GB762008A - Analysis of gases - Google Patents

Abstract

<PICT:0762008/III/1> <PICT:0762008/III/2> The concentration of a key component gas in a mixture of gases is continuously measured by passing the test gas mixture and a reference gas mixture alternatively, and preferably periodically, past a thermal transducer. The reference gas mixture differs from the test gas mixture only in the amount of key component gas, and both mixtures are brought to the same temperature and pressure, whereby, owing to the heating effect of adsorption at the transducer surface of the key component gas, the transducer develops a fluctuating D.C. electric signal which is a function of the key component concentration differential between the two gas mixtures. The transducer output signal may be increased by locating on or near the transducer, material selectively adsorbent with respect to the key component gas. The transducer may be a resistance thermometer or thermopile and may be used for measuring the concentration of water vapour in gases such as nitrogen, air, carbon-dioxide, propane, and ethane; the concentration of butane, isobutane or propylene in a carrier gas such as propane, and the concentration of benzene in a carrier such as air. Glass-wool, adsorbent cotton, soft paper tissues, silica gel, or activated alumina may be used for the adsorbent when water vapour is the key component gas. As shown in Fig. 1, for measuring the concentration of water vapour in air, a portion of a moist air stream at 10 is lead directly by tube 13, a valve 22, and heat-exchanger 3 to a cell 7 containing a bed 4 of silica-gel and a thermopile 16. Another portion of the moist air stream at 10 is dried at 1 or 2 to remove substantially all water vapour and passed over tube 15, valve 22 and the heat-exchanger 3 to a cell 6 similar to cell 7. Periodically, either manually or through a time controlled switch 20, the four way valve 22 is moved to switch the moist and dry streams from one cell to the other. The thermopiles in the two cells are arranged so that the signals generated in each are additive and serve to operate a meter or recorder 9 which may be calibrated to read directly the water concentration. The periodic time of the switch 20 may vary from 0.1 to 10 minutes. A two minute period is preferably used and depends on the time for the transient signal produced at each switch over of the air streams to reach its maximum. A further valve 11 allows driers 1, 2 to be used alternatively while the other is being regenerated, and the heat-exchanger 3 brings both air streams to the same temperature before passing them to the cells 6. 7. In another embodiment, Fig. 3, a portion of the test gas stream at 60 is dried at 61; the dry and moist stream at 62, 33 are metered at 63, 64 and passed by a valve 30 alternatively through the heat-exchanger 35 to a single cell 37. The fluctuating output of the thermocouple 40 is amplified at 43, rectified at 45, e.g. by a commutator driven synchronously with the valve 30 by a synchronous motor 50, filtered at 46 and fed to a D.C. recorder.