working w/ K Thermocouples was: EGT Sensors

[Steve Ciciora]

Just wanted to pass on a couple of things I have learned about Type K
Thermocouples.  

I always put ferite beads on the Thermocouple wires just before the
conditioner chip.  I've had people tell me about problems they have had due
to noise that ferite beads made go away.

Once, I was using a thermocouple to measure the temperature of an arc lamp.
 The high voltage spike to turn it on induced high voltages into my AD595
T.C. conditioner chip, causing it to latch up, heat up, and die.  I put on
ferite beads, and to 1N4148 diodes 'back to back' accross the input to the
AD595 (pins 1 and 14).  The TC voltage never got high enough to foward bias
the diodes, so the AD595 didn't really notice that it was there.  What it
did was short out any spikes greater than about 0.7 volts.  Spark plug
wires might make enough noise to cause problems.

If you make your own TC conditioner board, (such as one using an AD595),
you can solder Type K thermocouple wire to the PC board using stainless
steel solder flux.  It comes as a liquid.  I dip the wire in the flux, then
tin it with solder.  I do this 3 or 4 times.

Finally, even though the data sheet for the AD595 says it puts out 10mv per
deg. C, the chip does not linerize the output.  If absolute temperature is
important (+/- 5 deg. or so), keep this in mind.

Oh, if you have never worked w/ thermocouples, keep in mind, you need to
use special wire (the + lead and the - lead are made of two different types
of wire, Chromel and Alumel).  To measure temperature accuratly, you need
to know the temperature of where you go from thermocouple wire to copper
(such as on the conditioning chip).  The AD595 measures this temperature
(assuming you run thermocouple wire right up to pins 1 and 14) and does the
'cold junction compensation'.  I guess if you are measuring 2000 deg.,
being off 20 degrees won't make much difference.

- Steven Ciciora