GB805051A - Improvements in and relating to the measurement of moisture or other vapour concentrations - Google Patents

Abstract

805,051. Electric indicating systems. COUNCIL FOR SCIENTIFIC & INDUSTRIAL RESEARCH. Nov. 28, 1955 [Nov. 6, 1954], No. 32189/54. Class 40 (1). A moisture-sensitive capacitor is made by subjecting an aluminium support to two stages of electrolytic oxidation or anodizing, the first stage producing an oxide layer of high porosity and the second an inner non-porous layer which is highly insulating. A further layer of aluminium thin enough to be pervious to vapour is then applied to act as the second plate of the condenser. Alternatively the outer electrode may be a coating of graphite applied to the moisture-sensitive layer in an aqueous solution. During manufacture a 15 cm. length of <SP>3</SP>/ 32 inch aluminium welding rod is degreased with carbon tetrachloride; at least 10 cms. is abraded with a steel wool pad to remove the air-formed aluminium oxide film and etched in hydrochloric acid, dipped in sodium hydroxide and then washed. A 9 cm. length is then electrolytically oxidized by immersing it vertically in an acid bath of dilute sulphuric acid, a lead plate acting as the other electrode. A 50 cycle A.C. voltage is used and a current of 310 milliamps maintained for 25 minutes. This produces a porous aluminium oxide layer. Six cms. of the anodized end is then anodized in a solution of boric acid, ammonium borate, or sodium phosphate. An aluminium plate is used for the electrode and about 12 volts D.C. applied for 30 minutes when the current reaches a minimum value (which is less than 5 microamps) the rod is cooled and 5 cms. cut off, leaving 1 cm. free for connection purposes. This process produces a high resistance dielectric beneath the porous layer. All except 4 cms. of the device is then coated with a thick viscous solution of acrylic resin, e.g. " Perspex " (Registered Trade Mark) in chloroform and coloured by the addition of methyl red. The whole of the vapour sensitive surface is then immersed in a solution of dilute nitric acid for 1 minute to increase its sensitivity. A thin film of aluminium is then deposited on the vapour-sensitive surface of the rod by rotating the rod near a source of aluminium vapour in a vacuum chamber. A shield prevents deposition of aluminium more than 5 cms. form the tip. Bared copper wire is then wound tightly on the aluminium which overlaps the " Perspex " coating and the wire together with the shank end from the negative and positive connections respectively. Specification 732,969 is referred to.