If the leads are Alumel / Chromel , not only is the polarity important, but
the total length of the harness as well, as it will affect the calibrated
reading. Some use trimming resistors for calibration, (the old fasioned wire
wound type).
Dave

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com]On Behalf
Of George Goff
Sent: Sunday, December 28, 2003 7:28 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: What makes a pyrometer's cable so special?

In a message dated 12/28/03 6:24:38 PM, lalofs@RCN.COM writes:

<< the type of wire can be important.  The thermocouple probe itself is a
junction where two
different metals meet, typically just melted together in a small bead.
It produces a VERY small voltage roughly proportional to temperature. >>

Any extension conductors from the thermocouple head to the associated
instrumentation/controller are ALL important and must be the same metals as
the
thermocouple.  This can be an expensive proposition considering the noble
metals
which are needed for certain types of thermocouples.  Swapping the leads as
sometime happens because of color coding variations, can wreak more havoc
than
running copper wire to the thermocouple.  The potential produced is very
small
and very nonlinearly proportional to the temperature.

George