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Posted by David Sherman on Saturday, September 27, 2008 at 10:50:19 :
In Reply to: That does make sense.... thankyou posted by Bruce in BC on Saturday, September 27, 2008 at 00:34:24 :
So long as you know what "type" of thermocouple it needs, which is usually a "J" or "K", any will work. The "type" of a thermocouple indicates what the metals or alloys of the two wires are. "J" uses iron and constantan. "K" uses chromel and aluminum. There are half a dozen more types in use. Each has advantages and disadvantages in terms of temperature range and accuracy. "K" is probably the most common. To play around, you can make a thermocouple just by twisting together tightly the ends of a copper and iron wire and hooking the free ends up to a volt-meter set to its lowest scale. Heat up the junction with a lighter and you'll see that it generates voltage. You can solder or weld the junction together for stability, but it won't change the reading.
Omega Engineering is the major industrial source for all sorts of temperature measuring stuff and they're happy to sell single quantities direct. If you know your gage needs a type K thermocouple, and you're building the system from scratch anyway, you might check to see any of their would fit easily. You probably want one that's encased in a stainless-steel shroud to protect it from corrosion, although exposed junctions respond much faster. You can get them with all sorts of mountings (flanges, pipe threads, compression fittings, etc.) and with various kinds of wiring. When I get around to putting a pyrometer on my good M35s, which I'd like to do because I upped the fuel rate and it's no doubt running pretty hot now, I'll go the Omega route. I did add a turbo boost gage, which is pretty handy just by itself. One thing to keep in mind with thermocouples is that every wiring junction is its own thermocouple, and the signal you're looking at is milli-volts. For the best accuracy run the thermocouple leads directly to the gauge and don't try to splice in extra pieces of wire. Especially avoid wire nuts and crimp connectors.
Something like the Omega TC-K-NPT-U-72 would probably be good. It has a 304 stainless steel shielded junction, which should resist hot exhaust gas, mounts through a 1/4" IP hole, so to mount it all you have to do is drill and tap one hole, has fiberglass-insulated stainless steel braid-covered wire to resist the heat, a strain relief, and is only $34, which seems pretty reasonable for a ruggedized industrial unit.