[Nut-upsdev] zero-prepended values - why oh why?

[Arjen de Korte]

> I seriously doubt that there are any UPS devices that can do accurate
> voltage/current measurements with two decimals.

I'm not so sure about that.

> A Fluke 87 multimeter (which is a rather expensive one) has an
> accuracy of +/- 0.7% plus 2 digits when measuring AC voltage, +/- 1.0%
> plus 2 digits when measuring AC current.

When it comes to absolute measurent accuracy, you're right. But in many
cases, this is not all that matters, otherwise Fluke wouldn't give it a 4
1/2 digit display (20000 counts). Don't mix up measurement resolution and
accuracy, these are two very different things.

> 0.7% of 120V is 0.84V, so showing a second decimal makes little sense.

By your standards, Fluke might have gotten away with a 3 1/2 digit
display, right? Now I wonder why they put in that extra digit... ;-)

> As a reference, cheap multimeters usually have +/- 2% +/- 2 digits.
> This is probably more like what you'll find in a UPS.

I doubt it. Most digital multimeters use double slope analog-digital
conversion, which usually has excellent linearity. In a UPS I would expect
more something like a 8 or 12 bit successive approximation analog-digital
converter of the variety that is found in microcontrollers for embedded
systems. You'll get nowhere near the linearity figures of even the
cheapest multimeter with those. But since they'll probably don't measure
true RMS, there is little need for anything more sophisticated anyway. ;-)

> Given this, it's probably no use to display a second decimal for the
> current either (yes, I assume that a small/cheap UPS has less exact
> measurement capability ;).

Even if the absolute value may be off by more than 2%, the relative values
measured will still tell something. More often than not it is trends that
matter and not the absolute value.

> Frequency measurements are easier to get more exact, so two decimals
> there probably makes sense.

Frequency measurements are mostly a gadget (because they are so easy/cheap
to implement) but in reality have very little practical value (unless
you're running of a diesel powered generator or are living in a Third
World country with a lousy grid). In the western hemisphere the mains
frequency fluctuations are way below what is detectable with cheap
circuitry found in a UPS. I would seriously question any value more than
0.1Hz off from the nominal 50.0Hz here in Europe for that matter.

> In general, the trend seems to be that expensive devices (like our
> 120kW APC Silcon UPS) tend to be rather honest with the accuracy of
> the measurement capabilities and provides voltage like integers and
> frequency with one decimal. Some cheap devices however spits out
> measurements with two decimals but the minimum step seems to be
> multiple-integer... So there's a big difference in what the ups
> outputs and the actual accuracy. I find it better to be rather
> conservative.

I don't think we (as NUT developers) should worry about that and interpret
whether the precision of a measured value is reasonable or not. If the UPS
is outputting it, the driver should make that value available verbatim.
Whether or not to display it like that is open to debate, but I really
think we should stay away from making assumptions about the credibility of
values received from the UPS. I already envision the questions we'll
receive on ups-users from people complaining that the bundled Windows
software with their UPS will display the mains voltage with milliVolt
resolution, while NUT will only display Volts. Even though (and I agree
fully with you here) the accuracy will be terrible and most of the time
not relevant for operation anyway.

Arjen