View Full Version : Apology to Pasofino

03-21-2005, 01:26 AM
Yesterday I replyed to your post about EM fields. I was wrong to sugest, and my sugestion was incorect. For this I apologize. You should never listen to anything I have to say. I am for the most part a babling monkey.
It was late and I was very tired. I was thinking RF fields and not EM fields.

The following is offered as hypothetical discusion only! This is not a recomendation!

Lead is a good stopper of particle energy. Ultra short wave lengths. X-ray, Gama and Beta But I also believe it will block most RF all the way down to meter lengths.

To block EM from an alternating source grounded copper or steel would be best. So in theroy!!! If one was to ground a copper lined vest in paralel to his welder he would very effectivly block most EM. Though some EM would still be able to enter through exposed body. A grounded copper box is the only way to completely block all EM.

EM from permant magnets and DC sources are different than AC generated EM. To block EM from a DC source one would need a set of Helmholtz coils so an opposing field can be established which exactly cancels the magnetic field in a region inside the coils.

One could also also can use the so called μ-Metal. It is very very expensive but is very efficient shielding the magnetic DC.
It is an alloy which has a very high permeability constant and the magnetic field lines are easily driven in it.

The following was in no way a sugestion but only intended to offer an opinion.

Once again to Pasofino, I am sorry for providing you the wrong information yesterday.

03-21-2005, 02:22 AM
Babling Monkey? That conjurs up a horible sight. I wouldn't let it get to ya. We've all poked ourselves in the eye here a time or two. http://bbs.homeshopmachinist.net//biggrin.gif

03-21-2005, 03:26 AM

Don't be so hard on your self. You weren't the only one offering advice. My only point was, as others said, get qualified medical advice.

As for low frequency EM fields mu metal will do the job. Copper isn't that bad either.

For an interesting trick try dropping a super magnet through an aluminum tube. It will drift slowly through the tube like it is dropping through syrup. It is inducing eddy currents as it falls that oppose the motion of the induced field.

Unfortunately, those induced eddy currents can still affect a pacemaker.

03-21-2005, 04:15 AM
jimfun - Don't kill yourself, we've all made the occasional oops. Even me. http://bbs.homeshopmachinist.net//redface.gif And anyone taking medical advice without checking it out first is an IDIOT. Especially from an internet forum that's not even dedicated to medical topics.

So - I am not an electrical engineer, cardiologist, or have any such related training, so this is not medical advice; and you should always talk the matter over with your doctors and the pacemaker's manufacturer - particularly their engineering department.

However, I did play one once on TV - OK not really, but this is what I know about the subject (well, think I know).

Muscles are current operated devices, not voltage operated ones. Also, they are moderately conductive (as is the rest of your bodily tissues), and do a good job of attenuating the high frequency (Khz on up) EMI components - if you don't believe me, get a bunch of your friends and gather all around a laptop that's got WiFi; and watch the signal level go to zero. DC, and low frequencies however, do pass through - and these components tend to have much more power to them then high frequency EMI does. However, the power level involved will not generate electronically dangerous amounts of current, nor voltage, across your muscles - if they did, you would go into spasms as soon as you turned something (welder, etc) on - or worse, the levels would actually burn you (comparatively speaking, it takes a lot to fry a power transistor). What it can do, is confuse the heck out of the pacemaker, causing it to fire inappropriately, or develop sufficient potential across the leads to cause your heart to react inappropriately. Depending on your condition, the programming of the pacemaker, and the nature of the EMI, the effects of this may range from unnoticeable to fatal.

One of the reasons why I hate going to the barber is that they always - even if I have told them not to - use that electric clipper on my head; and I can feel quite clearly the EMF that thing is emitting - and it's not very powerful compared to a 220VAC arc-welder. So, yeah, I would avoid the arc-welder - besides, what's wrong with doing everything out of billet? http://bbs.homeshopmachinist.net//biggrin.gif

EDIT: ahem - make that "taking medical advice", not "taking medical advise". http://bbs.homeshopmachinist.net//redface.gif

[This message has been edited by hammerhead74000 (edited 03-21-2005).]