From: Jim Yanik


snip

bimetallic thermocouples generate millivolt signals,and paralleling
them would not work.Standard practice is to series-connect them,and
compare to a reference junction.Do these assemblies include signal
processing to convert the mV signal to a digital form,which could
then be sent on a parallel bus?

--
Jim Yanik
jyanik-at-kua.net

On every aircraft I ever worked on thermocouples were in parallel if
there were two or more.

Dan, U.S. Air Force, retired


Well,I guess they aren't bimetallic thermocouples,then.
Anyone have any speculation on what sort of signal will work with a
parallel connection?

--
Jim Yanik
jyanik-at-kua.net


All thermocouples are bi-metallic.

OK, try this on for size. Jet engines use chromal-alumal (type K) and
recips use iron-constantan (type J) thermocouples.

The different bi-metal combos generate different volts/degree.

The different combinations are for different temperature ranges. As I said all
thermocouples are bi metallic. The metals are determined solely by the
temperature being measured.


The wires are also
made of the same type of material. If you mount several thermocouples
in parallel then all plus wires match and all minus wires match. As
far as the cold junction end is conserned the hot end has one
thermocouple.



Just connecting them to a copper wire bus makes another bi-metal
junction,too,although it's not in the area being measured,and thus more
stable.

The discussion is about aircraft. In aircraft the wires that run to the
indicator is of exactly the same type as in the thermocouple. The total wire
resistance is 8 ohms in ALL aircraft. There are a pair of wire spools in one
leg that are used to adjust this. Copper wire is never used.

If you ever get a chance to look at a jet engine look at the EGT ring
of thermocouples. You will see they are in parallel.

Dan, U. S. Air Force, retired

You wire thermocouples in series and the voltage outputs add
together(sum).Parallel them,and one voltage bucks against the other.
Paralleling thermocouples is like parallelling two batteries of different
voltages.One works against the other.

I worked on thermocouple calibration while in the USAF,as a PMEL
technician(Precision Measurement Electronics Laboratory).

--
Jim Yanik
jyanik-at-kua.net


PMEL calibrated the test equipment I used on aircraft such as the jet cal
tester I used to test thermocouple systems. I can see why you might use copper
wire since it is more flexible than the chromel-alumel or iron-constantan we
used on aircraft. I don't know if you had to silver solder the leads to the
pins of cannon plugs, but on aircraft they had to be.

Dan, U.S. Air Force, retired