In a dynamic osmometer flow of solvent is measured and a counteracting pressure is created to stop the flow. Flow rate of the solvent is measured by the movement of an air bubble in a capillary tube of the solvent. The pressure of the solvent compartment is directly changed by raising or lowering a reservoir of solvent connected to the solvent compartment. The pressure difference between the two compartments is the osmotic pressure. This can be calculate by measuring the change in height or measured directly with a flexible diaphragm. Since the pressure is directly changed, an accurate measurement of osmotic pressure can be achieved in 10 – 30 minutes.