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Saturday, February 26, 2011

77 - which value is right?

Dear Reader,

I have a problem which I'm hoping will be addressed. It's this. Energy measurement is based on the product of voltage and amperage over time. And energy is measured in Joules which, in turn, is based on wattage which, as mentioned, is vi*dt.

Now. We've been the happy recipient of the use of some really zut DSO's. The one gives us data dumps in the half million and the other in the million sample range. Hugely detailed. Each sample range under observation is really thoroughly accounted. And the dumps are right out of the moment that the sample was captured. How the different DSO's measure their mean averages, or anything else, depends on that initial sample capture. It's that dump that represents an actual record. And we can access that record of samples - right out of the DSO.

Typically on the multiple channels that these instruments provide - it's possible to measure the different points on each circuit. So it is that the battery voltage and the shunt voltage are shown simultaneously. Therefore is it possible to measure them both - to estabish vi - in real time. As they occurred. So. One can take a record of that sample range and then transpose it to the spreadsheet for analysis and do a moment by moment computation of those measurements. For example, one can take the voltage across the shunt, divide it by the resistive value of the shunt and get the instantaneous current measurement. Then one can multiply that current by the measured voltage and that will give the actual measure of that instantaneous sample as it happens, so to speak. And one can do that sum for each of those 500 000, or 1 million samples - as required.

Alternatively, one can take the sum of all those voltages over that entire sample range and divide it by the number of samples to get a mean average of the current flow and a mean average of the applied source voltage and one will then get the average of the amount of energy applied over that time period related to the sample range.

Here's the kicker. The sum of the instantaneous wattage computed against each sample is never the same as the mean average. Those numbers never relate to each other.

I do have an answer - but I'm not sure if it's classical. Poynty, - if you're reading here - or anyone. I'd be glad of some kind of explanation. Which of those two systems is right? Certainly they're NEVER in agreement with each other.

Why this is relevant is because the math trace is the instantaneous product of both the shunt and the battery voltage. At higher wattage outputs the mean average of the shunt voltage defaults to positive but not that instantaneous product - not that math's trace. This remains negative. Interestingly - possibly because of the higher voltages, the battery voltage first dips by a half a volt or thereby and then steadily climbs back to its previous value.

It's puzzling.

Kindest regards,
Rosemary