On 9/22/2017 1:29 AM, T i m wrote:
On Thu, 21 Sep 2017 21:41:44 -0700, mike wrote:

On 9/21/2017 8:13 PM, wrote:

End rant.

He's making a one off for use at home, not commissioning a new design of military hardware.


The task is the same no matter who the customer.

It can and maybe should be, in an ideal world. I say that because I
hate having to deal with 'fools who rush in' or those expecting me to
make technical / choice decisions on their behalf (like getting them
some 'size 10, brown, lace up leather shoes'). ;-(

The more steps you skip, the more problems you have.

'May have'.

The more you think like a project manager, the better
managed your home projects.

Agreed. I like to understand as much about something as I can.
However, my ability to do so, time available to do so and the
pertinence of doing so don't always allow me to do so or to do so to
the level I would like. Sometimes you just have to get stuck in. ;-)

Once you acquire the habit, you'll find that it adds
little to the time in the beginning and greatly simplifies
the end.

Agreed 100%.

And it wastes far less time for rocket scientists in residence.

Quite ... however, this is a 'discussion' group and so does have an
element of that, along with the hard science / fact stuff.

Part of the 'human' bit is not making a post (especially the initial
one) too long (I fail most of the time) as it will put people off
reading it and if there were any further points that need refining
they could be done with a simple Q&A.

e.g. I initially outlined the fairly close detail at a level relevant
to my needs and understanding (IMHO anyway). I didn't expand on what I
was going to actually use it for because it didn't really matter to
the question. I left it open to the reader to ask any supplementary
questions as they felt relevant. ;-)

Cheers, T i m

Metrology is an interesting topic.

When someone asks me how to measure something, my first thought is,
"exactly how will the answer to your question improve YOUR life tomorrow?"
Sometimes I actually say it.
The "jolt" to their thought process forces them to express what they're
trying to accomplish...or admit to themselves that they have no idea.
I find that almost all questions end up in the "won't affect them at all"
category and I can dispense with trying to teach them something they'll
never use. If the answer won't change the future, don't bother with the
question.

It's similar to pointless social interaction.
People you don't know ask questions, like "how are you?"
They think they know what you'll say.
Respond with something like, "I have this rash on my dick and
think I need to find a different street corner... got any ideas?"
to see how much they really care about how you are. ;-)
But I digress...

If I had an electric boat, the ability to measure voltage to six
decimal places would not be my priority.
What I want to know is, "can I get back to the dock?"
You can do that more accurately with a sharpie and nonlinear
meter than you can react to a change in wind direction.

The river current and wind and the shape of the curve of distance covered vs
velocity and temperature would be far more important variables to me.
Amp hours consumed would be important.

Battery voltage that would be strongly impacted by whether I'm going
up or down stream at what net velocity is a poor substitute.

An integrating current meter, a water speed speedometer and a GPS would
much more interesting. If you knew the water flow direction and speed,
you probably wouldn't have to measure anything else other than
what you can get from the GPS. Drifting downstream for a minute
would give you those two numbers.

But, since you're dead set on measuring voltage...
The basic problem is that the voltage you want to measure is
the biggest voltage you have to power your circuit.
That's why I split my original post into the concept (battery)
and the implementation (zener).
A series zener needs to draw ZERO current at 10V and still
be 10V at MUCH higher current ratio. You will have to live
with some amount of nonlinearity.

You could stack an AA-cell or two on top of your battery system
and alleviate that problem.

I maintain that a digital voltmeter is the best option.
They're dirt cheap on EBAY.

All the above is based on an extreme extrapolation of
very limited information. That's why the system spec is so important.