> From: John Woodgate > Sent: Thursday, August 23, 2007 2:11 PM > > Tarver, Peter" writes: > > >In a practical sense, for solder be effective, you probably need a > >solder pot, a flux and a short, neatly twisted tip at the end of the > >thermocouple wires. > > Sacred cow say, 'Do not twist'. No, I don't know why, either. > Only the contact between the wires at the extreme 'inboard' > end of the twist is effective as the thermojunction, though. > The rest only adds mechanical strength.
The twist could be as little as a half turn and is only intended to bring the two conductors into contact with one another, prior to soldering. The heating effects of the solder pot could allow the wires to be drawn away from one another, due to either different coefficients of expansion or stresses in the wires due to local hardening (from processing or being bent in handling ...) I don't twist the ends when I weld thermocouples, but I've seen instructions on welding that say to do so. The point nearest the 'free' ends of the thermocouple wire is where the temperature will be measured and will act as the junction. If a twist is a cm long, there's probably 9 mm of irrelevant (to temperature measurement) wire at the end. Possible oxidation of the conductors and relatively high ohmic contact increases the inaccuracy of a measurement. As Bob Johnson pointed out, open-circuits are a big problem, but on some occasions, especially when I've leant thermocouples out, the insulation is abraded during removal. If care is not taken, it's very easy to not notice bare wires near one another. An anecdote: I once created a set of sixty+ 20' long, 30AWG, Type T thermocouples to measure temperatures in a hydrogen fuel cell to be placed in a large environmental chamber (20' left about 3'on the outside of the chamber). Made from scratch, it took about five hours to make this batch. The customer insisted on removing the thermocouples, rather than allowing one of our staff to do so, to whom I gave instructions to be careful with them. Their method: yanking on the wire where it exited the enclosure. I was not pleased with what was returned to me. Some sections I cut out were four and five feet long. Regards, Peter L. Tarver, PE ptar...@ieee.org CONFIDENTIALITY This e-mail message and any attachments thereto, is intended only for use by the addressee(s) named herein and may contain legally privileged and/or confidential information. If you are not the intended recipient of this e-mail message, you are hereby notified that any dissemination, distribution or copying of this e-mail message, and any attachments thereto, is strictly prohibited. If you have received this e-mail message in error, please immediately notify the sender and permanently delete the original and any copies of this email and any prints thereof. ABSENT AN EXPRESS STATEMENT TO THE CONTRARY HEREINABOVE, THIS E-MAIL IS NOT INTENDED AS A SUBSTITUTE FOR A WRITING. Notwithstanding the Uniform Electronic Transactions Act or the applicability of any other law of similar substance and effect, absent an express statement to the contrary hereinabove, this e-mail message its contents, and any attachments hereto are not intended to represent an offer or acceptance to enter into a contract and are not otherwise intended to bind the sender, Sanmina-SCI Corporation (or any of its subsidiaries), or any other person or entity. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
