Abstract: Alarm-indicating gas detection apparatus including a first semiconductor gas-sensing element having one electrical property which varies significantly with increasing concentrations of a first undesirable gas and less significantly with increasing concentrations of a second undesirable gas and a second semiconductor gas-sensing element having the same electrical property which varies significantly with increasing concentrations of the second gas and less significantly with increasing concentrations of the first gas. The electrical property of both gas-sensing elements tends to vary similarly with age and with changing conditions such as temperature and relative humidity. A compensation system is used to cancel out errors due to variations in the electrical property. Threshold circuits are used to distinguish between first and second undesirable gases and to generate alarm signals.

Abstract: Dual-beam comparison of gas absorption characteristics in which a single source and single detector of radiant energy and single chopper disc are used in sample gas and reference gas paths. Ambient atmosphere or gas directed through a sample gas chamber can be analyzed. Multiple gas analyses, of a plurality of gases or a plurality of constituents of a gas, can be performed simultaneously utilizing the single chopper disc with a plurality of optical gas assemblies. Each such assembly includes a single source of radiant energy, means for directing the radiant energy along sample and reference gas paths to a single detector for such radiant energy. The multiple units are mounted so that the single chopper disc cyclically interrupts radiant energy in the respective sample and reference gas paths in predetermined phase relationship. The combined sample gas and reference gas path response of the single detector for each gas analyzing unit is directed to phase-sensitive signal-separation circuit means.