a backwash system including a dispensing cylinder and valve arrangement for directing diluent as a backwash from a source to a sampling valve utilized to make dilutions thence to a sampler in the form of an aspirator tube . the sampling is performed by means of the aspirator tube introduced into a fluid for sampling and a vessel is provided for catching the backwash fluid and aspirating same to waste . the vessel and the tube are positioned relatively to enable the backwash to be caught in the vessel only when diluent is dispensed as backwash . the aspirator tube and collector vessel are returned to their normal relative condition either by movement of the tube relative to the collector vessel or vice versa . suitable controls are provided to assure that backwash occurs only when the tube and vessel are conditioned to receive same .