Measuring and testing – Liquid level or depth gauge – Hydrostatic pressure type
Patent
1991-01-02
1992-09-15
Yasich, Daniel M.
Measuring and testing
Liquid level or depth gauge
Hydrostatic pressure type
73302, 73714, G01F 2318
Patent
active
051467836
DESCRIPTION:
BRIEF SUMMARY
PRIOR ART
The invention is based on a liquid container with a hydrostatic level gauge, in particular a fuel tank with a tank level indicator, of the type defined hereinafter.
There are many known versions of level gauges in automotive engineering for monitoring the contents of the fuel tank. As fuel tank shapes become increasingly complicated, the level gauge that uses a float is increasingly being abandoned for more flexible, nonmechanical measuring systems. For instance, electrothermal tank level indicators or tank level indicators or level gauges operating on the piezoelectric, acoustical or hydrostatic principle are already known.
In hydrostatic level gauges, the hydrostatic pressure of the liquid to be gauged is measured and from this a conclusion as to the level is drawn by including the liquid density. To be able to measure the hydrostatic pressure of the liquid in the interior of the container, the differential pressure between the liquid pressure at the bottom of the container and the air pressure above the surface of the liquid is measured; a differential pressure sensor is generally used for this. A substantial disadvantage of this differential pressure method is the temperature dependency and the long-term instability of the zero point in differential pressure sensors available on the market.
ADVANTAGES OF THE INVENTION
The liquid container according to the invention having a hydrostatic level gauge has the advantage that because of the structural design according to the invention, a zero calibration of the differential pressure sensor is readily possible and can be performed at any time. For the zero calibration, the two measurement inputs of the differential pressure sensors are exposed to the same pressure. The electrical signal output by the differential pressure sensor under these conditions is stored in memory, and in the ensuing level measurement the output signal of the differential pressure sensor is corrected upward or downward by this memorized value, depending on the algebraic sign (+or -) of this value. From the output signal, thus compensated for in terms of zero point drift, the instantaneous level inside the container is then determined by the evaluation unit, taking into account the liquid density and the acceleration due to gravity. In this way, not only is high measuring accuracy attained, but cost savings also become possible, since zero point and aging stability are no longer required of the differential pressure sensor and it can accordingly be made at much less cost. Because the zero point stability is dispensed with, more economical differential pressure sensors made by thick-film technology can now be used.
Moreover, a predetermined disposition of the differential pressure sensor on the container bottom is no longer compulsory; instead, like the air pump it may also be disposed outside the container. Only a very small tube is needed as a transmission route for the hydrostatic pressure. The pump can be kept small, keeping the technical expenditure low.
The freedom of liquid in the evacuated meter tube can be maintained in various ways. In a first embodiment of the invention, a shutoff valve is provided in line with the air pump; it opens to flood the meter tube when the pump is shut off, and it is closed, with the pump running, after evacuation of the meter tube; after that, the pump is shut off again. Flooding of the meter tube may be done by opening the shutoff valve first, prior to each measurement of the level, or only at relatively long intervals, if a zero point drift is suspected because environmental parameters (temperature) have changed.
During these long intervals, the compression of the air column in the meter tube (for example from acceleration of the liquid or from temperature fluctuations) would cause liquid to rise in the meter tube, making the outcome of measurement incorrect. This error is kept small if in a preferred embodiment of the invention the lower end of the meter tube is widened so extremely in a short axial end segment that the volume of th
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Dukart Anton
Jansche Walter
Zabler Erich
Greigg Edwin E.
Greigg Ronald E.
Robert & Bosch GmbH
Yasich Daniel M.
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