Ian, I am assuming you are connecting the reference source to the calibrated meter and to the meter to be calibrated (DUT). I am also assuming that the reference is a variable voltage source covering the range you need to calibrate. Draw a diagram of the system and there and list the parts and the errors. Some errors will be systematic (known) and other errors are random. Errors:
reference source, meter, leads, added noise, and temperature, time. Depending on what has a lower traceable uncertainty, the source or the meter, the calibration uncertainty will revolve around that piece. Say the source is not traceable but the meter is. Then the starting point is the uncertainty of the meter. This is a random error. The leads affect the measurement in a systematic way. There is the voltage drop and a thermally generated offset voltage. The voltage drop of the leads will most likely be in the nanovolt range. Since this is a systematic error you can add or subtract it from your measurement as needed. Same thing with the thermal voltage- this will be in the tens of microvolt range. Unless you are calibrating a 6-1/2 digit meter at low voltages your calibration uncertainty becomes the uncertainty of the calibrated meter. Is the lab in the temperature range in which the calibrated meter spec is valid? The next part is what is the confidence level of your calibration? Most meters do not specify if the spec is for 2 sigma, 3 sigm! a or what. Fluke for example, uses 2.6 sigma. So if they say the accuracy is 0.0026% then the one-sigma accuracy is 0.001%. Now you need to decide if you need a guardband. The rule-of-thumb is if the measurement equipment spec exceeds the spec of the gear being measured by 4X (4:1 TUR) then no guardband is needed. If you don't have a 4:1 TUR (Test Uncertainty Ratio) you can still do the cal but you will need guardbands. But enough rambling. Here is a link that says it better than I can: http://metrologyforum.tm.agilent.com/uncert.shtml Dave Cuthbert Micron Technology From: Gordon,Ian [mailto:[email protected]] Sent: Wednesday, September 24, 2003 9:44 AM To: 'IEEE EMC-PSTC GROUP' Subject: Measurement uncertainties for voltage measurement Everyone, This may seem to be a little out of the groups remit but someone may have had experience of this. I am trying to determine the measurement uncertainty associated with calibrating an instrument which is essentially a voltmeter. This is done using a reference voltage source and a second, calibrated voltmeter. Does anyone have any suggestions as to possible sources of uncertainty of measurement in this arrangement? Ideally, someone will know of an on-line spreadsheet which I may use. All measurements are less than 15V DC. Thanks Ian Gordon _____________________________________________________________________ This e-mail has been scanned for viruses by MCI's Internet Managed Scanning Services - powered by MessageLabs. For further information visit http://www.mci.com This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
