Re: making thermocouple temp probes?


JazzMan wrote:
> I've got some 1/8" OD 316 stainless tubing on order from
> McMaster-Carr, already have some thermocouple wire rated
> for 2K°F, and will be buying some fittings for mounting
> the probes in the exhaust manifold of my engine. My thought
> was to TIG one end shut on a 2" length of the tube and use
> a stainless compression fitting to lock the probe in place.
> I have several questions. One is, do I need to pot the
> thermmocouple wire inside the tube? If so, with what? Would
> it matter if the potting agent was conductive? Do I need to
> somehow weld the thermocouple junction to the end of the
> tube being used as the probe body? Or would simple contact
> be suffient?
>
> For reference, the engine's been modified and I've been
> editing the fuel maps in the ECM. Knowing the EGT at
> individual cylinders allows me to address flow balance
> issues in the intake manifold, for instance.
>
> JazzMan
> --

No, you do not need to pot the wire to the tube but it would be a good
idea to keep out water, grease, etc.

If the potting agent is conductive and if it causes a low impedence
between the 2 wires then you could get a secondary junction there. I
doubt that will be a problem with what you're doing.

You do not need to weld the TC junction to the tube cap although that
would probably give you a faster and more accurate measurement assuming
electrical noise is not an issue. A grounded junction would have less
susceptability to thermal gradient measurement errors in this
application.

Make sure you are getting sufficient immersion depth into the exhaust
stream otherwise you could get an error in your measurement due to
thermal shunting. You might try locating your sensor into the stream
and then move it slightly to get more or less immersion. If slight
changes in immersion depth change the indicated temperature by too much
it could mean you have insufficient immersion into the exhaust stream.

If you are getting an indication that you have insufficient immersion
you could try exposed junctions where you do not cap weld the tube but
rather let the wire junction be exposed to the exhaust stream and just
use the tube for mechanical support. You would need to seal the tube
at the cold end to prevent exhaust leaks or use a ceramic potting at
the hot end.

Bill Schuh
Watlow

.

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