnetic fields in
alternators and generators produce very
significant electron motion that keeps our
instrument panels lit up.
CHEMICALLY induced electro-motion:
Conductive plates pasted with two different compounds
of lead submerged in mixture of sulfuric acid will
offers a potential for flow of electrons . . .
what's more it's reversible. But if the plates
are identical, no joy.
Pieces of zinc and copper pushed into a lemon
produces a measurable voltage between the
two metals; but use two copper pieces and
the effect goes away.
THERMALLY induced electro-motion:
put any two dissimilar metals in intimate
contact with each other and they will produce
a voltage based on temperature.
I had not picked up on the phenomenon Dick
cited because it's a layer down from
the practical physics that applies to our
craft.
About 1821 this observant guy Seebeck discovered
the thermo-electric effects in a loop of two
metals wherein their joinings (junctions) were
at different temperatures. The energy flow generated
in that loop of wire was directly related to
the magnitude of temperature difference between
the two junctions. If the alloys were carefully
controlled and tiny voltages accurately measured
that energy would represent real temperatures.
Hence the foundation of the thermocouple driven
instruments spanning over a century of aviation
practice.
ELECTRICALLY induced thermo-motion:
A few years later in 1834, another sharp fellow
Peltier observed that the thermo-electric generation
was reversible. I.e. if you replaced the thermocouple
measuring instrument with a current source, those
same two junctions would assume temperatures above
and below ambient. Effects difficult to observe
due to the very tiny effects. But they were real
and could be detected.
It wasn't until years later in 1854 that another
sharp fellow Thomson identified thermo-electric
effects within conductors that explained how
Seebeck and Peltier's discoveries worked.
The longitudinal thermo-electric property
within conductors was named after Thomson. The
Seebeck, Peltier and Thomson effects combine
to complete a story of how thermocouples
function to monitor engines and thermoelectric
coolers refrigerate beer.
At the ripe age of 22, Thompson was awarded
a professorship in natural philosophy (physics).
Thompson was only 30 years old when he
identified and quantified this longitudinal
behavior of thermodynamics in a single
conductor. Thompson was better known by his
British title of station: Lord Kelvin.
Dick cited the next level down in the
physics of Seebeck and Peltier effects;
accurate but not generally included
in practical explanations for the two
technologies. It's kinda like dissecting
the physics of lubricity and viscosity
when considering suitability to task
for lubricating oils in engines.
His only error was to identify it as the
'Seebeck effect' as opposed to the
'Thomson effect'.
It was an interesting excursion to
a time when I probably should have
been paying more attention in class!
Thanks for the trip Dick!
Bob . . .
-----------------------------
Bob Nuckolls
AeroElectric Connection
P.O. Box 130
Medicine Lodge, KS 67104-0130
Mobile: 316-209-7528
Web: http://aeroelectric.com
------------------------------
>Dick
>
>
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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