The short answer is that you can use regular copper wire to extend your
thermocouples, but you'd have to make sure that the adjacent splices were at the
same temperature. You can buy thermocouple wire at
www.omega.com and I'd recommend that. That
way any temperature differences won't matter to your measurement of OAT.
You'll find that fewer than one in a thousand people actually understand
thermocouple physics. Now I'm not saying that I'm a genius -- hardly --
but I did have the opportunity to work in a research lab using
thermocouples.
Thermocouples work by having two junctions between different
materials. A Type K thermocouple uses chromel and alumel (whatever the
heck they are) as the two materials. A Type J thermocouple uses iron (at
least I know what that is) and constantan (huh?). At least I think it's
that way but I may have it backwards. Anyway, for the purposes of this
discussion, it doesn't matter.
So we have three sections of wire. Material A at each end with
material B in the middle.
AAAAAAAAA - BBBBBBBB - AAAAAAAA
The hyphens represent the joints (junctions) where the wires are
connected.
Okay. Now we put one junction in ice water, and the other one on what
we're trying to measure the temperature of. You will get a voltage
difference -- usually in millivolts. That's one of those laws of nature,
called the "thermoelectric effect". Note that if the junctions are at the
same temperature then you'll get zero volts. You could make a thermocouple
out of aluminum and steel, or lead and gold, or uranium and plutonium if you
wanted to, but the voltages you'd generate would be too small to measure.
Now, you've probably noticed that the electronic gizmos we use in our
airplanes don't actually use a bath of ice water like we do in the
laboratory. They somehow synthesize a cold junction. (It's BFM to me
-- that's "black f**king magic" -- and maybe Brent Regan or Greyhawk can
educate us on this topic. After all, Scott seems to have a lot of time on
his hands, for which I am insanely envious, and Brent Regan IS a genius...but I
digress)
Also note that you can run these backwards -- if you apply a voltage
difference then you will create a temperature difference. Put the cold
junction on your beer can, blow air over the hot junction to take the heat away,
and shazam! You have a thermoelectric refrigerator and cold beer.
Nothing esoteric about this -- you can buy them at Wal-Mart. Put the power
in one way and you get a cooler. Put it in the other way and you get an
inefficient heater. Before you get excited and decide to turn your VMS
1000 into a beer cooler for your hangar, you should know that they don't do this
with a single thermocouple junction; they make up little plates with hundreds of
junctions on them.
But I digress again...
What you were proposing is like this, where C is copper.
AAAAA - CCCCC - AAAAA - BBBBB - CCCCC - BBBBB
(Note that the AAA and BBB at the ends may very well be the terminals on
your electric gizmo.)
You can definitely make this work. If the junctions between the
copper and the thermocouple materials are at the same temperature then you'll
get the proper readings. That doesn't mean both junctions need to be at
the same temperature -- but where you splice the copper into the circuit should
be the same temperature on both sides. Putting it yet a third way, the
splices to copper near your gizmo should be the same temperature as each other,
and the splices from copper to the thermocouple wires should be the same
temperature as each other, but the splices near your gizmo do not have to
be the same temperature as splices by the thermocouple. You could, for
example, connect the copper wire to your electronic gizmo in the cabin, and the
copper wire to the thermocouple in the engine bay. You can probably make
this work by wrapping the two splices in the engine bay together, or by
using a connector where the parts are basically all at the same
temperature. But if you want to remove all doubt, use thermocouple wire
for your extension cord. It's not expensive like airplane parts....
- Rob Wolf
Lancair 360
81% complete
Parker Colorado