I buy thermocouple wire on separate spools so that I can control the twisted pair, then cut the twisted pairs into 1.5m segments. Then I choose a pair from the first third and a pair from the last third of the spools to verify. No complaints from any auditors to date. For calibration, look at ASTM E207/E220/E563 - but there are easier, more reasonable ways to verify instruments and temperature sensors.
I have not seen any particular requirement in 29 CFR 1910.7 for NRTL thermocouple-based measurements/calibration. I would be very interested in any additional information that could be provided for NRTL temperature measurement requirements. Let us talk about this concept called 'temperature', because I have seen some non-credible temperature data from CBTLs and NRTLs. Temperature measurements are recorded for a small surface area of a larger mass, for a single instance in time. Temperature is a scalar quantity. Do not think of temperature measurements as vector quantities, and do not consider a temperature measurement to represent a 'constant' characteristic. The NIST polynomials' accuracy for the common stuff (J, K, T) has a theoretical yield of about than 0.1 degC error through the full scale. The NIST polynomial error can be improved an order of mag for a delimited temperature range. The reported accuracy of some instruments that are used for typical engineering measurements are about the same is the resolution (about 0.1 degC), which is not practical, and maybe not possible. For any of these common t/c types, the voltage gradient across a thermocouple wire pair is on the order of 100s of microvolts or perhaps 10's of millivolts for most product safety Type Test measurements. So the sensitivity of the thermocouple and its variable lead resistance and t/c attachment thermal impedance and thermal shunting and non-isothermal routing of t/c leads, the accuracy of the instrument, and the ambient noise conditions all conspire to make the resultant temperature measurement uncertainty, at best, 2 degC. And a lab that reports temperatures to 0.1 degC is delusional. There are some research/academic labs that may be able to control a test so the measurement uncertainty is 1 degC or better - but the practice is not reasonable for the engineering measurements in a product safety lab. Let the shouting begin. Brian From: emc-p...@ieee.org [mailto:emc-p...@ieee.org]On Behalf Of American Idle Sent: Tuesday, June 16, 2009 8:38 AM To: john.merr...@us.schneider-electric.com Cc: emc-p...@ieee.org Subject: Re: T-Couple Calibration 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, <john.merr...@us.schneider-electric.com> 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. To post a message to the list, send your e-mail to <emc-p...@ieee.org> 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 <emcp...@socal.rr.com> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
