Patent GB-1487179-A (GB)
Title: Method and apparatus for detecting polar vapours in gases
IPC: G01N27/62
Filed: 1974-12-12
Abstract: 1487179 Detecting polar vapours DEFENCE SECRETARY OF STATE FOR 12 Dec 1974 [5 Dec 1973] 56347/73 Heading GIN A detector for polar vapours in a gas stream comprises a pair of opposed dissimilar absorption surfaces 24, 25 carried by conductors 14, 15, anionizing source 10 upstream of the surfaces to render the gas conductive, and means forsensing the potential difference between the adsorption surfaces. In Figs. 1, 1A conductors 14, 15 are slidably mounted in slots 17 in a p-t-f-e holder 13 which is held in a cylindrical housing 1 by end cap 2. Conductors 20A, 20B each consist of a spring and two end balls biased apart by the spring and connect conductors 14, 15 to end cap 2 and to coaxial socket 23 respectively. The ionizing source 10 is an Am<SP>241</SP> alpha source, and a heater may be provided in the inlet duct to lower the relative humidity of the gas. Conductors 14, 15 may be of stainless steel, and one of them may carry a gold foil adsorption surface while the other carries a platinum foil or a silica adsorption surface or alternatively if water is to be detected, itself acts on the adsorption surface. A silica adsorption surface is provided by reacting silicon monoxide and oxygen to deposit silica on to a gold foil mounted on the stainless steel conductor. The adsorption surfaces are aged before use. In the modification of Fig. 4, one adsorption surface is provided on a wire 44 supported by a p-t-f-e holder 53 and connected to a coaxial socket 32 bolted to cylindrical housing 31, and the other is provided on a coaxial stainless steel cylinder 47.  The potential difference between the conductors may be measured accurately by an electrometer provided with the facility of applying a biasing voltage to cancel the potential difference produced by air without the polar vapour. Alternatively, the potential difference may be used to operate an automatic alarm system. To prevent the alarm being operated as a result of drift of the potential difference, the potential difference may be amplified, and the amplified signal used to control a motor driven potentiometer to cancel the drift. Alternatively, the potential difference may be used in an amplifying circuit including a C-R differentiating circuit, Fig. 8 (not shown), which may separate two integrating amplification stages. Fig. 10 shows a detector having two sensing cells 72, 74 of the type shown in Fig. 1, the air passing to cell 74 passing through a glass filter 73 which acts to delay high molecular weight polar vapours but not low molecular weight ones such as ammonia. Each cell 72, 74 is connected to an amplifier 76, 80, Fig. 11 (not shown) which operate triggers 77, 81, Fig. 12 (not shown), trigger 77 operating an alert warning 78 whenever any polar vapour passes through cell 72. The triggers 77, 81 are connected to an alarm circuit 82, Fig. 13 (not shown), a 3¢ second delay being placed between trigger 77 and alarm circuit 82. The alarm circuit 82 operates the alarm only if a signal is received from trigger 77 before one is received from trigger 81, thus operating the alarm in response only to a high molecular weight polar vapour which is delayed by the filter 73. The detector of the invention is said to be suitable for detecting toxic or polluting polar vapours, and tri-ethyl phosphate is given as an example. Other suitable materials for use as adsorbent surfaces are germanic oxide, carbon, boric acid, lithium sulphate, calcium stearate and paraffin wax.