GB1421742A - Apparatus for controlling the flow of gas - Google Patents

Abstract

1421742 Gas chromatography PYE LTD 2 March 1973 [4 March 1972] 10210/72 Addition to 1241867 Heading B1H [Also in Division G3] The invention of the parent Specification 1241867 wherein gas is passed through a long flow passage of a control element which is heated to change the viscosity and therefore the flow rate of the gas, is modified essentially by the provision of a gas flow transducer in connection with the inlet of the control element to produce an output signal which is a continuous function of the gas flow rate. The output signal can control means for changing the temperature of the control element. The transducer may be a hot-wire or ionisation anemometer, a piezoelectric crystal, a thermistor, a thermocouple, a resistance thermometer, or a differential heatexchange device responsive to the heat transferred 'between the gas and an element of the transducer. One or more such elements of the transducer may be formed by resistive arms of a Wheatstone bridge circuit, which arms may be formed by winding a wire around a tube along which the gas flows or formed by the walls of the tube itself, Fig. 2 (not shown). The control element may be heated by an electric current passed through the tube walls as disclosed in the Parent, or the current may be passed through an electrical conductor in thermal contact with the control element, e.g. an electric resistance wire wound around the element. Both the output signal current and the flow controlling heating current may be passed through the tube walls or through the conductor, one of the currents being a.c. and the other d.c. The control element can be immersed in an inert gas, Fig. 1 (not shown). The element may be a tube made of platinum, nichrome, stainless steel; glass, ceramics or P.T.F.E., or a tube filled with powder formed, e.g., from particles of glass, ceramics plastics or carbon, or filled with glass beads, graphite fibres, wire wool, fritted glass, or sintered metal. Alternatively, it may comprise a plurality of tubes arranged as disclosed in the parent, or it may comprise a block of porous material, e.g. sintered metal, bonded glass frit, porous carbon or graphite or a rod of porous pottery material, the cylindrical surface of the rod or some of the faces of the block being covered or coated to an impervious state. The control element may be used to control the flow of carrier gas in gas chromatography apparatus. Such apparatus may be arranged to automatically monitor atmospheric pollution over extended time periods in which case the control element can be associated with an additional length of tube, e.g. a capillary tube, in the carrier gas supply or in the column, and a small heating current is applied to the additional tube to reduce the flow rate of the carrier gas during those periods when no analysis is being made.