Patent GB-505628-A (GB)
Title: Improvements in or relating to apparatus for testing gas
IPC: G01N27/416
Filed: 1937-11-11
Abstract: 505,628. Determining physical qualities. MINE SAFETY APPLIANCES CO. Nov. 11, 1937, No. 30997. Convention date, Nov. 12, 1936. [Class 37] A gas is tested for the presence or concentration of a constituent thereof by exposing to the gas a portion of an electrode chemically reactive to the constituent, the remainder of the electrode being immersed in an electrolyte adapted to displace the reaction product therefrom, and measuring the e.m.f. thereby created between the partially immersed electrode and a second, preferably wholly immersed electrode. The e.m.f. may operate an indicator, such as a milliammater, directly, or it may operate audible, and/or, visible signals, recording, or control mechanism through a relay or amplifier. The e.m.f. is a function of the concentration of the constituent and is measured after equilibrium has been attained between the reaction between the electrode and the gas and the displacement from the electrode of the reaction product by the electrolyte. This equilibrium may be accomplished by capillary action drawing the electrolyte up into the region of the electrode acted upon by the gas, or by using a rotary partially immersed electrode. The displacement of the reaction product by the electrolyte may be effected, for example, by chemical decomposition, dissolving or desorption. The electrodes are solid conductors or semi-conductors which react directly or after being wetted with the electrolyte and may be formed of gauze or of imperforate material. They may be of metals, alloys, compressed powdered salts or impregnated charcoal. For example, in testing a gas for the presence of oxygen, the electrodes may be of pure copper and the electrolyte a mixture of ammonium chloride and ammonium hydroxide to which a small amount of ammonium carbonate is preferably added. The two electrodes may be of different materials which produce an e.m.f. in the cell when both electrodes are submerged, appropriate adjustment being made in the measuring circuit. A cell 1 of insulating material, synthetic resin, glass, rubber &c. is provided with similar ring electrodes 10, 11, in a compartment 3 joined by passages 5, 6, 7, to a compartment 4 in which a plunger 12 is reciprocated or rotated by a crank handle 16, 17, 18, to vary the level of the electrolyte 19. With the plunger 12 in the lower position, shown, both electrodes are submerged and no e.m.f. is generated. When the plunger is raised the electrode 10 is partially exposed to the gas under test which is drawn into the cell by an aspirator bulb 22 through tube 20. If the constituent is present it reacts with the exposed portion of the electrode 10 and the electrolyte is drawn up by capillary action to displace the reaction product. When equilibrium has been reached the e.m.f. is constant and is measured in any known way. Measuring e.m.f. As shown, the e.m.f. across the electrodes 10, 11, is applied in series with a milliammeter 23, a fixed resistance 50 and rheostat 26 across the points A, B, in a bridge circuit energized by a cell 59. The meter is brought to zero when both electrodes are submerged by operation of the potentiometer wiper 27, and the meter scale is calibrated directly in terms of concentration, for example, by using air when the oxygen is the constituent to be tested for. Tests for hydrogen sulphide are effected by using magnesium electrodes and an electrolyte of weak sulphuric acid solution, while for detecting chlorine, tin electrodes and an electrolyte of a weak aqueous solution of ammonium chloride and ammoniumstannic chloride is used.