Campbell Scientific CS110 Overview - Manual (Page 3)

The fair weather diurnal pattern is believed to be caused by variations in world-wide thunderstorm activity
which affects a global electric circuit.
The presence of charged clouds results in dramatic increases in the magnitude of the local electric field as com-
pared with fair weather fields, with the sign indicating the dominating charge polarity. Figure 3 displays the
local atmospheric electric during a local thunderstorm. Deviation from and return to fair weather field condi-
tions are observed at the beginning and end of this storm. The abrupt electric field change observed at approxi-
mately 6:12 am was due to a cloud-to-ground lightning discharge within 1 mile of the electric-field meter.
Although no universal warning criteria based on electric-fi eld measurements exists, two levels that have been used
are
⏐1000 V/m⏐ [LPLWS] and ≥⏐2000⏐ V/m [NAVSEA]. Obviously the lower the level used the more risk reduc-
tion available, at the expense of increased down time for operations suspended for lightning hazard warning.
The CS110 Versus Traditional Field Mills
Atmospheric electric fi elds have been measured for decades by electric fi eld meters dubbed “fi eld mills”. Tra-
ditional fi eld mills employ a continuously rotating vane (rotor) electrically connected to ground potential. The
grounded spinning rotor alternately shields and exposes sense electrodes from the electric fi eld to be measured,
resulting in a modulation of induced electrical charge. A charge amplifi er, or pair of charge amplifi ers, convert
the modulated charge into an AC voltage. Further signal conditioning results in a low frequency (
≤10 Hz) voltage
proportional to the electric fi eld.
A variety of methods have been employed to make electrical contact with the spinning motor shaft of rotating
vane fi eld mills, all of which suffer from wear out mechanisms. Furthermore, traditional fi eld mills do not have
an inherent way to compensate for electronic drift with time and temperature, along with changes in leakage
currents across sense-electrode insulators, all of which induce measurement errors. Frequent scheduled cleaning
of insulators is often necessary in critical fi eld-mill applications, especially in coastal environments, to minimize
measurement errors due to insulator leakage currents.
Figure 3. Atmospheric electric field monitored by a CS110 during a local thunderstorm.
Electric Field (V
olt/meter)
Mountain Standard Time (1 sample per second)
August 2, 2005 Thunderstorm at Logan Utah
3