Offset voltages V
off1
and V
off2
are zero field reference measurements made when the shutter is
closed, and used to accurately estimate voltage ΔV when the shutter is completely open.
Electronic offset voltages, surface potentials between various metallic parts and leakage currents
on the charge amplifier input result in non-zero values of V
off1
and V
off2
. An electronic reset of
the charge amplifier is performed prior to the measure of V
off1
to keep the charge amplifier
output near zero volts when the shutter closed. The measured electric field E, as determined from
the charge amplifier output is as follows:
E = kΔV = = k[V
open
– (V
off1
+ V
off2
)/2]
Where k is a constant determined by electrode geometry and electronic gain. The resulting
algorithm effectively eliminates measurement error sources that vary slowly with respect to the
time between zero field reference measurements, which is approximately 140 ms. Measurement
noise due to 50 or 60 Hz AC power can be suppressed by using the 50 Hz or 60 Hz noise
rejection measurement capability of the data logger.
Current source I
leak
in Figure 7-1 (p. 13) represents leakage currents across the Teflon insulators
supporting the sense electrode, along with the input bias current of the operational amplifier.
Deleterious effects of I
leak
are compensated for in the determination of ΔV as given in (eq. 1).
However, it is desirable to minimize the difference between V
off1
and V
off2
to preserve dynamic
range for large magnitude V
open
voltages. Hence a leakage-current compensation circuit is used
to generate the current Icomp in Figure 7-1 (p. 13) such that I
comp
= I
leak
. The leakage-current
compensation algorithm determines I
comp
for the present measurement based on Ileak from the
previous measurement, which is determined as follows:
I
leak
= Cf·(V
off1
– V
off2
)/ΔT + I
comp
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. The CS110
electric field measurement is a single shot measurement of the average electric field over the
integration interval, with a sin(x)/x frequency response with the first notch determined by the
integration length of either 1/(60 Hz) or 1/(250 µs), depending upon the user chosen integration
parameter. The resulting measured electric field is useful for applications requiring an absolute
electric field measurement, such as lightning warning.
CS110 Electric Field Meter 14
To Next Page
Loading...
