Why 0V when thermocouples are placed in an ice bath

The voltage is produced at the junction between the dissimilar metals, not by their individual bulk. So if your setup is

- A - B - A -

And both the AB and BA junctions are at the same temperature they should produce the same magnitude of voltage. Since they are wired opposite each other, the voltages in your circuit will be opposite and cancel.

Ice bath isn't magic - for this purpose anything that insures consistent temperature might work. But ice bath is a good reference to leave one junction in and compare the other against, because as long as the bath contains both ice and water it's temperature is going to depend on the purity of the water and not much else (ambient temp, pressure, etc)

There's also a nifty circuit out there that behaves like a reference junction in a virtual ice bath. I forget how it works, but omega's temperature measurement handbook might tell you.

I tell you exactly what I was doing. I have a type s thermocouple I was doing a quick reference check on. I hooked copper wires from a hp 3458, which has copper inputs, I hooked the copper wires to the thermocouple and placed the junction in the ice bath. I placed the thermocouple in the ice bath. It read 6uV so I know its good. I'm just trying to understand why. The thermocouple should produce its own emf of x, having platinum and rodium. But wouldn't the platinum to copper and the rodium to copper produce a different emf on each junction?

The EMFs cancel out since the temperatures at all three junctions are the same (well, your 6uV indicates an error of perhaps 1 degree C, a mismatch with extension leadwire alloy, or some combination of the two).

Best regards, Spehro Pefhany

Aack! The old thermocouple junction myth rears it's ugly head again! You have this exactly backwards - thermocouple voltage is produced by the temperature gradients in the bulk of the wires, not by the junction. The junction merely provides an electrical connection which allows the bulk gradients in between the measurement junction and reference junction to be measured. In the above example with the reference junction (the connection of the thermocouple wires to the copper meter leads) at the same temperature as the measurement junction (both in the ice bath), the net temperature gradient between the junctions is zero degrees, regardless of any different temperatures in the middle of any of the wires.

There is no known possible physical mechanism which can produce thermocouple voltages at the junction. Voltage gradients and temperature gradients are associated because the high thermal conductivity of good electrical conductors is due to transport of high energy electrons from the hotter side to the colder side, balanced by lower energy electrons returning to hot side. Same current both directions, but the hotter electrons "see" more resistance, hence higher voltage drop in the hot to cold direction, amount differing between metals but not direction. See also Wiedeman-Franz law.

Oops, you're right, and I actually had heard that before, but managed to forget. It's crossing the thermal gradient with different metals that gives rise to the difference...

In terms of what the correct theory clarifies that the flawed one misses, the most obvious practical implication would seem to be that you can use a mix of metals in your voltmeter as long as the meter comes close enough to being at a uniform (if arbitrary) temperature throughout - or do I have that wrong as well?

The junction of dissimilar metals produces no voltage, regardless of the temperature of the junction. It is temperature change along the length of a conductor that produces voltage, and each alloy has a different amount per degree, and a different curve of voltage per degree versus temperature. So any number of junctions of any combination of alloys produces no voltage, as long as the whole thing is at the same temperature.

Not quite correct; the platinum to copper connection was "at" room temperature, and the rodium to copper connection was "at" room temperature - the temperatures could be reasonably different, but fairly "close" to each other. Only the platinum to rodium connection was in the ice bath (*read* what he said).

Your interpretation is incorrect, or the measured voltage would have been about 115uV, not 6uV. He clarified in his second post that all three junctions were in the ice bath when the voltage measurement was made:

"I hooked the copper wires to the thermocouple and placed the junction (sic) in the ice bath. I placed the thermocouple in the ice bath."

Best regards, Spehro Pefhany

You have that correct; no temp gradient, no voltage. Of course it is difficult to find/build a decent meter with no power dissipation, which would be required for no temperature gradients, so in practice you try to make sure that temperature gradients are as small as practical and occur in the same materials so they cancel.

But why is there a voltage difference then when you heat the ends. Is it because a temperature gradient in the middle would null itself out?

Because the two different metals passing from the junction temperature to some other common temperature (where the temperature is measured) generate different potentials when passing between those two temperatures.

To have a calibrated temperature measurement that ignores the temperature at the meter, you subtract the difference produced when a similar couple passes from the reference ice temperature to the meter temperature.