On Thursday, March 6, 2014 7:38:31 AM UTC-5, Johny B Good wrote:
On Thu, 06 Mar 2014 05:21:37 -0700, RobertMacy

wrote:



On Wed, 05 Mar 2014 19:15:58 -0700, amdx wrote:



...snip....

Oh, I don't see how that can work with both wires running through it.

Mikek









repost a reply here....



Fields from a SINGLE long wire drop off as the inverse distance, 1/S. Ah,

but usually you have two wires, one with current + and the other with

current - If the wires are REALLY close together you can see how the plus

and minus end up with almost zero field.



However, most wires have a bit of size to them, thus kind of close to them

you get 1/S for the positive flow and -1/(S+dS), where dS is that smidgeon

of extra distance. now you see, NOT zero field...



What you get here is

1/S - 1/(S+dS) = (S+dS-S)/(S^2+dS*S), for really small dS, throw it away

and you get

dS/S^2, thus everybody tells you how the field drops off as the inverse

SQUARE of the distance.



Now, with judicious measurements made in a 'real' scenario; it is possible

to find the current flow in the two wires just by examining the magentic

field around the pair, albeit a bit tricky. For example, measuring close,

and further, you can find the 'distance' between the wires and calibrate

your measurement. In other words, measure current without knowing the

exact structure of your wires. and determine the structure of the wires

all at once. But, as I said, a bit tricky.



Most clamp on meters use an 'iron' core to distort and 'short' [as in

concentrate] the field around whatever it is clamped onto. That type meter

dtermines the TOTAL current flowing through whatever it's clamped onto, so

if clamped on both AC wires, you get zero.



However, what I described is measuring the field WITHOUT distorting the

field [well, not distorting too much]





As an extreme example of the effect of 'spatially' measuring fields to

then determine current flow:

My magnetic field measuring circuitry has extremely low noise floor. Using

that circuitry I once designed a non-contact 'clamp on' multi-sensor

system for measuring the independent current flows inside a 25 pair cable.

...for tapping any phone line you want. Each pair when 'active' is

anywhere from 10-75mA and produces a noticeable field out to around 3 ft.

If your noise floor is low enough.



So, a bunch of electronically selectable and 'steerable magnetic

detector arrays' plus a ****load of electronics then?



I suppose it might be do-able today (Hell! who'd have thought we'd

have microprocessor chips with transistor counts measured in hundreds

of millions (last figure I saw was 135 Million nearly a decade ago).



Seems to be an incredibly 'over-engeered' solution' just to save the

need to isolate the conductors to take a current measurement though.

--

Regards, J B Good


The part that makes it hard to believe is that this is apparently
some little company that doesn't even have a real product to sell
yet. It looks like a small, cheap device targeted at consumers.
They don't say anything about any new technology, patents, or how
they do it. You would think that if this were possible to do with
any reasonable accuracy and cost all the pros in the field, eg Fluke would
have been out with it long ago. A simple clamp on that you could put
around any power cord/cable like that would be an instant seller.