At 05:43 PM 6/12/2006, you wrote: >What will happen if you use the thermocouple welding technique? It seems >that the thermocouple is based on the peltier effect of the dissimilar >junction between the iron constantan or chromel alumel or what ever. If >other metals are introduced, like tin-lead, for example, the peltier >characteristics will be altered and likely not follow the normal curve. I >have never heard of anyone soldering a thermocouple but I suppose some >people might do that. I have seen the two separate thermocouple wires >affixed to a common substrate where the errors cancel. If soldering is a >common practice I would sure like to hear about it and know if the soldered >junction tracks the TC curve exactly. Soldering thermocouple junctions is a common practice. Welding of thermocouple junctions is probably the most common practice, as you don't have the problems with soldering a variety of metals together. There are special electrical welding apparati for such a purpose available. Omega probably sells them. The property is called the "thermocouple effect": i.e., generating a voltage between dissimilar metals based on temperature. The "Peltier effect" is the reverse: i.e., generating a temperature difference by driving a voltage through a dissimilar metal junction. I have made practical thermocouple junctions by twisting two wires together, peaning them flat together, and soldering them. Welding them together can be done easily using a car battery and jumper cables: remember type K wire is similar to nichrome resistance wire. Don't be too concerned about other metals present at a junction. So long as the junction is small, all extraneous thermocouple effects will cancel. Also note that solder is heat conductive as well as electrically conductive, so the joint is very uniform in temperature. You should be more concerned about flexure stress points along the thermocouple wire (type K is not too flexible), which may create measurement junctions in annoying places. Any two metals can be twisted together to make a thermocouple junction with a millivolt or two of output. The most commonly available wires are iron, nichrome and copper. Nichrome and copper is very similar to type T wire. Iron and copper is type J. Commercial thermocouple wire is used because of 1) known response equations for their particular alloys, 2) high EMF output, 3) tight linearity specifications. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causas scire" ================================================================ -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist