The invention provides an indirect pressure sensing system for non-invasive measurement of primary pressure in a sealed container, which communicates primary pressure changes from within the container, via a flexible diaphragm, to a secondary chamber wherein there is a defined relationship between the primary and secondary pressures, which enables a pressure sensor in the secondary chamber to generate a signal representing primary pressure in the sealed container, but to remain isolated from the contents of the sealed container. The pressure sensor can provide electrical outputs representing the pressure detected, and the outputs are fed to data processing means capable of producing a measurement of primary pressure. The system can have a liquid culture of cellular material (eg. micro organisms, plant tissue cells, animal cells etc) partially filling the container, whereby the metabolism and/or growth of cellular material causes gas exchanges between liquid and headspace, which can result in primary pressure changes.