EMI / RFI Affects On Humans

[Roger Ramjet]

Years ago whilst servicing an house alarm & became bemused upon seeing bits of aluminium foil stuck to various walls. The customer explained that she could strongly sense electrical or magnetic energy in the house & the foil tended to mitigate the effect. Interestingly, I noticed stray voltages appearing on my DMM despite not being connected to anything ? Along with the customer enthusiasm, we did a tour of the house and were amazed to see changes in voltages across all rooms with a reduction where foil had been stuck to the wall.

Clearly the DMM test leads were acting as an antenna so three possibilities came to mind:-

1. Induced EMI from the house wiring ?

2. Induced RF from a nearby MOD Radio Station ?

3. Local granite outcrops may have resulted in pockets Radon Gas which being radioactive may have affected the DMM ?

Any feedback or further explanation would be of great interest.

Rog

[agardiner]

Was the house anywhere near overhead pylons?

[Maarten]

Radioactivity won't show up on a DMM. It must be an electric field, and a pretty strong one at that.

[GrimJosef]

Originally Posted by Roger Ramjet

... Clearly the DMM test leads were acting as an antenna ...

Or something in the DMM was picking up an alternating magnetic field (I take it you were measuring an alternating voltage rather than a static one).

Cheers,

GJ

[Keith956]

Back in the early 90's I used to visit a certain well-known Swedish telecoms company in Kista a lot. One of the lead designers was a chap who, I was told, was allergic to electromagnetic fields. Apparently his bedroom was lined with aluminium foil to form a faraday cage, he drove a diesel car (no spark plugs) and his phone had tubes to the headset so he didn't have to hold an electric handset.

We had to give a software demo to him once, which involved a large projection TV for the display at one end of a 20 foot corridor and him sitting at the other end, well away from any electromagnetic fields. He was certainly taken seriously there, and who were we as visitors to question his 'condition'.

[The Philpott]

Customer may have been sensitive to magnetic fields, and if said fields were fluctuating they could have produced the electric field you seem to have detected. It's not safe to say that subterranean sources of magnetism don't rapidly fluctuate- very occasionally they have been found to do exactly that.

Magnetic fields can do some very strange things to perception- hallucination, feeling of presence, and in one experiment a group of people isolated in a room with zero magnetic field and an intermittently flickering light were unable (after a while) to detect whether the light was flickering or not. When the earth's poles lose their field (or even reverse) in a couple of hundred years our descendants will very likely be in trouble.

DMM's of 10meg (or even 1meg) input impedance can throw up anomalous readings of a few mV of AC even by being moved towards a mirror- so it's feasible the aluminium foil may even have been contributing to the phenomena..

Dave

[The Philpott]

(Now i note you actually advised that readings REDUCED in the region of the aluminium foil...possibly it was shielding the house wiring from the meter.. i note that my cheap DMM had a sheet of foil glued inside the rear of it's casing)

Dave

[Roger Ramjet]

Thanks for the responses everyone.......

Re Post 2 - There were no overhead pylons near the property.

Re Post 4 - It was so long ago, I cannot remember if the DMM was on DC or AC scale. I would assume that AC range would give a better reading noting the EMI field would have been alternating to induce a response. That said, maybe rectified AC via DC scale would also have given a reading.

I would guess we are all surrounded by this EMI / RF radiation, its just that some people are permanently tuned into it ?

Rog

[Hybrid tellies]

I would suspect on the other side of the wall you would find a neighbours Plasma TV which were notorious for putting out very high levels of RFI/EMI.
So the aluminium foil might have offered some shielding from it.

[Craig Sawyers]

I used to work at Oxford Instruments, and the test lab staff used to talk about "magnet head". Particularly when shimming a high uniformity superconducting magnet, which might take well over an hour, any attempt to move quickly in the fringing field could induce metallic taste, slight nausea, and a sensation of flashing lights.

That was almost certainly a dB/dt effect causing eddy currents in the brain.

Now that was an extreme situation, in fields of a few Tesla.

Typical magnetic fields in a domestic setting are very small indeed by comparison https://www.gov.uk/government/public...s-and-exposure suggests small fractions of a microTesla in domestic environments - so typically a millionth or less of the exposure from a superconducting magnet - or an MRI machine which will be between 1 and 4 Tesla.

So could people be sensitive to domestic AC magnetic fields? Well, other than anecdotal evidence, a double blind trial would be appropriate to find out one way or the other.

Craig

[lesmw0sec]

Originally Posted by Craig Sawyers

I used to work at Oxford Instruments, and the test lab staff used to talk about "magnet head". Particularly when shimming a high uniformity superconducting magnet, which might take well over an hour, any attempt to move quickly in the fringing field could induce metallic taste, slight nausea, and a sensation of flashing lights.


Craig

I have been a volunteer occasionally, for tests with the Bangor university which meant being thrust into an MRI machine for a brain scan. It always left me with a slight feeling that I had drunk too much the previous day (although that was not the case!)

[GrimJosef]

I once had an fMRI scan (functional magnetic resonance imaging, for anyone allergic to acronyms ) which involved spending upwards of half an hour with my head in a multi-Tesla magnetic field. I couldn't move my head much, but there was no strong internal sensation as I was slid into the tube or as the machine clunked and whirred. Given that, as Craig has said, the background fields in our homes and ordinary workplaces are orders of magnitude less than the MRI fields, you'd have to believe that the sensitivity to magnetism varies hugely from person to person if you want to explain how some are affected and others aren't.

The belief, or otherwise, in sensitivity to EM fields has spawned the 'tin foil hat' debate which has been going on for quite a while now https://en.wikipedia.org/wiki/Tin_foil_hat.

As someone who worked with large pulsed EM fields for a couple of decades I have to say that any risk from them came a long way down my list of everyday concerns.

Cheers,

GJ

[Radio Wrangler]

Let's go back to the situation reported in the first post. It's unlikely that there was an MRI scanner next door, or a superconducting magnet factory, so the strength of magnetic field needed to produce known effects isn't going to explain things.

High-impedance multimeters do produce readings when you wave the probes around. If you've got DC volts selected, then triboelectric effects of waving a probe tip through air can produce several volts particularly if it is a very high input impedance. Effectively measuring the output of a static electricity generating mechanism. Some of the Fluke and Keithly ones have extremely high impedance on ranges of a few volts.

On AC volts, with a high Z, they can respond to 50Hz electric fields from house wiring. Most often twin and earth wiring has the return close to the power conductor, so there is self-shielding effect, but in many houses, lighting wiring is partially done with double-insulated single wire. No shielding effect and you can see the radiated 50Hz field from them with high impedance voltmeters and with a bit of wire in the grabber of an oscilloscope's probe. Remember also the hand capacitance effect from yourself to one probe more than another can increase the apparent level.

What is missing from the first post is of any report of what was seen when the same multimeter was waved around in other homes and back at the workshop.

When you touch the input of an audio amp and get the usual loud buzz, no-one is surprised. You get many mV into what's usually a substantially lower-Z sensor.

The multimeter did nothing surprising.

Theorising that it was RFI implies the meter is sensitive well beyond its frequency response of its AC volts range. This is unlikely in the normal way, BUT if there is RF and at high volts/metre field strength, then the RF can get self-rectified in the first semiconductors it meets, and produce DC offsets as a result. This effect is real and repeatable. I have an AVO EA113 electronic multimeter. These things are notorious for weird readings anywhere near a transmitter. I can key-up the amateur radio rig and tune the antenna and demonstrate readings going all over the place on the little AVO.

As for theorising about Radon gas.... in decaying, Radon emits alpha particles which are positively charged. Alpha particles unless very energetic (particle accelerator) are easy to stop and not terribly dangerous. Inhaled Radon is dangerous. In your lungs, it is surrounded and you cop every alpha particle emitted. These sorts of things can be measured electrostatically, with an electrometer grade voltmeter and a large-area electrode, and a high level of alpha emission. Proper alpha measuring equipment has an expensive sensor tube with a fragile window... I doubt if multimeter probes would do the job, and any effect would be lost in triboelectric effects. If this mechanism produces large readings, then I'd run like hell.

David

[Cobaltblue]

Thanks to GJ RW and the other contributors for the explanations of these Phenomena

What I love about Electronic in general is every day is a school day and David's explanation of triboelectric effects is one I will tuck away for future reference.

Its time to put this back into its screened box

Cheers

Mike T