A bubble point test is a test designed to determine the pressure at which a continuous stream of bubbles is initially seen downstream of a wetted filter under gas pressure.

To perform a bubble point test, gas is applied to one side of a wetterd filter, with the tubing downstream of the filter submerged in a bucket of water. The filter must be wetted uniformly such that water fills all the voids within the filter media. When gas pressure is applied to one side of the membrane, the test gas will dissolve into the water, to an extent determined by the solubility of the gas in water.

Downstream of the filter, the pressure is lower. Therefore the gas in the water on the downstream side is driven out of solution. As the applied upstream gas pressure is increased, the diffusive flow downstream increases proportionally.

At some point, the pressure becomes great enough to expel the water from one or more passageways establishing a path for the bulk flow of air. As the result, a steady stream of bubbles should be seen exiting the submerged tubing. The pressure at which this steady stream is noticed is refered to as the bubble point.