making thermocouple temp probes?

Like your motto, Jazz.

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.

Good luck Chris

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.

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

"JazzMan" wrote in message news: snipped-for-privacy@airmail.net... ....

I would not worry about the potting agent, unless it was sold as a moldable conductor. The aluminum is probably an oxide, or particles that are oxidized on the surface, and not particularly conductive. The thermocouple acts as a low voltage, low impedance source, so a little stray current would not change the sensed voltage enough to matter anyway.

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

In message , Chris writes

Don't know if this info is of any help but I made up a lower temperature probe, based on a Pt100 sensor & thin walled 3/16" brass tubing, by brazing in a very short length of brass rod to block off one end of the tube. Slid the sensor into the tube as far as it would go and back filled the rest of the tube with a very fine, dry sand, then sealed open end of tube with high temperature epoxy adhesive. The sensor leads & joints are protected by some lengths of narrow high temp sleeving and the sand was simply garden sand, washed thoroughly, dried in an oven, sieved through a fine grade kitchen sieve & finally ground very fine in the pestle & mortar usually used for herbs & spices (best to do this when herself is out of the house !). To get the sand past the sensor leads & into the tube fix a small paper cone around the open end of the tube, fill it with the fine sand and then gently tap the tube ......... I've been using this arrangement for some time as the sensor for a PID temperature controller & it works very well, strong mechanically & with a fast thermal response time.

The exact TC you need are available commercially, normally they are sold with a plug connector.

Jack

Got a link? I've looked through the McMaster-Carr catalog and typically their sensors are far too long (clearance issues in the engine bay around the manifolds) and too large in diameter. I want to run as small a diameter as possible so as not to affect exhaust flow. My perfect sensor would be 1" long and less than 1/8" diameter, with a 1/16" NPT base so that I could drill and tap the edges of the manifold flanges at the heads and install the sensor directly into the exhaust stream. I've never seen anything like that except in photos of complex test equipment setups.

JazzMan

I hate to send you to a competitor but for a one off order try Omega.com ($25).

I'd suggest you buy a 0.063 or 0.125" diameter mineral insulated probe whatever length you need with some lead wire. These probes can be bent to fit into the engine spaces as needed.

Buy the NPT fitting you want in a plug form and drill it out to accept the probe then braze the fitting onto your probe sheath at the desired immersion depth. I'd use a plain steel fitting instead of a SS fitting since if you don't design for the thermal expansion of your metals you could run into some problems. Use a lot of antiseize on the threads.

Bill Schuh Watlow