Method of measuring a property of entrained gas in a hydraulic fluid and fluid-working machine

A fluid working machine comprising a working chamber of cyclically varying volume, a low pressure manifold and a high pressure manifold, a low pressure valve for regulating communication between the low pressure manifold and the working chamber, an electronically controlled high pressure valve for regulating communication between the high pressure manifold and the working chamber, and a controller operable to actively control at least one of the low pressure valve and the high pressure valve to determine the net displacement of fluid by the working chamber on a cycle by cycle basis characterised by entrained gas measurement means to measure entrained gas within working fluid received into the working chamber, the controller being operable to take into account measured entrained gas when determining the timing of the active opening or closure of at least one of the low pressure valve and the high pressure valve.

Computer software comprising program code which, when executed on a fluid working machine controller, causes the fluid working machine to determine the timing of the active opening or closure of one or more actively controlled valves taking into account received measurements of entrained gas.