EMF meter (C) Daniel Friedman EMF Measurement Sensitivity & Accuracy Problems & Solutions
     

  • EMF MEASUREMENT INSTRUMENT ACCURACY - CONTENTS: Sources of error and variability in electromagnetic field strength measurement surveys. What are the sensitivity & accuracy of EMF electromagnetic field exposure survey measurement instruments? How to handle the position-sensitive nature of some EMF instruments - and an explanation of the field shape of electromagnetic fields
  • POST a QUESTION or READ FAQs about how to hold, position, & use EMF field strength measurement instruments to account for variations in sensitivity and accuracy of Instruments for EMF power line and other source surveys
  • REFERENCES

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EMF measurement instrument sensitivity & accuracy: this article explains 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.

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Sensitivity & Accuracy of EMF Measuring Instruments

Safeco EMF meter  (C) Daniel FriedmanBecause RF and EMF measurement tools need to be properly chosen to measure the particular type and frequency of RF or EMF signal that is of interest, be sure to also
see EMF RF FIELD & FREQUENCY DEFINITIONS for a simple explanation of different types of radio frequency (RF) and electromagnetic frequency (EMF) types and where they are found.

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.

    We describe the position sensitivity problem in more detail at EMF Polarity & Shape Affect Measurements. We describe how to do so at Recommended EMF Measurement Procedure.

    We illustrate the EMF field shape at EMF/EMR Measurement Procedure Overview.
  • EMF measurements are sensitive to distance from the source of the EMF.
    See EMF MEASUREMENT DISTANCE AFFECTS STRENGTH.
  • EMF measurements are sensitive to interference from local EMF sources that may swamp or distort EMF from a power transmission line.
    See EMF LOCAL 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.
  • Because RF and EMF measurement tools need to be properly chosen to measure the particular type and frequency of RF or EMF signal that is of interest, be sure to also
    see EMF RF FIELD & FREQUENCY DEFINITIONS for a simple explanation of different types of radio frequency (RF) and electromagnetic frequency (EMF) types and where they are found.
  • See Definitions of Gauss vs Milligauss for details about gauss and milligauss and definitions of these terms

EMF meter (C) Daniel FriedmanInconsistency 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.

See EMF Polarity & Shape Affect Measurements for further explanation.

Position-sensitive EMF Measuring Instruments - Why and How They are Used

Safeco EMF meter  (C) Daniel Friedman

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 EMF MEASUREMENT WORKSHEET you'll see that provision is made for recording raw data points as well as the individual mG readings.

See EMF Polarity & Shape Affect Measurements (just below) for further explanation, and

see EMF MEASUREMENT WORKSHEETfor the proper calculation method used to combine multiple position-sensitive EMF measurements into a single field strength number for a location.

Readers should also see Comparing Gauss versus Milligauss Field Strength Measurements where we explain the greater accuracy of low-level EMF readings when using the milligauss scale on an ELF EMF meter.

EMF Polarity and Shape, not just Distance, Affect some EMF Measurements

bar magnet shows the typical shape of an electromagnetic fieldElectromagnetic 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.

We discuss the problem of electromagnetic field shape, polarity, and instrument testing sensitivity to instrument orientation or position at Sensitivity & Accuracy of EMF Measuring Instruments.

Instead of contacting us with a request to perform EMF Electromagnetic or RF Radio Frequency Field Strength measurements, in most cases it is more economical and convenient for a property owner to purchase their own instrument, making measurements under varying conditions. In this series of articles we describe how to make measurements using a consistent approach and using good documentation.

See Recommended EMF Measurement Procedure for details of how to collect EMF measurement data.

Following good procedure and using instruments properly are two steps towards making accurate, repeatable EMF measurements. But because the signal transmission for RF sources such as radio, TV, or cell towers, the load on a power transmission line is not under control of an individual property owner, and because the EMF strength varies as the power transmission line load varies, it is important to have an idea of that condition as well when attempting to characterize EMF exposure at a specific location.

In contrast, EMF measurements are quite accurate and repeatable at other EMF sources such as close to electrical appliances and service entry cables.

 

Continue reading at EMF MEASUREMENT INSTRUMENT USE TIPS or select a topic from the More Reading links shown below.

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