Abstract: An instrument to accept multiple microbal samples and measure the gaseous products produced thereby as a function of time. Each sample module provides a closed reactional vessel communicating to a liquid reservoir from which displaced liquid passes to a vertical measuring column wherein it activates a float which moves a marker responsive to volume of gas produced in the reaction vessel. Plural sample modules are combined so that markers of all move in a parallel direction upon a recording sheet moved perpendicularly thereto to provide a record of gas formation as a function of time. The instrument is particularly adapted to measure the activity of a yeast activated fermentation process as in bread making.