Improvements in pressure differential measuring means for suction dredging instruments

Claim 1

In a dredging system of the type including a vessel at the surface of a body of water; a suction dredging assembly carried by said vessel and including suction pump means having an inlet conduit projecting downwardly into a body of sand located below the surface of the water and a discharge conduit for directing a suspension of sand in water to a sand recovery region; and pressure-measuring means comprising purgative pump means having an inlet for water and a pressurized water outlet having at least two branches, one branch discharging at a first discharge point located below the surface of the water such that the pressure at said first discharge point is influenced by the suction pressure in said inlet conduit of the suction pump means, and the other branch discharging at a second discharge point located at a level different from said first discharge point, and pressure difference pickup means having a first water-filled line communicating with said one branch at a first pressure pickup point and a second water-filled line communicating with said other branch at a second pressure pickup point; the improvement wherein: said Pressure difference pickup means is located on board said vessel and said first and second water-filled lines communicate respectively with said one and said other branches at levels sufficiently higher than the levels of said first and said second discharge points that the pressures at said first and second pressure pickup points remain always at values greater than the vapor pressure of water, and the ratio of (1) the value of pressure decrease at said first pickup point due to flow losses in said one branch to such first pickup point to (2) the value of pressure decrease at said first discharge point due to flow losses in said one branch to such first discharge point being equal to the ratio of (3) the value of pressure decrease at said pickup point due to flow losses in said other branch to such second pickup point to (4) the value of pressure decrease at said second discharge point due to flow losses in said other branch to such second discharge point, whereby the pressure difference measured by said pickup means is independent of the output of said purgative pump means.