Question regarding thermocouples and a voltage terminal block

[Rich Grise]

I can't agree with that view at all. The verbiage "thermocouple
voltage produced by the junction" is complete nonsense; how can it
possibly help anyone's understanding? It does not take much
investigation to actually understand the physical basis for the
function of thermocouples, which in my opinion is far better than
relying on a completely fictitious explanation even if that fiction
can be used to make correct calculations.

It behaves as if it's caused by the junction - for example, do you
have to compensate for the length of the leads? ;-)

Thanks,
Rich

 

[Glen Walpert]

It behaves as if it's caused by the junction - for example, do you
have to compensate for the length of the leads? ;-)

Only if the impedance of your meter causes significant IR voltage drop
in the leads.

It may seem to be caused by the junction if you don't think about it
much, but when you try to understand why a junction could cause
thermocouple voltages there are no answers. What exactly is the
physical effect? How does the junction know what the temperature at
the other junction is? Where does the energy come from? In a 3-metal
junction, where the thermocouple leads are separately spot welded to
the part being monitored or where copper meter leads are attached, it
is known that the third metal in the junction has no effect on the
measurement - why is that?

Once you understand that it is the total temperature gradient between
junctions that causes the voltage, the thermocouple actually makes
sense. It is a heat engine which obeys all laws of thermodynamics;
the energy which produces the voltage is driven by the conduction of
heat energy from the hot end to the cold end. No temperature gradient
in the third metal of the 3-metal junction means no voltage
contribution.

When a temperature gradient exists in a metal most of the thermal
energy transport (heat conduction) is done by movement of free
electrons; higher energy electrons travel from the hot end to the cold
end and lower energy electrons travel at the same rate from the cold
end to the hot end. The higher energy electrons have a higher
electrical resistance associated with their current flow from hot to
cold than the lower energy electrons have associated with their
identical current flow from cold to hot; therefore (in the absense of
externally imposed current) the hot end is always more negative than
the cold end in metals (or semiconductors where electrons are the
majority carriers - opposite for holes). The magnitude of the effect
is different for different materials, and it varies with temperature
in a complex non-linear way which is different for different
materials, but no problem, the results for metal pairs that
thermocouples are made of have been tabulated for your convienience.

This has all been explained much better in the various references I
have posted links to over the years, and I think anyone with any
interest in thermocouples would benefit by reading them and/or
additional reading on the subject.

http://www.electronics-cooling.com/Resources/EC_Articles/JAN97/jan97_01.htm

I guess I just have a weakness for explanations that make physical
sense .

Regards,
Glen

 

[Rich Grise]

Only if the impedance of your meter causes significant IR voltage drop
in the leads.

It may seem to be caused by the junction if you don't think about it
much,

That's true - but to the guy building the diff amp at the cold junction,
the physics of how the wires work isn't really an issue.

but when you try to understand why a junction could cause
thermocouple voltages there are no answers. What exactly is the
physical effect?

I thought it was something similar to the way a battery or dissimilar
rectifier might work, until I saw them making TCs from scratch by
welding chromel wires to alumel wires. There's no "junction" in a
(proper) weld - the metals merge and form an alloy, with a gradient.

How does the junction know what the temperature at
the other junction is?

Well, to the uninitiated, it doesn't care, because it's the "hotness"
that causes the mumble mumble volts at the output! ;-)

The rest, I've snipped - you present a pretty good case - the only
thing I was pointing out is that it _looks like_ it's coming from
the junction - I left out the "things are sometimes not what they
seem" part. ;-)

Cheers!
Rich

 

[DJ Delorie]

I thought it was something similar to the way a battery or dissimilar
rectifier might work, until I saw them making TCs from scratch by
welding chromel wires to alumel wires. There's no "junction" in a
(proper) weld - the metals merge and form an alloy, with a gradient.

Thermocouples 101

A length of wire, with each end at a different temperature, produces a
voltage difference between the two ends.

Wires of different composition produce different voltages for the same
temperatures.

The length of the wire, or the connections at the end, are irrelevent.

The way a thermocouple works is by taking two wires of different
metals, connecting one end of each together, and measuring the voltage
across the remaining ends. If you know the voltage, and the local
temperature, you can calculate the remote temperature.

Example: If an Al wire is producing 10uV at the hot end, and a Cu wire
is producing 7uV at the hot end, measuring across the cold ends
results in 3uV (the difference).

So, it's not the junction that's doing the work, it's the whole length
of thermocouple wire. Also, this is why you need to have the
thermocouple wire reach back to the sensor board - the board needs to
know the temperature of the cold end, using an extension cord puts the
cold end too far from the board's temperature sensor.