Patent Number: 
Section: claims

1. A device for filtering or straining a liquid with a filter or strainer and assessing the degree of clogging of the filter or strainer, comprising a filter or strainer having a first side for facing a liquid mass and an opposed second side; first and second substantially identical vessels positioned substantially in a common horizontal plane on opposing sides of said filter or strainer; first and second gas pressure indicators or sensors associated with respective of said vessels and each sealing connected via first and second gas-filled conduits to an upper part of a respective corresponding vessel, each vessel having a cross-sectional area, said conduit and vessel both having an interior volume whereby the interior volume of each vessel is between 4 and 7 times larger than the interior volume of the corresponding conduit, each vessel having an inlet for communication with a common liquid mass having a surface level which inlet has a cross-sectional area that is smaller than the cross sectional area of the corresponding vessel and which is located near the bottom of the corresponding vessel, whereby said device is arranged such that liquid entering the vessel via the inlet establishes a surface level within the vessel whose position varies with the level or pressure of the surrounding main liquid mass whereby the gas pressure within each of said vessel and conduit varies in response to changes in the liquid level within each said vessel while said conduits remain filled with gas, said first sensor or indicator establishing a base level representative of the liquid pressure within the common liquid mass, the inlet of the second vessel being connected to a strainer or filter, for straining or filtering liquid entering the second vessel from said liquid mass, the gas pressure within said second vessel being indicative of a pressure drop resulting from said filtering or straining operation, said pressure decreasing as said filter or strainer becomes clogged. 2. A device according to  claim 1  wherein liquid within said second vessel exits said vessel via an outlet conduit. claim 1 3. A device according to  claim 1  having a secondary tube conduit between said filter or strainer and said inlet. claim 1 4. A device according to  claim 3  wherein said secondary tube conduit is generally horizontal. claim 3 5. A device according to  claim 1  wherein the interior volumes of the first and second vessels are each between 5 and 6 times the interior volumes of the corresponding first and second conduits. claim 1 6. A device according to  claim 1  wherein the conduit has an inner diameter of between 8 and 20 mm. claim 1 7. A device according to  claim 6  wherein the conduit has an inner diameter between 12 and 13 mm. claim 6 8. A device according to  claim 1  wherein the inlet of the first vessel is for communicating with the liquid mass through a tubular member having a wall and closed distal end and a plurality of apertures within said wall to admit liquid into said inlet, said apertures having a sufficient size to limit clogging of at least some of said apertures by impurities. claim 1