Abstract: A container having a port for holding a liquid medium and a reactant that interact to produce a change in gas levels within the container. A removable stopper for insertion into the port creating an airtight seal. A tube extending through the stopper into the container having a distal end for being submerged in the liquid medium for receiving the liquid medium and monitoring a change in the level of the liquid medium within the container. A controller is provided extending through the stopper into the container for setting the liquid medium at a level within the tube, wherein the controller is operable to direct the liquid medium into the tube to a desired level so that a change in gas levels caused by the interaction of the reactant with the liquid medium can be monitored through a change in the level of the liquid medium within the tube.