Thanks John, I didn't qualify my contribution properly. It's probably worth adding that it is convention to regard a manufacturers stated tolerance from an equipment specification sheet as a rectangular distribution. Luke
>>> On 09/09/2009 at 11:00, in message <[email protected]>, John Woodgate <[email protected]> wrote: In message <[email protected]>, dated Wed, 9 Sep 2009, Luke Turnbull <[email protected]> writes: >As Derek said, UKAS document Lab 34 would be a good read for more >information: > >http://www.ukas.com/library/Technical-Information/Pubs-Technical-Article >s/Pubs-List/Lab34.pdf Yes, this is quite helpful. > >This document will (I think), tell you to divide each number by sqrt(3) >before adding them using root-sum-squares. Only if the probability distribution is rectangular (i.e. all values within the tolerance range are equally probable). For Normally-distributed variation (I write 'N' to indicate that this is a special meaning), the variation is theoretically unbounded, so it has to be curtailed to something that has a high level of confidence - probability that it does not hide an error. The accepted confidence level is 95%, and for that the uncertainty value is divided by a constant k, whose value is 2. (95 % (0.95) is the fractional area under the Normal distribution curve between sigma = -1.96 and 1.96, approximated to +/-2. This isn't explained in Lab34, nor in CISPR 16-4-2; it just 'appears'.) For U-shaped distribution, the divisor is sqrt(2), and for triangular distribution it is sqrt(6). But, as usual, nothing beats reading the document yourself. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk Things can always get better. But that's not the only option. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - 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. 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]> Conekt is a trading division of TRW Limited Registered in England, No. 872948 Registered Office Address: Stratford Road, Solihull B90 4AX - 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. 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]>
