working w/ K Thermocouples was: EGT Sensors

[Steve Gorkowski]

What type material do you use? I have only small amounts of help with ferrite
beads . I use single end grounded shielded thermocouple wire . Interested in
your process.

Steve

Steve Ciciora wrote:

> 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