Campbell Scientific CS110 Electric Field Meter - Manual (Page 17)

CS110 Electric Field Meter
Ileak = Cf·(Voff1 – Voff2)/ ΔT + Icomp (eq. 2)
Where Cf is the value of feedback capacitor used in the charge amplifier, and
Icomp is the leakage current compensation value implemented during the
measurement.
This charge amplifier input leakage current increases with degradation of
insulation of the sense electrode insulators due to moisture or other surface
contamination. Consequently, the measurement and reporting of Ileak is useful in
determining if or when insulators should be cleaned.
The reciprocating motion of the CS110 electric field meter is limited to
approximately 5 Hz, which is adequate for lightning hazard warning, where 1
minute averaged data is often used. 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 2, 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 f
3dB
=
(2 ⋅π⋅R⋅C) -1 = 2.4 Hz. In this mode the instrument is a field change meter and
the charge amplifier output can be sampled by the datalogger 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. Detailed information
regarding the slow antenna mode of the CS110 is given in Appendix E and
Section 8.3.
4. Site Requirements and Recommendations
4.1 Power Requirements
Field mills typically consume many watts of power because their motors are
operated continuously. In the reciprocating approach, the stepper motor is
powered off much of the time, resulting in low power consumption. The
current required by the CS110 powered from 12 V DC is shown in Figure 4.
As depicted in the figure, the average electric field meter current is a function
of the desired measurement rate, which is user-controlled by means of the
datalogger program, making economical remote solar power feasible. Variable
sample rates based on measured results can also be implemented to conserve
power in solar powered applications. For example, the datalogger can be
programmed to measure electric field at a 10-second rate during fair weather
conditions, and then automatically switch to 1-second measurements during
threatening conditions. An example variable sample rate program is given in
Appendix F. Figure 4 does not include the current required for peripheral
devices necessary to communicate with the CS110 site. Like the stepper
motor, communication devices that are turned off when not needed, can offer
low average power consumption.
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