Patent GB-615623-A (GB)
Title: Gas testing apparatus
IPC: G01N27/18
Filed: 1945-05-15
Abstract: <PICT:0615623/III/1> <PICT:0615623/III/2> In a gas - testing apparatus of the type in which the reduction in temperature of an electrically heated wire caused by contact with a gas is measured by determining the change in resistance, the resistance wire being disposed in a measuring cell, flow-resisting means are located in the conduit leading to the cell and upstream thereof and suction means are located downstream thereof, the arrangement being such that the gas pressure is lowered to less than half atmospheric, e.g. 3-50 mms. In Fig. 1, gas is drawn from pipe 30 through a constriction 31 and a conduit 29 broadened at 38, by a suction pump 32.  A passage 28 leads to a test cell 20 containing a resistance 21 which is part of a Wheatstone bridge circuit 22.  A valve 33 may be used to maintain constant pressure in the apparatus.  The modification shown in Fig. 4 obviates variations due to pressure change by using a reference gas, e.g. air, at the same pressure as the test gas.  Two test passages, 38 and 45, are provided, one for each gas, and two test cells communicate with each passage.  The two resistances for each gas are connected in diagonally opposite arms of the bridge 42.  A valve 37 enables the bridge to be balanced by initially passing air down both test passages.  For <PICT:0615623/III/3> <PICT:0615623/III/4> <PICT:0615623/III/5> <PICT:0615623/III/6> <PICT:0615623/III/7> <PICT:0615623/III/8> determining changes in the fuel/air ratio of an internal-combustion engine 54 comprising also a carburetter 57, supercharger 56 and exhaust pipe 60 (Fig. 7), the exhaust gas is tested against the air in inlet pipe 62 by means of a bridge 64, when the position of minimum thermal conductivity represents the perfect fuel/air ratio.  In use, the meter 65 is then applied (by switch 66) to bridge 63 which compares the thermal conductivity of the fuel/air mixture in connecting pipe 55 with air in pipe 62. The meter will now record variations from the perfect mixture by a positive or negative movement of the needle for a rich or lean mixture respectively. Cross-over switch 68 reverses the movement of the needle in the case of a fuel gas whose thermal conductivity changes in the reverse direction to that used for setting this apparatus.  Engine 54 may operate suction pump 32 which discharges into pipe 55.  The fuel/air mixture from pipe 55 may pass through a tube containing a reducing agent positioned in the exhaust pipe for heating purposes.  The exhaust gas from pipe 60 may be similarly treated.  Alternatively, as shown in Fig. 12, the exhaust gas alone may be treated by dividing it into two streams, p  passing one over an oxidizing agent in tube 101 and the other over a reducing agent in tube 102, and comparing their thermal-conductivities.  For use, the meter 85 is first connected by switch 86 across shunt resistance 111 to be in series with the battery 93 and variable resistance 94, and the current adjusted to a suitable value.  Valve 103 is then turned counterclockwise to admit gas from tube 101 to both pipes 95, 96 and the meter 85 connected across the bridge by switch 86.  After the meter reading is adjusted to zero by potentiometer 92, valve 103 is turned to the position shown in the diagram, when the meter will then show positive and negative variations of thermal conductivity. Three-way valve 98 is positioned as shown in use, but when the engine is shutting down, may be turned to admit air, through a constriction 98a, to the test system.  The test cell shown in Figs. 16 and 17 comprises two metallic blocks 120, 121 defining cell chambers 88, 89, 90 and 91. Test passages 95, 96 in block 120 communicate by slots 1411 in plugs 140 with the cell chambers.  Lead-in wires 142 pass through four Kovar seals 144 and support four resistances 141, one in each arm of the cell chambers.  The two resistances in each chamber are connected in diagonally opposite sides of a bridge.  Guard grooves 153 surrounding the cell chambers are connected to passages 95, 96 by pipes 154 to protect the vacuum in the chambers.  A compressible gasket 1202 may be provided between the blocks.  To maintain constant temperature in the blocks a thermostatic switch 149 and electric heater 150 are provided.  Valve 103 in block 120 is protected against loss of vacuum by interconnecting grooves 155, 156, 1032 connected via pipe 157 and T-piece 1101 to the suction pipe 97.  To prevent excessive asymmetry of the meter scale, a second hair spring 180 (Fig. 15a) is adapted, through pivoted arm 1805 to influence the meter indicator needle on one side only of the zero position.  Fig. 19 shows a self-adjusting constriction comprising a body member 190 mounted in the wall of a gas pipe 192, and in which a helically grooved plug 194 is slidably mounted.  The suction through pipe 138 causes the plug to move against the action of spring 200 to shorten the effective length of groove 195.  A constriction may alternatively be used in which the plug 194 is fixed, and a filter is provided in the gas passageway.