Hi All,
In the '70's, I used to weld the cores of a mineral insulated, stainless steel sheathed thermocouple. The OD of the thermocouple was as small as 2mm. I worked on the design of the capacitor discharge welder. The thermocouple pair were Pt/PtRh. A rough outline of the process is as follows: The thermocouple would be trimmed and the mineral insulation (MgO) removed >from around the thermocouple cores. The cores would be bent over so that they touched. Leads were attached to the far end of the thermocouple pair to pass current through the junction. Argon would be released around the thermocouple and a discharge initiated that would weld the two cores together.==Note the Argon== Next the insulation would be checked to ensure that the sheath was isolated and, all being well, the sheath would be back-filled with MgO to insulate the couple. The powder would be packed down by hand using a small probe that fitted inside the sheath and a stainless steel end-cap inserted. This would be welded in a similar manner (but using probe to touch the cap) and the insulation checked again. The thermocouple sheath-cap weld would be checked using a helium leak test. The TC would then be "dried out" and the tail end of the thermocouple would be sealed using a gland and potting compound to keep out moisture. These were used in non-fossil fuelled power stations. We used to make many other types of 'couple but, as far as I can remember, these were the only ones that were produced in an inert atmosphere. These were "the best" that we made at that time - the quickest response, most accurate and toughest (for the corrosive environment). The cheaper 'couples (on large OD versions) were produced by pushing the ends together and putting an oxy-acetylene torch to them! All our 'couples underwent "calibration" against fixed points (solid CO2 in methylated spirit and the other steam) and were also checked at other points against a "standard" 'couple. The cheaper 'couples produced without an inert atmosphere did produce a greater deviations against the standard TC voltage tables and so I suppose that this is the effect introduced by the oxides. I hope that this helps. Tim Haynes ******************************************************************** This email and any attachments are confidential to the intended recipient and may also be privileged. If you are not the intended recipient please delete it from your system and notify the sender. You should not copy it or use it for any purpose nor disclose or distribute its contents to any other person. ******************************************************************** This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
