which branches into a first branch 22 a and a second branch 22 b at a branch point 21 a measurement region 20 is defined in the measurement duct 16 and is associated with a detector 19 to sense a predetermined characteristic of particles in the measurement region 20 two opposed of bubble valves 100 a and 100 b are positioned in communication with the measurement duct 16 and are spaced opposite each other the bubble valves 100 a 100 b communicate with the measurement duct 16 through a pair of opposed side passages 24 a and 24 b respectively liquid is allowed to partly fill these side passages 24 a and 24 b to form a meniscus 25 therein the meniscus defines an interface between the carrier liquid and a gas in the reservoir of the associated bubble valve 100 an external actuator 26 is also provided for actuating the first bubble valves 100 a which momentarily causes a flow disturbance in the duct to deflect the flow therein when activated by the actuator 26 the second bubble valve 100 b serves as a buffer for absorbing the pressure pulse created by the first bubble valve 100 a the first side passage 24 a is hydraulically connected to a compression chamber 70 a in the first bubble valve 100 a so that if the pressure in this chamber is increased the flow in the measurement duct near the side passage is displaced inwards substantially perpendicular to the normal flow in the duct the second side passage 24 b positioned opposite of the first side passage 24 a is hydraulically connected to a buffer chamber 70 b in the second bubble valve 100 b for absorbing pressure transients this second side passage 24 b co operates.