Analysis tools that use living cells as the primary sensor element require that samples be adjusted to physiological osmolarity levels prior to evaluation. Measurements presented show the effects of sample osmolality variations on cardiac cell action potentials (APs) as a motivation for the in-line osmometer. Finally, this work demonstrates an in-line osmometer for the purpose of evaluating the baseline osmolarity of a sample prior to adjustment. The operational range of the device presented is 0–500mOsm/kg with a resolution of 3mOsm/kg and an accuracy of ±8mOsm/kg.