Chemistry: physical processes – Physical processes – Crystallization
Patent
1978-06-19
1981-01-27
Marcus, Michael S.
Chemistry: physical processes
Physical processes
Crystallization
324 61R, 340629, 340632, 422 98, G01N 2722
Patent
active
042472997
ABSTRACT:
An air pollutant and/or fire combustion sensing apparatus includes a sensing electrode having an electrically non-conductive or dielectric sensing layer which has a surface resistivity in excess of 1.times.10.sup.10 ohms/square (and preferably 1.times.10.sup.15), and a bulk resistivity in excess of 1.times.10.sup.12 ohm-cm, and preferably 1.times.10.sup.15 at 50% R.H., and which is essentially free of dipole-hydrogen bonding forces such that the surface energy component is primarily due to dispersion bonding forces and, if at all, only incidentally as a result of dipole-hydrogen bonding forces. The latter appears to be a principal factor and desirably has a value of less than 5.0 ergs/cm.sup.2 and preferably less than 1.0 erg/cm.sup.2. The gases detected have a large Van der Waal gas "a" constant and dipole moment. This sensing layer adsorbs air borne polar constituents to alter the charge on the electrode. Optimum results are obtained with polytetrafluoroethylene (Teflon TFE), perfluoroalkoxy (Teflon PFA), fluorinated ethylene-propylene (Teflon FEP), polystyrene or polyethylene. A high input impedance detecting circuit with good electrometer characteristics responds to the change in the charge, particularly on the surface. A thermally stabilized field effect transistor operating as a voltage amplifier with an input impedance on the order of 10.sup.10 ohms and preferably greater than 10.sup.12 ohms is satisfactory.
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Klein Carl F.
Thoma Paul E.
Johnson Controls Inc.
Marcus Michael S.
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