Don't know what ICNIRP Guidelines are, but as I recall the
old Swedish MPRII rules were on the order of 200nT over the
band of 5Hz to 2000Hz and 20nT over the band of 2KHz to
400KHz. [From memory, so numbers could be off]
As I recall, basically, 2mG became the magic limit for
"safety" concerns of the environmentally squeamish.
My measurements were taken (inadvertently at first) in the
driver's seat of a 1972 Mercedes 280SEL. And their magnet
puts out a lot of field. At the chest was just above
limit, and at the hands was 5 to 10 times limit, depending
on hand positions.
Uh,...50uT, or 40A/m, is the earth's field here in
California. It can get stronger, or weaker, depending on
the concentrations of iron objects around you.
Back in the 80's during the work to develop a low cost,
accurate meter, I would see 'spikes' of over 2 microTesla
as vehicles like a truck would pass me while I was standing
alongside 1st street in San Jose. At 20mph the movement of
the vehicle moved the field deflection right into the
bandwidth of my little meter. Buses were more dramatic.
PS the meter was my own invention and had sensitivity of
1V per microTesla with flat response from 5Hz to over 1
MHz. I was amazed how easy it was for the earth's 50uT to
get into that bandwidth with very small movements of the
meter.
For an NDE application to replace noisy Hall Effect and GMR
sensors, I extended the range of the meter down to 0.001
Hz. The circuits took all morning long to "turn on".
Hated that delay. But with a 2 minute time constant and
0.1% accuracy, what are you going to do?
40 year SHORT course? hmm....all is relative, right?
What is the reasoning behind the ICNIRP Guidelines at
around 1/4 earth's field? Who is requiring the guidelines
be met?
- Robert -
On Fri, 24 Feb 2006 21:12:22 +0000
John Woodgate <[email protected]> wrote:
> In message <[email protected]>, dated Fri, 24
> Feb 2006, Robert A. Macy <[email protected]> writes
>
> >No, from the little magnet inside the speedometer
> spinning around in front of you.
>
> H'mmm: I don't understand that; the magnet isn't very
> strong and it's a long way from even the driver's hands.
> The ICNIRP Guidelines are of some significance to me at
> present, and I just don't see 10 A/m (12.6 microtesla)
> there.
>
> I'm (slowly) building an omnidirectional magnetometer for
> fields up to 1000 A/m, but I don't have an eddy-current
> speedo to test. Maybe an excuse to buy a classic car on
> the company account?
> >
> >You're right, I wasn't clear about the origin of the
> magnetic field. Main reason I'm not allowed to write
> legal specifying thingies.
>
> Try a short 40-year course of writing standards. (;-)
> >
> >Regarding the earth's field, the steel tends to "short"
> out the earth's field.
>
> It gets in through the windows, though. And although it
> varies from place to place, a value of 40 A/m is not
> uncommon.
> --
> OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and
> www.isce.org.uk
> 2006 is YMMVI- Your mileage may vary immoderately.
>
> John Woodgate
-
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