Abstract: Method and apparatus for providing a concentration study of particles in a large system of continuously flowing fluid which is coupled to a sampling chamber that has a detector of the Coulter type therein. A regulator is provided to maintain constant the pressure in the sampling chamber irrespective of pressure changes in the large system. The regulator establishes a boundary surface between fluid from the sampling chamber and a column of liquid, such as mercury, immiscible with said fluid and of greater density than said fluid; whereby, a predetermined pressure is produced at said surface and permits sample fluid to pass through said column liquid as the pressure on the fluid variably exceeds the pressure at the boundary surface.