Monday, 10 February 2003: 4:00 PM
A low-power electric-field meter with reciprocating shutter
Electric-field meters of the type commonly known as “field mills” have been used for many years to measure the atmospheric electric field near the ground for thunderstorm research and safety purposes. Although the electric field is a unique indicator of atmospheric electrical hazards, routine observations of ambient electric field have not been included in the canonical suite of meteorological data sets. This may be due to prohibitively high costs of acquisition, installation, and maintenance of field mills. Field mills employ a motor-driven metal rotor connected to ground potential to alternately expose sensing electrodes to and shield them from the electric field. In most field mills, the voltage output from one or more charge amplifiers is synchronously rectified and then filtered to arrive at a DC output voltage proportional to the applied electric field. Long-term reliability of field mills has been limited by the reliability of the ground-potential connection to the rotating motor shaft and the degradation of electrical insulation of the sense electrodes. Under a license agreement with the University of Oklahoma, Campbell Scientific Inc. has developed an electric-field meter (patent pending) that employs a reciprocating shutter, instead of a spinning rotor. In significant departure from conventional rotating contacts, the reciprocating shutter allows for a direct electrical bond between ground potential and the motor shaft. DC measurement errors due to contact potentials, electronic offset voltages, and insulator leakage currents are eliminated by measuring the charge amplifier output voltage with the shutter closed. Self-diagnostics have also been incorporated to limit maintenance to “only when required.” The reciprocating shutter action also provides for low power consumption, as the motor is de-energized most of the time. The resulting electric-field meter is suitable for deployment on solar-powered automated meteorological stations, singly or in networks.