K-type thermocouple

[Phil Allison]

"Bob Parker"

From http://www.sensoray.com/support/tcapp.htm .....

"There is a misconception of how thermocouples operate. The misconception
is that the hot junction is the source of the output voltage. This is
wrong. The voltage is generated across the length of the wire.

** This is a pedantic and misleading comment - the hot tip IS the source of
voltage in a thermocouple used in the normal way.

Or, more precisely, voltage is only generated where there is a temperature
gradient along the pair of wires coming from the joint ( or tip) to the
meter.

The magnitude of this voltage depends ( in a non-linear way ) only on the
actual temperature difference - so the length of these wires becomes
irrelevant.

So - if the tip it at a certain temp, you will get the same voltage output
at the meter as long as NOT all the wire leading to the meter is at a
different temp to the meter.



...... Phil

 

[-=Spudley=-]

IAWD.

I don't understand this at all, it seems like a lot of gobbledegook.
The voltage generated is related to the difference between the hot and
cold junctions. In most modern measuring devices the cold junction is
compensated for within the device. A long thermocouple may have some
resistance but with modern high impedance measuring devices the effect is
minimal. The voltage is generated at the junctions.

 

[Bob Parker]

"Bob Parker"



** This is a pedantic and misleading comment - the hot tip IS the source of
voltage in a thermocouple used in the normal way.

Or, more precisely, voltage is only generated where there is a temperature
gradient along the pair of wires coming from the joint ( or tip) to the
meter.

The magnitude of this voltage depends ( in a non-linear way ) only on the
actual temperature difference - so the length of these wires becomes
irrelevant.

So - if the tip it at a certain temp, you will get the same voltage output
at the meter as long as NOT all the wire leading to the meter is at a
different temp to the meter.



..... Phil

It seems that the theory's more complex than it first appears.
According to some 'experts', the dominant voltage is generated by the
temperature gradient along the dissimilar wires, plus there's a smaller
voltage generated at the junction at the tip, plus other minor factors
affect the reading.
Anyway I just thought I'd throw that in to get people discussing the
subject. I'm no expert on thermocouples.


Bob

 

[Davo]

It seems that the theory's more complex than it first appears.
According to some 'experts', the dominant voltage is generated by the
temperature gradient along the dissimilar wires, plus there's a smaller
voltage generated at the junction at the tip, plus other minor factors
affect the reading.
Anyway I just thought I'd throw that in to get people discussing the
subject. I'm no expert on thermocouples.


Bob

Why would you get a voltage caused by a temperature gradient along a
wire? Copper-Constantan thermocouples are fairly commonly used, do you
get a voltage generated by a temperature gradient along a copper wire?
None of the copper wires I've ever seen develop a voltage from a
temperature gradient. What are these people talking about?

 

[Phil Allison]

"Davo"

Why would you get a voltage caused by a temperature gradient along a wire?

** It's a fact of nature - very likely a quantum effect.

Copper-Constantan thermocouples are fairly commonly used, do you get a
voltage generated by a temperature gradient along a copper wire?

** Yes.

http://en.wikipedia.org/wiki/Thermocouple#Attachment

None of the copper wires I've ever seen develop a voltage from a
temperature gradient.

** They do.

But only when you have dissimilar metals, in a series loop, will the effect
show up.

Contacts made of dissimilar metals show the same effect, but it is normally
only a few uV.

Plays havoc with some types of strain gauges and their pre-amplifiers.



....... Phil

 

[[email protected]]

Would indeed have to be continuously stirred. Ice itself may well be muchbelow
0C.

Grahams right (for once) but if the bath is continuously stirred so
that the water is moving all the way through the ice (or a least
through the ice in the immediate vicinity of the sensor) and the water
is pure (or at least de-ionised) the temperature is 0C plus or minus a
few thousandths of a degree Celcius/Kelvin.

If you dig through the American National Bureau of Standards web pages
you could find a more detailed exposition (I did the last time I
looked but that was about five years ago).