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Post Time: 30/01/2009 10:58:51 |
neuroki
Total Forum Posts: 13 |
I was wondering if anyone could clarify a couple of points for me.
1. With regards to 50Hz electric wiring. Am I right in understanding that when using a meter with an earth reference lead, the result measures the electric field (magnetic field also?)of the appliance or wiring that you are trying to measure etc, but if you use a meter without an earth reference lead it measures the electric field (magnetic field?) that the person holding the meter is being subjected to at that moment in time? If that all makes sense!
2. What is the difference between "magnetic field strength" and "magnetic field density"? |
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Post Time: 08/02/2009 21:07:46 |
alasdairP
Total Forum Posts: 173 |
Apologies for the delay is responding. The answer is necessarily technical.
"Purists" believe that the "absolute" value of the electric field is what matters and say that it should be measured with a meter with a "displacement current sensor" (usually two flat parallel electrodes separated by an insulator) mounted on a non-conductive, preferably plastic, tripod. Nothing is connected to Earth. The Holaday 3604 meter is an example of this sort of meter. It is best suited to measuring e-fields under high-voltage powerlines outdoors.
We believe that it is the electric field your body actually experiences that matters as regards to possible adverse health effects. So the EMFields Pro meter calibration level is based on many tests with someone holding it. It gives a good measure of what the field is between the environment where the meter is pointed towards and the user's hand. This can lead to confusing measurements if you don't understand the principles, especially upstairs in normal houses where most of the e-field you experience often comes from the floor (due to the downstairs lighting wiring) and then "returns" to earth on a radiator or taps in a bathroom which legally have to be "earth-bonded". So you are then effectively the "source" of the e-fields coming upthrough your legs!
The Gigahertz 3030 meter comes with an earthing lead. We recommend that it is not used when measuring the e-fields your body is actually experiencing. It is useful to find the actual source of the electric fields as it is referred by to an absolute earth. So it will read high when held close to the upstairs floor (when using the earth lead) whereas the EMFieldsPro will read low if you are also standing on the floor.
Magnetic field strength and magnetic flux density are (incorrectly) used interchangeably.
Magnetic Field Strength (H) units are actually Amps/metre, but common usage means that Magnetic Flux Density (B) units in tesla (or microtesla or milligauss in the USA) are universally quoted these days, including by WHO.
In fact, FYI, B = uo * ur * H, where uo (mu-0) is the permeability of vacuum (4*pi*10^-7 Henries/metre!) and ur is the permeability of the medium that the magnetic field is in at the time (usually about 1 for air, water and most biological materials, but can be many thousands for ferromagnetic materials).
Microtesla and milligauss are the two normally accepted units for "magnetic field" measurements these days.
I expect you wish you had never asked that one!
Best wishes
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