|Title of Invention||
"A SELF CORRECTING PRESSURE BASED WATER LEVEL GAUGE USEFUL IN CLEAR AND TURBID WATERS "
|Abstract||A self-correcting pressure based water level gauge useful in clear and turbid waters which comprises at least two pressure transducers  and  separated by an appropriate known vertical distance and essentially interconnected to an electronic interface module  consisting of a multiplexer , a control, computing and data storage means  wherein the said interface module during each sampling of pressure values, estimating the effective depth-mean density of water wherein the said density value which being applied on the measured pressure signal from the lower transducer  estimate the sea level measurement arising from an uncertainty in the effective depth-mean density which exists in hitherto known single pressure transducer based sea level gauges.|
|Full Text||The present invention relates to a self-correcting pressure based water level gauge useful in clear and turbid waters.
More particularly, this invention relates to a multiple pressure transducer based water level gauge useful in clear and turbid waters, allowing for automated estimation of effective depth-mean density of natural waters in its vicinity during each sampling, resulting in the improvement of water level measurements.
In hitherto known pressure based water level gauges, V. B. Peshwe, S.G. Diwan, A. Joseph and E. Desa [in "Wave & Tide Gauge" in IJMS, Vol. 9, pp. 73-76 (1980)] describes a wave and tide gauge, which utilizes a strain-gauge pressure transducer. The transducer with associated electronic circuitry gives an output voltage proportional to the pressure head of the water column above the transducer. Appropriate filtering by analogue signal processing circuits is applied to emphasize the measuring parameter (i.e., wave/tide), and to trace the signal as a function of time on strip chart recorders. A drawback of this system is that a change in water pressure head arising from variations in water density is not taken into account. Consequently, the device is susceptible to inaccuracies in water level measurement, arising from tide-induced diurnal variations and river-flux-induced seasonal variations in seawater density.
In an alternate pressure based water level gauge, A. Joseph and E.S. Desa [in "A microprocessor-based tide measuring system" in J. Phys. E: Sci. Instrum, Vol. 17, pp. 1135-1138 (1984)] describes a fully automatic self-recording tide gauge which uses a quartz pressure transducer and a microprocessor in a single-board computer. The water pressure data acquired
at pre-programmed time-intervals are translated to water height using an assumed value for the water density. The time-tagged water level data are recorded in erasable programmable read only memories (EPROMs). User friendly features built into the instrument permits the user to inspect or obtain hard copy outputs of data at the recording site. This system also suffers from inaccuracies in sea level measurements, arising from uncertainties in the assumed water density.
In another pressure based water level measurement system, D.T. Pugh [in "Tides, Surges, and Mean Sea Level: A Handbook for Engineers and Scientists", John Wiley and Sons, New York (1987), and in "The Physics of Pneumatic Tide Gauges" in Int. Oceanogr. Rev., Vol. 49 (2), pp. 71-97 (1972)] describe a gas-purged bubbler gauge. In this device, a compressed gas (air or nitrogen) is bubbled freely into the seawater from a submerged fixed end of a tube. The pressure in the entire length of the tube, irrespective of the elevation of its other end, is approximately equal to the pressure head of the seawater column over the bubbler's orifice. The gas pressure at the landward end of the tube is measured by a differential pressure transducer, which responds to the difference between the system pressure and the atmospheric pressure, so that only the water head pressure is recorded. Here also, translation of measured water pressure to the height of the water column above a clearly defined datum (i.e., bubbling point) is performed by a microprocessor based system. The depth-mean value of water density at the measurement site is fixed for an entire season, based on some typical values of the density profiles measured from the said site over a day. This system
also suffers from inaccuracies in sea level measurements, arising from semi¬diurnal variations in the fixed value of the water density.
In yet another pressure based water level measurement system, R. Spencer, P. Foden, and J.M. Vassie [in "Development of a Multi-year Deep Sea Bottom Pressure Recorder" in Electron. Eng. Oceanogr, EEO-394, pp. 175-180 (1994)] and R. Spencer and P. Foden [in "Data from the Deep Ocean via Releasable Data Capsules" in Sea Technol., Vol. 37(2), pp.10-14 (1996)] describe a self-contained instrument package, which records the sea level using a precision quartz pressure transducer mounted on a frame, which rests on the seabed. A data logger stores hydrostatic pressure and temperature on the seabed and also transfers the data to four buoyant releasable data capsules (RDCs), using a micro-controller and infrared links. The RDCs can be periodically released, and can be recovered by ship or the data can be transmitted via satellite. The ability of this system to achieve time-series sea level measurements from the deep ocean is unique. Measurements being made in the open sea, depth-mean water-density variations may not be as severe as in coastal or estuarine waters.
In a recent development, A. Joseph, E.S. Desa, R.G. P. Desai, V. Kumar, E. Desa, and V.B. Peshwe [in "Development of a Sea Level Recorder for Measurements at the Harbours and Jetties" in Proceedings of the International Conference on Trends in Industrial Measurements & Automation, pp.205-213 (1999)] describe a cable-driven pressure-based water level recorder. The water level sensor is a temperature-compensated user-programmable quartz differential pressure sensor, which provides time-averaged water pressure in RS-232 format. The pressure inlet attaches to a
parallel-plate geometry to abate measurement inaccuracies arising from the influences of flows, waves, and a combination of flows and waves as shown by A. Joseph, J.A.E. Desa, E. Desa, P. Foden, and K. Taylor [in "Flow Effects on a Pressure Sensor" in Proceedings of Ocean Electronics (Sympol-'95), pp-11-16 (1995)], and by A. Joseph, J.A.E. Desa, E. Desa, J. McKeown, and V.B. Peshwe [in "Wave Effects on a Pressure Sensor" in Proceedings of the International Conference in Ocean Engineering (ICOE'96), pp. 568-572 (1996)], and further by A. Joseph, J.A.E. Desa, P. Foden, K. Taylor, J. McKeown, and E. Desa [in "Evaluation and Performance Enhancement of a Pressure Transducer Under Flows, Waves, and a Combination of Flows and Waves" in J. Atmos. Oceanic Techno!., Vol. 17(3), pp.357-365 (2000)]. This gauge records pressure and displays sea level at programmed intervals. Translation of measured water pressure to water level is accomplished using depth-mean water density value, which is obtained from vertical density profiles measured from the vicinity of the gauge at approximately weekly intervals, using a portable CTD profiler [as reported by A. Joseph, V.B. Peshwe, V. Kumar, E.S. Desa and E. Desa in "Effects of Water Trapping and Temperature Gradient in a NGWLMS Gauge Deployed in Zuari Estuary, Goa" in Proceedings of the Symposium on Ocean Electronics (Sympol-'97), pp. 11-16(1997)].
Hitherto known pressure gauge/bubbler gauge devices used by various researchers in several parts of the world have the objects of measurement of water levels or their oscillations in bore-wells, canals, estuaries, fjords, the seas, and so forth. The value of the density of the water column above the pressure transducer is usually assumed to be invariant, or
that obtained at a site measurement is uniformly applied to all the measured water pressure values to estimate the water level or its oscillations. However, the depth-mean density of water in many regions (e.g., estuaries, fjords, and coastal areas) undergoes significant changes with seasons as in Fig.1. Previous studies [by A. Joseph, E. Desa, E.S. Desa, D. Smith, V.B. Peshwe, V. Kumar, and J.A.E. Desa in "Evaluation of Pressure Transducers under Turbid Natural Waters", J. Atmos. Oceanic Technol., Vol. 16(8), pp.1150-1155 (1999)] indicate water level anomalies as large as 35 cm in water level estimates arising from an uncertainty in the effective depth-mean density of suspended-sediment-laden waters in the Hugli Estuary, Bay of Bengal, India. The observed errors are of a sufficiently serious nature to merit attention, as autonomous correction for this drawback has not been incorporated in a working device.
Thus, the drawback of hitherto known pressure based water level gauges is the introduction of an error in water level measurements as a consequence of an uncertainty in the effective depth-mean density of the water column above the pressure sensing point.
The main object of the present invention is to provide a self- correcting water level gauge useful in clear and turbid waters, which obviates the drawbacks as detailed above.
Another object of the present invention is to provide a means to measure the depth-mean density of a vertical water column which will be useful for translation of water pressure to water level.
Yet another object of the present invention is to provide a means to obtain a time-series of water density values over a given height of water column for several studies related to oceanography and meteorology.
The novelty and inventive steps of a self-correcting pressure based water level gauge useful in clear and turbid waters of the present invention provides for autonomous measurement of water density, together with water pressure, thereby providing an additional essential means required for translation of water pressure to water level. The system of the present invention allows real-time, time-series measurement of effective depth-mean density of a known vertical length of water column — especially important in regions such as estuaries where rapid temporal changes are encountered — which is hitherto not available in known water level gauges. The system of the present invention allows accurate measurements of water levels in clear and turbid waters. In the drawings accompanying this specification:
Fig. 1 shows the water density profiles in the Zuari Estuary (Goa) as measured by a density profiler for (a) monsoon season, (b) non-monsoon season during 1997-1998.
Fig. 2 illustrates one of the typical mounting schemes of a conventional pressure based gauge for water level measurements in shallow waters, which comprises primarily of an absolute or differential pressure transducer  with or without the logger electronics enclosed, and having a pressure inlet  which is open to water pressure. Another pressure inlet , which is present only in the case of a differential pressure transducer, is vented to atmospheric
pressure via a flexible hose  and an atmospheric pressure chamber  to compensate for the inverted barometric effect.
Fig. 3 shows the self-correcting pressure based water level gauge useful in clear and turbid waters, of the present invention consisting of at least two pressure transducers  and  mounted at an appropriate vertical separation, 'H1; each being vented to an atmospheric pressure chamber  via flexible hoses [9, 10] to compensate for the inverted barometric effect. The pressure signals from the transducers are taken to an electronic interface module  comprising of a multiplexer , control, computing and data storage means  which is interfaced to a display module  for visual inspection of data, and a laptop computer  which is used to initialize the gauge, and also as a data readout unit.
Fig. 4. shows a typical trace of the water density variations estimated by the present dual pressure transducer based sea level gauge deployed in the Zuari estuary, Goa together with the corresponding sea level oscillations measured by the said gauge.
Accordingly, the present invention provides for a self-correcting pressure based water level gauge useful in clear and turbid waters
which comprises at least two pressure transducers  and  separated by
an appropriate known vertical distance characterised in that the said transduces interconnected to an
electronic interface module  consisting of a multiplexer , a control,
computing and data storage  the said storage (13) is interface
to a display module (14) and a laptop computer (15) , horizontal parallel and plates (16,17)
bring provided below the said transducer (9).
In an embodiment of the present invention, the positive pressure inlet of each transducer is maintained at the center of, and flush with the horizontal plane of an end-plate , reducing thereby the undesired flow-induced and wave-induced dynamic pressure component in the measurement of the desired static water pressure.
In another embodiment of the present invention, the said end plate is selected from copper or copper alloy to inhibit its bio-fouling during prolonged exposure to euphotic waters.
In yet another embodiment of the present invention, the end plate is given a circular shape, thereby achieving good horizontal azimuthal response in presence of continuously changing directions of water flows and waves, which is often the case in sea waters.
In still another embodiment of the present invention, the periphery of the end plate is rounded or shampered, thereby minimizing the formation and shedding of flow-induced and wave-induced wakes which might adversely affect the measurement of static pressure at the pressure inlet.
In a further embodiment of the present invention, the diameter of the said end-plate is selected to be 3 times or more that of the transducer housing, thereby protecting the pressure inlet from the wake-field generated by the pressure transducer housing.
In a still further embodiment of the present invention, an additional plate of similar material and dimensions is attached to the said end-plate by means of three or more cylindrical stand-offs , the said additional plate being separated from the first plate by a distance approximately equal to the diameter of the transducer housing. Such a horizontal parallel-plate front-end device at the pressure inlet significantly reduces the undesired dynamic pressure in the measurements of the desired static seawater pressure in the presence of a combination of waves and flows [as in A. Joseph, J.A.E. Desa, P. Foden, K. Taylor, J. McKeown, and E. Desa in "Evaluation and Performance Enhancement of a Pressure Transducer Under Flows, Waves, and a Combination of Flows and Waves" in J. Atmos. Oceanic Technol., Vol. 17(3), pp.357-365 (2000)].
When two pressure transducers are deployed with a vertical separation 'H1 between their pressure inlets, the value of the effective depth-density pH of the water column of height 'H1 can be estimated, after each sampling, from the measured pressures PI (from the lower pressure transducer ) and Pu (from the upper pressure transducer ) using the relation:
where pH , PI , Pu , H and g (acceleration due to gravity) are all expressed in
the same system of units. Thus, simultaneous measurement of time-averaged
i pressures PI and Pu from two submerged pressure transducers deployed at a
precisely known vertical separation 'H' provides a means of estimating the
value of PH during each sampling. This enables measurement of sea levels
with an enhanced precision, from both clear and turbid waters. However, in situations where appreciable density stratification exists, this vertical
separation would need to be as large as is practical. In our measurements over a year in the Zuari Estuary (Goa), use of a 1-meter vertical separation 'H1 between the transducers  and  gave an estimate of PH which deviated less than 0.3% from the density of the total water column above . Tide-induced oscillations in depth-mean water density PH estimated by the device
of the present invention are shown in Fig.4.
The main advantages of the present invention are:
1. It provides a self-correcting water level gauge with the use of two or more
2. It provides an additional water density parameter as a bonus.
3. It can be used even in turbid waters where single pressure transducer
based water level gauges begin to produce large errors in water level
1. A self-correcting pressure based water level gauge useful in clear and turbid waters
which comprises at least two pressure transducers  and  separated by an
appropriate known vertical distance, characterized in that the said transducers
interconnected to an electronic interface module  consisting of a multiplexer
, a control, computing and data storage  ,the said a control, computing and
data storage  is interfaced to a display module (14) and a laptop computer
(15), horizontal parallel end plates (16,17) being provided below the said
2. A self-correcting pressure based water level gauge as claimed in claims 1 wherein
horizontal parallel plates having diameter preferably 3 times than transducers
3. A self-correcting pressure based water level gauge useful in clear and turbid waters
substantially as herein described with reference to Fig. 3 of the drawings
accompanying this specification.
|Indian Patent Application Number||488/DEL/1999|
|PG Journal Number||13/2008|
|Date of Filing||31-Mar-1999|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI-110001, INIDA.|
|PCT International Classification Number||G01F 23/ 14|
|PCT International Application Number||N/A|
|PCT International Filing date|