Sources of error and variability in electromagnetic field strength measurement surveys
How to select & use low cost EMF measurement instruments
Sensitivity and accuracy of Instruments for EMF power line and other source surveys
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Here we discuss the sensitivity and accuracy of EMF or ELF measuring instruments used for performing electromagnetic field (EMF) or
electro-magnetic radiation EMR measurements to measure EMF exposure levels in gauss or milligauss. We discusses
sources of error and variation in EMF measurements and we review and make suggestions for using several low-cost EMF
measurement devices to determine the instantaneous electromagnetic field exposure.
See ENVIRO-SCARE, EMF & Property Values if you don't know what EMF, ELF, or electromagnetic fields are or if you want a summary of the possible health effects of EMF exposure and the more likely effect on the property value of homes located very close to power transmission lines. Readers who intend to make their own EMF measurements should be sure to also see WORKSHEET for EMF MEASUREMENTS and also WORKSHEET for EMF MEASUREMENTS - Example.
The information provided here is for research and study purposes. The author makes no representation of unique
expertise on this topic, other than having field experience in EMF measurement, having studied technical literature and
having conversed with other experts and authors in the field for a number of years.
Sensitivity & Accuracy of EMF Measuring Instruments
Most hand-held low-cost EMF measuring instruments (see our list at Evaluation of Low-Cost EMF Instruments) can make measurements down to 1 milligauss. Because it is common for us to find background EMF levels of 1-3 milligauss in residential neighborhoods where local electrical power distribution lines and transformers are present, and because some studies examined EMF field strengths down to this low level, a do-it-yourself EMF measurement project should probably avoid using instruments that lack that level of sensitivity.
But as we explain next, an accurate, sensitive EMF instrument is insufficient for making accurate and repeatable EMF measurements.
Some instruments obtain EMF measurements that are sensitive to the orientation or position in which the instrument is held. Be sure to check that your instrument's readings are consistent regardless of instrument position, use a Tri-field instrument to avoid this source of inaccuracy, or if you prefer, as we do, to be able to see the field shape and source, be sure that your measurement procedure copes with this variation.
EMF measurements are sensitive to interference from local EMF sources that may swamp or distort EMF from a power transmission line. See Local EMF Sources May Exceed Power Line Strength for details.
EMF measurements are highly sensitive to hourly and seasonal variations in the load on power transmission lines; See Time of day & Season Affect EMF Strength for details.
EMF measurements of local EMF sources such as appliances are entirely sensitive to whether or not the appliance is turned on and in-use as well as to variations in electrical current depending on appliance settings.
Inconsistency in EMF measurement methods is a problem: In order to permit comparison of measurements (and studies) it is important not only to have line load data
available (for researchers) but also that site measurements are made in a very consistent manner from building to
building.
To do otherwise would make it impossible to compare conditions at one property with those at another, even if
both properties are equidistant from the same power line and even if both measurements were made at the same moment.
Position-insensitive EMF Measurement Instruments - best for the amateur
Some EMF measuring instruments, typically called "tri-field" or tri-position instruments, will give a consistent EMF strength reading in a given location regardless of how you are holding the instrument or where you are pointing it - providing the field strength has not changed between measurements.
We recommend this type of instrument for home use by "do it yourself" emf measurers.
Position-sensitive EMF Measuring Instruments - Why and How They are Used
Other EMF measuring instruments such as our Safeco (Photo at left) are very position-sensitive. The spreadsheets we provide allow for collecting EMF measurements
on three axes when using an instrument such as this one.
The disadvantage of the instrument is the requirement to make multiple measurements at each location during an EMF survey.
But the advantage of the instrument is that it allows us to see the shape, source, and even orientation of an electromagnetic field at a given location.
For Position-sensitive EMF measurement instruments, three readings are necessary.
Horizontal (spin through 360 degrees and record highest reading)
Vertical (same as above)
Pointed towards suspected source (e.g. distant power line)
To compute the actual point measurement, each of these numbers, once converted to mG, must be squared, the three
squares added, and the square root taken of the sum. This is because the measurement scale is not linear, so a direct
raw average would be incorrect. In the EXCEL worksheet which we provide at WORKSHEET for EMF MEASUREMENTS you'll see that provision is made for recording raw data points
as well as the individual mG readings.
EMF Polarity and Shape, not just Distance, Affect some EMF Measurements
Electromagnetic fields are created around power transmission lines by the passage of high levels
of current through the transmission line wires themselves. A power-line generated
electromagnetic field has polarity and shape, roughly
spherical around a power line.
The problem of the electromagnetic field having polarity and shape means that some early or low-cost
EMF measuring devices will give widely varying field strength measurements depending simply on the
physical orientation of the device when the measurement is made - that is, what direction you point the
instrument affects its reading.
But don't think that pointing directly towards the power line wires overhead
gives the maximum reading.
It may not, due to field polarity. More costly EMF meters have multiple sensors to
overcome this defect.
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Additional technical contributors & reference sources for this article are listed below.
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US Environmental Protection Agency, Office of Pesticides
and Toxic Substances, TSCA Assistance Office (TS-799), 800-424-9065
or 202-554-1404.
"Evaluation of Potential Carcinogenicity of Electromagnetic Fields,"
EPA Report #EPA/600/6-90/005B October 1990. EPA: 513/569-7562.
"Biological Effects of Power Frequency Electric and Magnetic Fields"
background paper, prepared as part of OTA's assessment of "Electric Power
Wheeling and Dealing: Technological Considerations for Increasing Competition,"
prepared for OTA by Indira Nair, M. Granger Morgan, H. Keith Florig, Department
of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA
15213
"Biological Effects of Power Line Fields," New York State Powerline
Project. Scientific Advisory Board Final Report, July 1, 1987.
"Extremely Low Frequency (ELF) Fields," Environmental Health
Criteria 35. World Health Organization, Geneva, 1984.
"Electric and Magnetic Fields at Extremely Low Frequencies:
Interactions with Biological Systems. In: Non ionizing Radiation Protection,
World Health Organization, Regional Office for Europe, Copenhagen, 1987.
"Electric and Magnetic Fields from 60 Hertz Electric Power: What do
we know about possible health risks?," Department of Engineering and Public
Policy, Carnegie Mellon University, Pittsburgh, PA 15213 1989.
"Electromagnetic Fields Are Being Scrutinized for Linkage to
Cancer," Sandra Blakeslee, New York Times, Medical Science section, April
2, 1991
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Electromagnetic fields are created around power transmission lines by the passage of high levels
of current through the transmission line wires themselves. A power-line generated
electromagnetic field has polarity and shape, roughly
spherical around a power line.
Environmental Concerns in Buildings,
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