On Mon, 25 Jun 2012 03:00:57 -0700, mike wrote:

On 6/24/2012 2:45 PM, Jim Wilkins wrote:
wrote in message
...
On 6/24/2012 11:35 AM, Jim Wilkins wrote:
...I monitored the 240V line current with a clamp-on current probe
jsw

I did exactly the same thing to log my water heater consumption. That
works fine for devices with power factor of 1, like a water heater.
It can give you a hint about relative consumption, but the actual
power you compute from RMS current * RMS voltage = VA is not real
power (watts) if
your power factor isn't 1. Many devices are .6 or so. Computers,
CFL,
anything with a motor, most electronic stuff. And even your current
number isn't right unless you use a real RMS responding clamp-on
meter.
Most aren't.

You must be new here. R.C.M. is the land of homebrew 3-phase rotary
converters with carefully balanced leg currents. Alt.energy.homepower
is where power factor has to be tediously explained, over and over.


That brings up a topic near and dear to my heart...Communication. What
is the appropriate level of response when you think you can correct a
misconception?

It's been my experience that much of the discourse, not to be confused
with communication, in the newsgroups is based on serious misconception.

Yup.

The ratio, signal divided by noise, in the newsgroups approaches zero.
And the previous sentence is a symptom. I started with "signal to noise
ratio",
but immediately recognized that some net cop was gonna argue with the
definition and start a flame war.

When people ask a question, I try to put myself in their shoes, try to
figure out what they really want to know and construct an answer in a
context I think is relevant to their task and that they might
comprehend. That's hard to do in
a half-duplex mode like newsgroups. There's lots of opportunity for the
thread to get sidetracked during the response delay time. I try to be
tactful, but making the point requires some directness. And the
"experts" start piling on before the question is even clarified.

Sometimes, the guy on the other end has a misconception. Or maybe he
just abbreviated the situation. Or maybe it's a typo. Or maybe I
misinterpreted. Or any number of other things.

The magic of USENET is that we can iron out misconceptions using
accessible language and examples in a friendly, conversational
way. Dealing with the noise is worthwhile.

When somebody sez, "I monitored the 240V line current with a clamp-on
current probe", what's a rational assumption? Since most people don't
understand power factor, and most clamp-on current probes plugged into
DMM's don't have
any means of phase output even if the DMM does have a serial port, and
you need phase info
to calculate power, should someone respond with a short discourse on
power factor?

That, and / or a link to a web page that presents the info clearly
and efficiently.

It's not just an argument over a term, it's a major error factor.

Yes. We must be careful about the purpose of our measurements, though.
A clamp-on multimeter can give us a number that represents an upper
limit on the amount of power that a device requires. As you said, a
given device can require far less power than the 'ohms law' numbers
indicate, because of waveform purity and/or phase displacement issues.

Sometimes the 'ohms law' numbers reveal 'good enough' information, (say
to discover the worst-case continuous current going to a branch circuit
for circuit breaker sizing).

As you say, applied to real power calculation, 'ohms law' numbers can be
woefully wrong. Obviously, when determining real power, we must include
PF or we can be sorely mistaken.

It matters not that everything you ever need to know has been discussed
over and over somewhere else. The only thing that matters is whether the
OP, or the lurkers, might benefit from a short discourse.

Also, for folks who uncover the info much later using a search engine.

So, if you think you can POLITELY help, should you?

Certainly. And thanks for asking.


--Winston