Fluid moves against gravity in an osmometer due to the osmotic pressure difference between the fluid and the bag. When an osmometer is set up, the fluid in the beaker is hypotonic relative to the solution in the bag, so the net movement of fluid is into the bag. As fluid moves into the bag, a net pressure is generated forcing the fluid up the column.
Osmosis occurs when two solutions of different concentrations are separated by a semi-permeable membrane, which is permeable to the smaller solvent molecules but not permeable to the larger solute molecules. The solvent tends to diffuse across the membrane from the higher concentration solution to the lower concentration solution. Osmosis is of great importance in biological systems, where the solvent is the water in which many of the essential molecules for biological processes are transported. The energy that drives this process is usually discussed in terms of osmotic pressure or strength. Osmometers are used to determine the osmotic strength of solutions. As such, they are useful for determining the concentration of dissolved salts or sugars in blood or urine samples. In addition, they can also be used to determine the molecular weight of unknown compounds or polymers.