At 12:55 PM 07/12/2010, you wrote:
>Sean Breheny wrote:
> > Is there really one reference junction, though? I am under the
> > impression that any joint of dis-similar metals in the thermocouple
> > circuit will cause a voltage difference which is temperature
> > dependent. This includes even things like bond wire connections inside
> > the IC used to read the thermocouple. So, I'm not sure that it makes
> > sense to couple the cold junction sensor so closely with the TC
> > connector unless you are also coupling it closely with the rest of the
> > electronics in the circuit which reads the TC.
>
>Temperature differences don't matter anymore when both conductors are made
>of the same material, like both become copper traces on a PC board.  For
>best accuracy you want both junctions, each between one of the thermocoupl=
e
>wires and the PC board, to be at the same known temperature.  The
>thermocouple voltage then tells you the difference between this temperatur=
e
>and the junction at the other end of the cable where the two dissimilar
>conductors meet.  Add that to the known absolute temperature and you get t=
he
>absolute temperature at the end of the thermocouple.

Because of nonlinearity, that's a bit of an oversimplification if you
really want accurate readings from a thermocouple.

To get an accurate reading you need to read the TEMPERATURE at the
cold junction, then transform that (nonlinear) into a VOLTAGE, which
is then added to the VOLTAGE read from the thermocouple, and then use
the inverse (nonlinear) transform to find the corresponding TEMPERATURE
at the hot end. The only fly in the ointment is a T/C which is not
monotonic (eg. B).

Analog implementations cut corners and assume linearity around room
temperature (a really poor assumption if things get cold, for many
thermocouple types) and limit variations about room temperature so
you get a sort-of accurate reading, but it's not exact and there is
little excuse for doing this sort of hack in the digital realm.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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