Yet when stringing multiple measurements together to look at dynamics of lightning discharges,
the relatively slow CS110 maximum sample rate of 5 Hz (200 ms) imposes serious limitations.
While the resulting time series presents the correct measured electric field at each measurement
time, what happens between measured data points is unknown, as the shutter is closed during
that time. Therefore, while good quasi-static (> 0.2 s) electric field data is provided by the CS110,
higher speed dynamic electric field information (< 0.2 s) is not provided. For a higher speed
measurements, the slow antenna mode of the CS110, as discussed in CS110 as a slow antenna (p.
61), can be used to provide 50 Hz (20 ms) electric field change data.
For applications desiring > 5 Hz, the CS110 reciprocating electric field meter can be configured as
a slow antenna (MacGorman and Rust 1998). The shutter would typically be left open indefinitely
in slow antenna mode and resistor R3, depicted in Figure 7-1 (p. 13), is switched in parallel with Cf
providing a 66 ms decay time constant for the charge amplifier. In the slow antenna mode, the
charge amplifier has a high-pass filter frequency response with the lower cutoff frequency
defined as f3 dB =(2πRC) - 1 = 2.4 Hz. In this mode, the instrument is a field change meter and
the charge amplifier output can be sampled by the data logger as fast as every 20 ms (50 Hz),
using 250 μs integration durations for the analog integrator. Voltage measurements using the
250 μs integration duration for an analog integrator, result in an upper 3 dB bandwidth of 1.8
kHz. Refer to CS110 as a slow antenna (p. 61) for detailed information about the slow antenna
mode.
7.2 CS110 lightning detection example
The following figure illustrates the atmospheric electric field monitored by a CS110 during a local
thunderstorm. In this figure, the atmospheric electric field changes dramatically from fair weather
conditions (approximately -100 V/m) during this thunderstorm. The abrupt electric field change
observed at approximately 6:12 A.M. was due to a hazardous cloud-to-ground lightning
discharge. As the electric field is seen to deviate from a typical fair-weather field and approach
levels capable of producing hazardous lightning discharges, a lightning hazard warning
algorithm would ideally issue an alarm, or perhaps various caution/alarm levels, during the
critical front-end portion of the storm illustrated here.
There is no universal hazard alarm level based on atmospheric electric field, although two levels
that have been used are ≥1000 V/m [LPLWS] and ≥2000 V/m [NAVSEA]. The lower the level used,
the greater reduction of risk, but this is at the expense of increased downtime for operations
suspended for lightning hazard warning.
NOTE:
Campbell Scientific, Inc. is not liable for the reliability and performance of the warning
algorithms implemented by users of our equipment. While lightning warning systems can
CS110 Electric Field Meter 15
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