Patent GB-1315509-A (GB)
Title: Method and apparatus for monitoring carbon dioxide
IPC: G01N27/416
Filed: 1970-09-15
Abstract: 1315509 Measuring physical properties electrically ABBOTT LABORATORIES 15 Sept 1970 [24 Sept 1969] 44086/70 Heading G1N In an arrangement for measuring the partial pressure of a gas e.g. carbon dioxide, in a liquid, e.g.intraveneous solution, the liquid is impinged continuously on the semi-permeable membrane of a pH ion sensitive measuring electrode. A small portion of production intraveneous liquid to which CO 2  has been added is continuously diverted from flow line L via pump 47 to impinger 42 to be discharged against membrane 48 of sensing electrode 18 in chamber 16, chamber 16 neck 24 and a major portion of nebulizer chamber 26 being filled with the sample liquid. After measuring the pH to give partial pressure of CO 2  the apparatus is calibrated by periodically opening valves 52, 56 and 66 to drain the sample liquid from chambers 16, 32 divert the sample liquid from impinger 42 and pass standard CO 2  into inlet conduit 34 of the nebulizer. The gas equilibruim cloud so produced is measured in chamber 16 and compared with the value it should be. Electrode 18 construction. Fig.2 Reservoir 78 between glass tube 74 and housing 76 contains a sodium bicarbonate buffer solution and the glass tube 74 also contains a measuring glass electrode 80, 88, 89 and reference electrode 86 e.g. mercury amalgam immersed in KCl solution. The end of the device is closed by membrane 48 permeable to CO 2  and separated from tip 89 of the measuring electrode by nylon mesh 98 saturated with bicarbonate buffer solution. Mesh 98 acts as a salt junction between measuring tip 89 and reference electrode 86 via aperture 106 in tube 74. To partially compensate for drift in electrode potential a small amount of gas of a fixed CO 2  content is bled into reservoir 78. Monitoring circuit. Fig.4 Stand by button is first depressed to move switch 176 and close supply switch 174 to supply power to a pen recorder 122 and meter 120, the input to recorder 122 being shorted out by switch contacts of 176. The standard button 178 is then depressed to reset stand-by switch 176 and close standard switch 180 energizing solenoid 186 which opens control valves 52, 56, 66, Fig.1 and substitutes the sample liquid in chamber 16 by the standard gas. Recorder 122 is now no longer shorted out and receives meter 120 output representative of the voltage generated by electrode 18. After equilibrium has been reached calibrate button 158 is depressed to energize transformer 142 and thereby activate recorder solenoid 138 to move the recorder switch into contact with contact 144 to impress a predetermined voltage set by micrometer 154 and which is a mathematical representation of the partial pressure of CO 2  in the standard across the recorder input 125. The difference i.e. compensating voltage in voltage between the predetermined voltage and two electrode voltage is "stored" in bridge network 160 by means of slide wire resistor 162 connected in one arm of the bridge and the setting of which is varied by clutch 194 energized by the difference in voltage. During initial standardisation the compensating voltage is zero but will not be subsequently due to the drift in output of the electrode. Measurement of a sample is then carried out by depressing button 196 which results in control valve solenoid 186 being de-energized and valves 52, 56 and 66 closing so that the sample liquid is introduced into chamber 16. The voltage measured by the electrode 18 is supplied to the recorder input in series with the compensating voltage from bridge 160 so that the correct voltage is recorded. Automatic operation The process of sampling and calibrating may be carried out automatically by depressing button 202 which utilises timers 210 and 228 and time delay relay 206.