The mixer and fluid filtering unit is connectable to a source of a first or base fluid under pressure and to a source of a second fluid or solute which may be a solute pumping unit. The base fluid flows into a mixer section through a first inlet, and a small proportion of the base fluid is diverted into a filter section where it is filtered and passed out as motive fluid for the solute pumping unit. The remainder of the base fluid flows through a compensating valve assembly and into the mixer section where it is mixed with solute by a fluid pressure operated ratio valve assembly which maintains a variable preset mixing ratio. A sensor connected to the compensating valve assembly senses filter pressure drop and adjusts the compensating valve assembly in accordance therewith.
