Johnson Controls Gmbh

Gerald E Weber

Paul E Thoma

Carl F Klein

An air pollutant and/or fire combustion sensing apparatus includes a pair of sensing electrodes separated by a free space to define a capacitance unit. At least one of the electrodes includes a corrosion-resistant, conductive material which interacts with air borne products to alter the permittivity and/or charge transfer characteristic of the unit. The material is in the form of a highly corrosion-resistant metal selected from a metal or metal alloy including at least one of the metals selected from the Group VIII, periods 4, 5, and 6 of the periodic table, particularly rhodium and stainless steel. Carbon and copper respond to produce a detectible output. An amplifying and alarm circuit is connected to the capacitance unit, and responds to either an increase or a decrease in the capacitance as a result of the interaction with the sensing electrode material. The circuit includes a thermally stabilized field effect transistor as a buffer input amplifier connected in differential configuration to produce a high input impedance and a high gain. The transistor is connected to a high gain DC operational amplifier having a feedback compensation network to compensate for thermal and low frequency drift change. A constant current source provides the bias supply to the input amplifier to improve common mode rejection and the thermal stability. A pair of programmable unijunction transistors are connected to the output of the operational amplifier and to the input of an alarm device to detect either an increase or a decrease at the amplifier output which may arise as a result of the decreased or increased capacitance characteristics of the air borne material.

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Ben Brook, TX, United States of America

Gerald E Weber