Campbell CS110, Инструкция по эксплуатации, Страница: 20 / 86

CS110 Electric Field Meter
10
4.4 Site Recommendations
Many factors can distort and/or change the electric field at a given sight. For
example, vegetation growth can reduce the effective height of an elevated
instrument above the ground and can created unwanted space-charge due to
corona discharge. Gravel rings or concrete pads around a given site are
recommended to prevent changes in effective instrument height due to vegetation
growth. Electric field meters used for lightning warning at Kennedy Space Centre
use a 25-foot radius gravel ring around each electric field meter [LPLWS].
Animals and people within the vicinity of an electric field meter can significantly
alter the measurements. Fencing off a given site may be best for some
applications. However, installing a
small
metal fence around an electric field
meter site may result in corruption of measurements at large electric fields because
of corona discharge from sharp metal points on the fence.
Aerosols, dust, and automobile exhaust should be considered when selecting an
electric field meter site, as they can affect the local electric field.
In theory, the effects of tall nearby objects can be accounted for in site correction.
Yet, because of possible corona current along with general field distortion, it is
recommended that electric field meter sites should not be located near tall objects.
Kennedy Space Centre site requirements stipulate having no objects protruding
higher than 18
°
above the horizon, as seen from the ground at the electric field
meter location [LPLWS]. Roof mounted electric field measurements are practical
if a site correction can be done to account for field distortions.
Also a good Earth ground connection to the CS110 and associated mounting
hardware is necessary to make a given site appear as a vertical extension of the
Earth ground. It is recommended that the integrity of this Earth Ground
connection be checked periodically by verifying that the resistance of the stator to
Earth Ground rod is <1
Ω
.
Although the list of factors that can impair electric field measurements is long,
experience has shown that useful electric field measurements can be made by
paying careful attention to the above mentioned details.
5. Factory Calibration and Site Correction
5.1 Factory Calibration
Electric field meters are typically factory calibrated using a parallel plate method,
where a uniform electric field is developed by applying a known voltage between
parallel conductive plates. The large hexagonal parallel plate electric field
calibrator illustrated in Figure 5 is used for factory calibration of the CS110
Electric Field Meter. The large physical size was incorporated to minimize non-
ideal fringing effects. Sharp corners were avoided in order to prevent corona
discharge. All metal parts of the calibrator are manufactured from stainless steel,
and the inside surfaces are polished to reduce the surface charges in order to
provide a stable zero electric field. All outer surfaces are electrically connected
and tied to Earth ground while the insulated inner plate is driven by a high voltage
amplifier. The high-voltage amplifier is calibrated out-of-house yearly against a
reference that is traceable to the National Institute of Standards and Technology
(NIST).