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Articulograph Field Strengths

Alternating magnetic fields generated by the Articulograph AG100 in the VLF frequency range were measured at several distances from each of the transmitter coils. The generated fields were 140-160uT on the surface of each transmitter coil. This level is below the exposure standard approved by the American National Standards Institute.

Magnetic field strength was measured using a Narda Model 8532 Precision ELF/VLF Gaussmeter.gif This meter uses an induction coil to measure the true RMS field strength of a magnetic field along a single axis, with an accuracy better than +/- 5% (typically 1%) over the specified frequency range. The frequency range switch was set to the VLF range (2kHz to 50kHz), in order to filter out contributions from ELF fields. All measurements reported in this article have been converted from milligauss to microtesla using the conversion 10mG=1uT.

The transmitter coils in the Articulograph were turned on more than two hours before testing, in order to let them reach a steady operating temperature, as recommended in the manual (Carstens, 1993). All three transmitters were set to operate at maximum power (E-value=255).

The strength of the field perpendicular to the midsagittal plane was measured at 1cm increments in the vicinity of each of the three transmitter coils, beginning with the probe coil touching the coil of the transmitter. The results are shown in table 2. The minimum distance in table 2, 3.5cm, is the sum of the 2.5cm radius of the transmitter coil and the 1cm radius of the probe coil.

Distance (cm) Blue Coil ($\mu$T) Green Coil ($\mu$T) Red Coil ($\mu$T)
3.5 150.0 132.8 131.1
4.5 100.0 93.4 100.0
5.5 66.5 65.3 70.0
6.5 48.0 48.0 45.0
7.5 38.2 33.6 35.3
8.5 26.5 25.3 22.0
9.5 20.0 18.4 18.4
10.5 15.7 16.3 13.7

 
Table 2: Table 2: Magnetic field strengths perpendicular to the midsagittal plane. Distances shown are the distances between transmitter axis and probe axis; a distance of 3.5cm is obtained by resting the probe on the surface of the transmitter.

Fields generated by the blue transmitter coil were also measured in two directions parallel to the midsagittal plane. These fields were much weaker than the fields perpendicular to the midsagittal plane, and also much more variable: over the course of several seconds, the strength of a parallel magnetic field sometimes varied by as much as a factor of two. Representative values at several distances are given in table 3. In this table, ``vertical fields'' were measured parallel to, and ``horizontal fields'' perpendicular to, a straight line in the midsagittal plane passing through the blue and green transmitters.

Distance (cm) Vertical fields ($\mu$T) Horizontal fields ($\mu$T)
3.5 10 15
4.5 3.4 4.5
6.5 1.9 1.8
10.5 0.4 0.4

 
Table 3: Table 3: Magnetic field strengths generated by the blue transmitter coil along axes within the midsagittal plane.

Field strengths perpendicular to the midsagittal plane were modeled using the formula

B = A/r²

where B is the field strength, r is the distance from the transmitter axis, and A is a constant chosen to minimize the model mean squared error. The constant for the blue transmitter is A=1910uT cm², and the best-fit constants for the red and green transmitters were both A=1750uT cm². Figure 1 shows the measured field strengths and best-fit model field strengths as a function of distance r.

 
Figure 1: Figure 1: Measured field strengths (solid line) and modeled field strengths (dotted) as a function of distance from each of the transmitter coils.


next up previous
Next: Discussion Up: Electromagnetic Exposure Safety of Previous: Relevant Standards

Mark Hasegawa-Johnson
Mon May 26 15:33:40 PDT 1997