| Contents Notes |
AT-cut piezoelectric quartz crystals coated with low vapor pressure materials are evaluated as digital transducers in completely digital detection systems for monitoring gas streams. Mathematical relations describing the detectors are developed and tested. Specific application is made to monitoring gaseous sulfur compounds commonly found in stack effluents of pulp mills. It is shown that detectors formed by coating such crystals with a liquid capable of dissolving the gas of interest exhibit reversible characteristics with subsecond response times. Furthermore, if the sorbate-sorbent interactions are physical the detector exhibits linear response to varying concentrations of sorbate over the entire working range. Curvilinear plots are obtained if there are stronger interactions between the sorbate-sorbent pair. In the extreme case where there is chemical reaction between the two, the detector becomes irreversible, exhibits slow response characteristics, becomes integrating in nature, and can be used as specific detector with very high sensitivity. Instrumentation for actuation and measurement of crystal frequencies and for interfacing such detectors to digital computers are developed and evaluated. It is shown that the above-mentioned functions can be performed satisfactorily with inexpensive instrumentation designed around readily available integrated circuits. A partition detector exhibiting reversible response characteristics is applied to on-line data acquisition from a gas chromatograph employing a large general-purpose, time-sharing computer and a remote terminal. Employment of simple, inexpensive interfacing along with the presently available computational facilities on the Campus, permit implementation of a completely automatic data acquisition system. This approach, as an alternative to the use of small dedicated digital computers, is worthy of serious consideration. Behavior of the partition detector as applied to gas chromatography is described mathematically. It is shown that such a detector is sensitive to carrier gas flow rates and therefore is a concentration detector rather than a mass flow type. Furthermore, since this detector is sensitive to partition coefficient of the sorbate gas it can be used for confirming identity of components suspected of belonging to a known homologous series. |
