Charge build-up and decay monitor

Abstract

A device for monitoring air ionization. The charge build-up and decay monitor uses two metal plates, a first 'floating' plate electrically isolated from the monitoring circuitry and a second 'sensing' plate, to form an air dielectric capacitor of known capacitance. In charge decay mode the floating plate is charged and a second capacitor is connected between the sensing plate and electrical ground, producing a capacitive voltage divider. In charge build-up mode the second capacitor is disconnected and a second air dielectric capacitor of known capacitance is formed between the sensing plate and the ground metal casing of the device, again producing a capacitive voltage divider. The monitoring circuitry measures the charge present on the sensing plate, which is proportional to the charge present on the floating plate, without using failure-prone chopper electrometers or other electromechanical means. Internal monitoring circuitry computes the absolute value of the charge present on the floating plate to avoid the need for parallel processing circuits for positive and negative charges, the accuracy of the measurements are enhanced by the use of a large-valued resistor as part of the monitoring circuit to slow greatly the discharge of the second capacitor. Additional accuracy is obtained by monitoring the decay time between two predetermined voltages occurring on a portion of the decay curve that changes relatively slowly. Using predetermined test voltages also avoids the need to set multiple switches and dials and thereby decreases the possibility of human error in using the device.