In a colloid osmometer, what physical quantity is directly measured to determine osmotic pressure?

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In a colloid osmometer, the osmotic pressure is determined by the hydrostatic pressure difference that must be applied across a semipermeable membrane to stop the flow of solvent. The membrane allows solvent to pass but not the colloidal particles, so solvent moves to the side with higher osmotic pressure until an opposing pressure is applied. A pressure sensor (or manometer) directly measures that opposing pressure, and that value equals the osmotic pressure of the colloid.

Temperature, volume, and light absorbance aren’t the quantities read to determine osmotic pressure in this setup. Temperature can influence osmotic pressure, but it isn’t the measurement itself; volume isn’t the direct reading of osmotic pressure, and light absorbance relates to concentration or turbidity in some assays, not the balancing pressure across the membrane.