John, I agree with you regarding the seebeck effect and its lack of importance with welded junctions, with some reservations; we must remember that if the thermocouple leads are shorted behind the junction, that point effectiely becomes the junction. If one shorts the leads of a type K (chromel/alumel) thermocouple with a copper wire, for instance, there are two junctions formed - chromel/copper and copper/alumel. The voltages of the two juctions add, and I believe the resultant voltage will be the same as if the leads were directly shorted (comments anyone?), assuming the equal temperature at both junctions. Just as with a "twisted" junction, however, the result is only as reliable as the connection, and since type K wire is rather unsolderable... In addition, I'd like to point out that the exact temperature at which the melting point "plateau" (due to heat of fusion) is observed can be significantly impacted by local variations in solder alloy, as well as superheating. And thermocouple error is certainly not to be disregarded. Bottom line is I prefer to mount thermocouples with a SMALL bead of high-lead solder (such as Pb97.5, Ag 1.5, Sn 1; mp 309C)and failing that (such as mounting to non-metallic substrate) with a non-conductive epoxy material (again, small amounts). I've also had great success mounting thermocouples in small holes in epoxy materials using high-temp thermally cured epoxies (such as Omegabond 100 or Omegabond 200. Although these are also electrically conductive to a degree, there is no detectable effect on accuracy of the thermocouple. Fritz Byle Rockwell Automation/Allen-Bradley At 08:29 AM 5/29/97 -0500, you wrote: > You are right to a point. The Seebeck effect come in to play with dissimilar metals in the conductive circuit. If you are using a thermocouple with good metal to metal connection, the solder can't penetrate this junction. It is true that the solder give an alternate path for the current, and does impact the measurement, but the effects are minimal. I have clearly seen the reflow of solder, which shows up on the profile as a short plateau, at 181 to 184. Is the error do the that fact that the thermocouple is mounted in the solder or error in the thermocouple itself? Personally, I don't know, but since that error is within the error associated with thermocouples, I don't care. > On the other hand, if you are in the habit of twisting your thermocouples together, than this could be a really big problem. > >John Guy >American Competitiveness Institute (EMPF) >317-655-3673 x130 >[log in to unmask] *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To subscribe/unsubscribe send a message <to: [log in to unmask]> * * with <subject: subscribe/unsubscribe> and no text in the body. * *************************************************************************** * If you are having a problem with the IPC TechNet forum please contact * * Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] * ***************************************************************************