The Helley-Smith bedload sampler operates on a pressure-difference principle and consists of an expanding nozzle, nylon-mesh sample bag , and external components for structural integrity and stability under flow conditions. The Helley-Smith bedload sampler has a hydraulic efficiency, the ratio of the mean velocity in the sampler nozzle to the mean ambient velocity, of approximately 1.54. Factors affecting the hydraulic efficiency include the geometry of the orifice and the open area of t he sampler bag .
The exit-to-entrance area ratio of the nozzle is the major factor influencing hydraulic characteristics of the sampler. For the nozzles tested, the hydraulic efficiency rapidly increased to a maximum value as the exit-to-entrance area ratio increased . The hydraulic efficiency increased from 1.06 to 1.54 as the exit-entrance area ratio increased from 1.00 to 2.62. With an exit-entrance ratio greater than 2.62 but less than 3.22, this efficiency remained constant at 1.55.
Filling the sample bag to 40 percent capacity with a sediment larger in diameter than the mesh size of the bag had no effect on the hydraulic efficiency. Particles close to the 0.2 mm mesh size of the sample bag plugged the openings and caused the efficiency to decrease in an undetermined manner.
