On Wed, 27 Feb 2008 19:06:14 -0600 Ben Miller wrote: | snipped-for-privacy@ipal.net wrote: |> If I measure a voltage with a voltmeter, I will express it as |> precisely |> as the device is capable of accurately measuring and precisely |> displaying. If it has an accuracy of 1/10 of a volt in a 200 volt |> range (rather good) I'll use that and might state the voltage as |> "119.1" or "121.0". But if | | Many mid to high end multimeters are more accurate than that.
I'm happy with a 4 digit voltmeter. But I want 1 Hz tuning steps on my SSB ham rig, even to the high end of the UHF band.
|> it only has an accuracy of 1 volt, I'll state it as "119" or "121". |>
|> So when you see me use a highly precise expression like "235.5589", |> it is coming from a mathematical calculation done with at least 6 or |> 7 digits |> of precision (probably more since I default to using the double type |> which has 14 or so digots), using a formula I believe to be accurate. |>
|> If I ever manage to make a real physical measurement with such |> accuracy, I'll be sure to let you know about that miracle device |> capable of doing such a thing. |>
| | You are only talking about 1ppm or 0.1ppm. A lot of equipment exists with | that accuracy. I have calibration instruments in my lab that will do it. | However, that is several orders of magnitude better than any field | measurements that I make. Even if you did make measurements with that | accuracy on an electrical power system , it would be meaningless from a | practical standpoint. It would be difficult to measure the difference in | winding temperatures, for example, with a voltage variation of 0.0001%.
If you measure the voltage and find it is 278.415 volts, it could be just a bit different in a few seconds. At this level of accuracy and precision we're into line noise levels :-) But, my mathematical model calculations don't have line noise to worry about (unless I am doing noise modeling).
| You are presenting a great mental excercize, but if you need to go that far | down to differentiate between the results of two formulas, then as a | practical matter in most power work, they are the same.
I want to be able to recognize them in the mathematical model sense, too.