[conspire] How accurate, etc.: AC Power (Watt)meter (What's Your Watt?/...)

[Scott DuBois]

On Sun, Apr 12, 2015 at 10:21:34AM -0700, Michael Paoli wrote:
> Well ... it varies.  ;-)
> 
> But yes, good points - not so much that "they" or "it" is necessarily
> inaccurate or that inaccurate, but good to generally be aware of how
> (in)accurate one's measurements are.
> 
> Most test instruments typically have specifications, that well spell out
> their accuracy.  And yes, the specifications generally are quite
> accurate - and especially from reputable manufacturing companies.  That
> doesn't mean the specifications are always 100% correct, but they're
> generally intended to be correct, and for the most part are.

Should we also consider those devices that come with NIST certifications against
those that don't as well though? Devices that are not "certified" as accurate or
have lapsed in their certification may not necessarily be accurate to the levels
that the manufacturer specifies. Parts wear, even electrical parts that may not
necessarily move, but perform some type of work regardless of how menial that
work may be.

[snip]

> Your Conserve Insight uses sensitive
> electronics to measure the amount of power
> flowing to your connected device. When
> the connected device is consuming a very
> small amount of electricity, it becomes
> very difficult to distinguish between the
> power being consumed and the electrical
> noise found at all power outlets. To ensure
> accurate readings, the Conserve Insight
> stops showing values below 0.5 watts and
> displays the '0-0.5 Watt' screen.

Ah, reminds me of my JC days learning how to find "phantom draw" from an
automotive battery that seems to mysteriously die regularly. The draw may be too
small to measure directly through inductive practices so one makes a loop of
coil with 10 winds to amplify the measurement by a factor of 10 (something that
_can_ be read.) Of course, this is 'A' draw, much easier to work with.

[snip] 

> Oh, also, I don't think, in general, one will improve the accuracy of
> measuring power consumption of a small AC load, by starting with a
> larger load, adding a smaller load, then noting the difference in total
> power consumption.  Most notably, for most typical loads, the load
> won't be constant - notably varying by Voltage - which does fluctuate.
> E.g. that 7.5W incandescent night light one may have (before someone
> makes 'em illegal or whatever), it's actual power draw will likely vary
> a fair bit with voltage fluctuations, so adding a load such as that to
> try and determine the power draw of a smaller load, may actually give
> one less accurate information about the power draw of the smaller load.
> A more accurate approach would be to apply multiples of that smaller
> load (plug in many such identical devices under identical use status
> and conditions) ... but alas, that may not be feasible or convenient.

Yeah, when things start to get "really" small, the inconsistant variables become
large. Not "all" light bulbs are made with the tightest of tolerances so that
becomes an introduced variable. The consistancy of the current supplied, the
accuracy of the measuring device, the resistance in the carrier (wire), the
resistance in the connections, etc. It all adds up and when the value to be
measured gets smaller, then such variables become greater factors.

At some point, I think one has to simply state that their acquired results is
only a guestimation based on the "known" variables in the system being measured.

-- 
Scott DuBois           "In college, before video games, we would amuse ourselves...
BSIT/SE	                                      ....by posing programming exercises."
EFF ID: 1731778                                                         -- Thompson


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