To reduce errors when measuring sizes of particles in light-scattering counters and phase-Doppler analysers, the hydrodynamic focusing of particle trajectories in relation to the centre of the measurement zone is used. This focusing is effected in a special running-type chamber. Equations describing the mechanical energy of liquid flows running through a running-type chamber have been compiled. By solving a system of such equations, relatively simple equations for engineering calculations of the coefficient of hydrodynamic compression of particle trajectories in relation to the centre of the measurement zone have been obtained. Results of experimental research are given and the application of the proposed technique for designing improved running-type chambers with one and two ＇＇sheaths＇＇ is demonstrated.