Maker Pro
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EDN garbage?


George Herold

Jan 1, 1970

Only in that he doesn't quantify how great the effect is.  One thing I've
learned about metrology is that _everything_ affects your measurements --
it's just that some things don't affect your measurements as much as
others.  Did you look at the paper he referenced?

I browsed the referedd to paper.. I didn't find anything about the
gradients along the wire.... I assume it says something to the same
effect somewhere.

To be honest the thermo-electric effect has always been a bit
mysterious to me. But I never heard of thermal gradients along the
wire making a difference. I guess maybe if you try and draw too much
current from the TC jucntion.

Did it quantify things

Yeah show me some numbers! Do a measurement.
I saw none of that.

Sorry for wasting both our times.

George H.

George Herold

Jan 1, 1970
Yes, he's full of it, assuming that the thermocouple wires are really of
uniform composition along their length, and that the ends of the leads
(where the amp is connected) are at the same temperature.  In general a
thermocouple measurement system has a bunch of thermocouple junctions,
most of which we want to be able to ignore--e.g. the terminal to the PC
trace, trace to pin, pin to wirebond, wirebond to chip wiring, chip
wiring to silicon...and back again.

Yup, thanks
Thermocouples stink for uses near room temperature.

Oh, now you tell me. :^)
I use copper-constantan TC as 'mostly' non-magnetic sensors. (fairly
fine gauge wire.) I'm not sure what other sensors would be non-
Maybe a little SMD diode? The TC's are very convenient though... just
stuff the wire where you want to measure. And cheap OTS

George H.


Jan 1, 1970

[calling the temperature gradients in wire an error source]

Well, the gradients are NOT an error source, but you'll
never convince him, because he can show you a counterexample.
Take a length of soft iron wire (sacrifice a coathanger, if you have to).
Connect the ends to an ammeter.
You'll see zero circuit current. Now fire up a torch and heat the wire
to make a hot spot. Still zero circuit current. Now slowly move the
hot spot.

When the hot spot in the wire goes one way, the current turns negative. Move
it the other way, the current goes positive.

The temperature gradient in the wire DOES make a thermocouple net voltage
here, because your hot spot is a different allotrope (sometimes called Austenite)
of iron crystal than the cold wire is. The Austenite-Martensite transition at
the trailing edge of your moving hot spot occurs at lower temperature than
the transition at the leading edge, because the transition has hysteresis. The
iron wire is thus two different metals, with connections at different temperatures.

So, if the thermocouple materials undergo phase transitions, the gradients
in the connecting wire do have a net effect on the circuit. That's a good argument
for knowing a LOT about the thermocouple wire materials before you measure
with them.