ETS-Lindgren HI-3603 VLF Survey Meter - Manual (Page 31)

HI-3603 VDT/VLF Survey Meter
© ETS-Lindgren, August, 2005
Revision E, Part #H-600042 31
has been recommended (Diffrient et al., 1981). Nevertheless,
because the value of 30 cm has been so often reported in the
literature, measurements should at least include this distance
among possible others.
VDT emission data reported in the literature show that in most
instances a fixed screen condition has been used to promote
more repeatable measurements. For example, a commonly
reported method involves filling the screen with a single
character such as an E or M and adjusting the brightness and
contrast controls to their maximum position. In contrast to these
precautions, it has also been reported that these measures often
seem to have very little, if any, impact on the resulting measured
values of electric and magnetic field strength (Roy et al., 1983).
Nevertheless, because of peculiarities of some VDT's, a check of
the effect of varying the brightness and contrast controls should
be made. Roy et al. (1983) suggest that CRT performance,
which decreases with age, and the type of video generating
system used are two possible factors responsible for this
phenomenon.
Electric field strength measurements are obtained by orienting
the HI-3603 sensor paddle so that the LCD screen faces away
from the VDT screen, enabling the user to read the value of field
strength. In this orientation, the emerging electric field lines
terminate on the surface of the displacement current sensor
plate.
It is important to realize that when the user holds the HI-3603,
there is generally a significant enhancement in the density of
electric field lines striking the sensor plate and thus the reading
will be in error relative to a free-field measurement. To reduce
this user perturbation effect, the HI-3603 should be held with a
non-conductive holder so as to avoid direct electrical connection
between the user and the instrument. Figure 5-1 illustrates the
positioning of the HI-3603 in front of a VDT using the support
device, Model 491001, which provides for the required degree of
decoupling between the user's body and instrument.