Ed, As Derek said, UKAS document Lab 34 would be a good read for more information: http://www.ukas.com/library/Technical-I formation/Pubs-Technical-Articles/Pubs-List/Lab34.pdf This document will (I think), tell you to divide each number by sqrt(3) before adding them using root-sum-squares. (The reason is that to add uncertainty contributions that have different distribution types e.g. rectangular, triangular, normal; you calculate the width of a normal distribution that has an equivalent spread to the e.g. rectangular distribution). You will then multiply this final number by 2 to get the uncertainty "for a confidence level of 95%". I hope this helps - it's not too easy to make sense of this in an email. Luke Turnbull Dr Luke Turnbull EMC Technical Manager TRW Conekt Stratford Road Solihull West Midlands B90 4GW
Tel: +44 (0)121.627.3966 email: [email protected] web: www.conekt.net <http://www.conekt.net/> >>> On 09/09/2009 at 00:16, in message 9d04b979323dcd428297dda95108893e0287f...@bb-corp-ex2.corp.cubic.cub>, "Price, Edward" <[email protected]> wrote: A quick question about calculating measurement uncertainty. For example, if I am doing a conducted emission test, and I know the calibration tolerance for each of the "modules" of my measurement system, how do I calculate an overall uncertainty figure? I have been under the assumption that, in the example I cite, that I have a tolerance for my probe (say +/- 0.5 dB), my cable (+/- 0.1 dB) and my analyzer (+/- 1 dB). I have been using an "rms" calculation, where I square each of the tolerances, sum them, and take the square root. Thus, square root of 1.6 dB equals 1.26 dB uncertainty. I recently had a customer say that this method implied that I was using "rectangular" shape factors, and thus, I should be taking the cube root of the sum. Rectangular? Cube root? Ed Price [email protected] <blocked::mailto:[email protected]> WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty - 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]>
