Hi Even *with* all the fancy stuff in my Fluke thermometers … they still are only rated for a bit worse than 0.1 C. When I send them in for calibration, the thermometer generally comes back “calibrated fine”. The thermocouple I send in with them often comes back with a note about “you need to buy a real thermocouple …”. On a simple lash up, you would use a thermocouple that is lying around as your cold junction. If you get yours from eBay (like I do) … who knows what you have.
Lots of gotcha’s. Bob > On Nov 11, 2016, at 9:16 AM, Scott Stobbe <scott.j.sto...@gmail.com> wrote: > > If you want sub degree precision, you will need to make your connections to > dissimilar metals on an isothermal boundary, a terminal block is better > than clips in free air. > > On Fri, Nov 11, 2016 at 8:28 AM, Bob Camp <kb...@n1k.org> wrote: > >> Hi >> >>> On Nov 11, 2016, at 8:02 AM, jimlux <jim...@earthlink.net> wrote: >>> >>> On 11/10/16 10:28 PM, Mike Millen wrote: >>>> It would work as well if you used a pair of regular copper wires to >>>> connect the meter to the thermocouple... >>>> >>>> The junctions created by all the new connections will cancel out. >>>> >>> >>> as long as the temperatures are "exactly" the same, >>> (Seebeck coefficient varies with temperature) >>> and the two metals at the junctions are the same, >>> (ditto, but the curves are different for different materials) >>> and the mechanical configuration is the same >>> (current density also affects it) >> >> The gotcha is that few of us weld copper directly to the thermocouple >> leads. The far more >> common approach is to grab clip leads. At least around here, the clips on >> the leads are >> not made of copper. They are some sort of (badly worn) plating over >> (oxidized) base >> material. >> >> I grab a “copper wire” clip lead and hook up to the thermocouple. There >> isn’t a lot of >> delta T in most bench situations. In this case you have a heated gizmo >> warming things up …. >> Who knows what the delta T may be or how small the contact area actually >> is. >> >> Simple answer: Don’t trust the first number you get. Try it a couple of >> times with *different* >> leads. Make sure you do indeed get within a degree or three on each of >> them. Depending on >> how you have your cold junction set up, that may also need the same >> treatment. >> >> Bob >> >> >>> >>> >>> For run of the mill "measure to 1 degree at room temperature" you can >> probably make that assumption. >>> >>> But if you're looking for precision, you need to take this stuff into >> account (that's what "cold junction compensation" is all about.. ) >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to https://www.febo.com/cgi-bin/ >> mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/ >> mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
