Re: making thermocouple temp probes?

Chris wrote:
>
> 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.

> Like your motto, Jazz.
>

Thanks! I stole it from someone else a while back. I have no
idea who the wendel guy is, but the saying just seemed to embody
an ideal I thought was worth knowing.

> Exposed thermocouple beads can be grounded at the point they're
> measuring (sometimes there's no other way), as long as your instrument
> measuring the thermocouple is floating (like a battery-powered
> thermocouple meter). As long as you've got a floating meter, a
> grounded bead and a SS compression fitting is a great idea. You might
> want to put a sliver of mica between the compression fitting and the
> T/C bead to ensure you're not shorting out the T/C by grounding it at
> more than one point. Other than that, you're in good shape.
>
> Sounds like you've got a good plan.

I appreciate the advice. The t-couple wire pair is insulated in
a single woven material, and each individual wire is also
insulated. I'll have to remove the main insulation to get
the two wires to fit inside the tube. The potting agent would
be used to seal the two wires going into the tube to keep
water and dirt out. The patting agent I looked at in the catalog
has aluminum in it, so it would presumably be conductive. I
don't know if the agent would soak through the woven insulation
around the individual wires.

JazzMan
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