Note that the hammer method does create a weld. Another method that can be used is solder. Purists complain that this creates two junctions. However when they are both at the tip where you are measuring the temperature, the intermediate metal is irrelevant. I weld with line voltage ac, a light bulb ballast and a carbon motor brush for the contact surface.
I get upset with some people who think it is their job to be difficult. This business of calibration of thermocouples is a good example. For the purposes of product safety testing, no-one has problems with the CALIBRATION of their thermocouples. But its an easy problem to fuss about. Your real problems will be shorts, opens, wrong thermocouple types or type settings, thermocouples detached from their measurement point, wrong polarity, reading and transcription errors, misidentified thermocouples, poor location choices, thermal gradients at the measurement point, etc. These can happen even with calibrated thermocouples. You can worry about calibration if you are doing milligram calorimetry. The problems with thermocouples are mostly open or short circuits. It is good practice to check your recorder once in a while against a boiling point and ice point, but the most important thing to do is begin your testing with a comparison of all the thermocouples before they are applied and again after they are applied but before the EUT warms up. I generally check the identity and function by putting the thermocouple in my mouth before applying it. It should rapidly change to about 34C (depending on how much you're drinking). Polarity errors are also easy to overlook (you may find your temperature rise is negative). You will find opens at the weld, at terminals and connectors or within the leads. Shorts somewhere along the length of the thermocouple leads can result in temperature measurement at the short instead of at the tip. Shorting against a chassis edge as they enter an EUT is common. This can be particularly insidious since they don't stop working, they just measure the wrong place. In rare cases intermediate metals in the path to the instrument can introduce errors. The error they introduce depends on the temperatures of the intermediate junctions. Everyone using thermocouples should be familiar with what the introduction of intermediate metals in switches and terminals might do to the measurements and how to control the effects. The accuracy needed for product safety measurements is pretty low. Why anybody records to a tenth of a degree is beyond me. Temperature limits for insulation and touchable surfaces are only approximations and errors of a few degrees may be arguable from a certification standpoint but are not significant from a safety standpoint. In many cases limits based on touch, measurement stickers or thermal crayons have enough accuracy to rule out thermal concerns. I have used an assortment of attachment methods. Tape is quick and easy. Superglue is also quick (but toxic). You apply a drop, insert the thermocouple and cure it instantly with baking soda or a commercial accelerator. Acetone can be used for removal. Sodium silicate (waterglass) is great (but slow) for difficult surfaces like glass or high temperatures. It creates a very hard cement and you usually have to abandon the junction by breaking off the wires when done. Solder works well on some metals but you may need special fluxes or alloys depending on the metals involved. With any attachment method, be aware of heat flow. If the junction vicinity is a uniform temperature, any adhesive will be suitable. In other conditions the cement can be an insulator or a heat sink. If there is a high thermal gradient near the junction you may have to be careful. That is the reason the tiny weld bead is a good measurement tool. Note the leads can have a significant cooling effect on the junction. Bob Johnson ITE Safety <http://www.itesafety.com> American Idle wrote: You could pull the whole spool off and make your 2nd junction from the last bit, then re-wrap the whole spool :-) I talked to a UL team lead on their DAP/ISO 17025 program and he had the following comments; -This requirement is based on a CTL decision -You must validate one TC from the beginning of the spool and one from the end of the spool with an RTL Calibrator or water bath method -You risk all your previous data if the last TC you make from the spool doesn't calibrate right He also stated that this requirement may change in the future because it doesn't make a lot of sense (and suggested that I bring this particular issue up for discussion if I happened to know anyone who sits on the Standards Commitee!). As another poster stated, you may be better off purchasing pre-made thermocouples. The only risk there is if your manufacturer goes out of business, your calibration certificates may become invalid. -Ken Arenella On Tue, Jun 16, 2009 at 9:50 AM, <[email protected]> wrote: While we are on the subject. Anyone have an inexpensive solution to the big NRTL's new Calibrated Thermocouple requirement? As I read the spec it requires calibrating the first and last T-Couple off of the spool minimum. Takes me a couple years to use 500ft. So I would technically only need to Calibrate one a year. Thanks in advance John Merrill Principal Product Safety Engineer Schneider Electric - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.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 <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. 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