Appendix J. Site calibration
using transfer standard on
CM310s
Using the Mparallel_plate from the factory calibration, the CS110 measures the electric field (e
field
)
induced on the sense electrode. When installed at the surface of the ground facing up, the CS110
measures the Earth true electric field at that point.
To measure the Earth true e
field
when the CS110 is mounted above the ground pointing down, a
site calibration must be done to determine a multiplier, known as the C
site
or enhancement factor.
The C
site
multiplier is applied to the uncorrected e
field
measured by the downward facing CS110
to obtain the Earth true electric field. The C
site
multiplier corrects for the height of the CS110 face
above the ground and its distance below the top of the grounded metal mounting structure.
Campbell Scientific calculated a C
site
for the configurations described in this document. Change
the e
field
and C
site
when the CS110 uses a different mounting configuration, when the mounted
CS110 is on a peaked roof or near the edge of a flat roof (magnifies the e
field
), and when the
mounted CS110 is near a taller object that obstructs more than the 18 degree above the horizon
(reduces the e
field
).
In most situations, if using a corrected e
fields
, the criteria for the lightning hazard alarm level can
be either the US Navy level of 2000 V/m or NASA 1500 V/m launch. Uncorrected e
field
require the
caution and alarm thresholds to be adjusted based on your observations of lightning danger.
Exceptions to the Navy or NASA level are hill and mountain tops installations, and cold storms.
Hill and mountain tops typically experience higher electric fields than flat locations even during
fair weather conditions. Free ions are attracted to a charge of the opposite polarity. Consequently
they congregate at the high points before bleeding off, sometimes in a corona discharge called
St. Elmo’s fire. As a result of the higher concentration of charge, even if the CS110 is mounted in a
standard configuration on high ground, you will likely have to adjust your alarm threshold based
on some history of observed lightning and measured electric fields. Second, the lowest charged
layer in an electrified cloud occurs at the bottom of the freezing level. Precipitation will lower
some of that charge to the ground. For a given charge in a cloud, the closer the freezing level is
to the ground, the higher the e
field
because the distance between the charged layer and the
ground is reduced; the number of meters in the denominator of V/m is reduced yielding higher
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