Title of Invention

"AN AUGMENTED ERROR-CORRECTING SEA LEVEL RECORDER"

Abstract

An augmented error-correcting sea level recorder An augmented error-correcting sea level recorder, as shown in figure 2 which comprises a self-calibrating air-acoustic gauge consisting of an acoustic transducer [1] located at the top of a sounding tube [2] and at a fixed elevation K from the chart datum (CD), characterized in that with two differential pressure transducers [8] and [9] being located below the end of the sounding tube [2] at vertical separations S1 and S2 respectively below the acoustic transducer head and each pressure transducer being vented to an atmospheric pressure chamber [13] to compensate for the atmospheric pressure variations and all the three transducers being accommodated within a protective well [4] provided with an array of perforations [10] on the entire surface of the protective well, the said sounding tube having a calibration hole [3] at a preset vertical distance L from the acoustic transducer [1]; the output data streams from all the above said transducers [1], [8],and [9] connected to known electronic interface module [14] consisting of a known data retrieval, display and storage means such as microcontroller based computing and data storage [15] having a display module[16] for data display, and a computer [17] for data retrieval.

Full Text

The present invention relates to an augmented error correcting sea level recorder. The present invention particularly relates to an improvement in the modern air-acoustic sea level gauges allowing for the automated correction of error in sea level measurement arising from temperature-gradient-induced anomalies and trapping of lower-density water in the protective well of the said air-acoustic gauge. Hitherto known air-acoustic gauges as described by P.Y. Dupuy [in "The SHOM Ultrasonic Tide Gauge" in Intergovernmental Oceanographic Commission Workshop Report #81 pp. 8-12 (1992)] and an improved self-calibrating air- acoustic gauge (Fig. 1) later developed by National Oceanic and Atmospheric Administration (NOAA), with the purpose of accurate sea level measurement to capture the weak climate signal relating to global warming effects have the drawbacks of site-related effects arising from:- 1. Temperature gradient in its sounding tube [as first reported by J.M. Vassie, P.L. Woodworth, D.E. Smith, and R. Spencer in "Comparison of NGWLMS, Bubbler and Float Gauges at Holyhead" in Joint IAPSO-IOC Workshop on Sea Level Measurements and Quality Control (IOC Workshop Report #81), pp. 40-51 (1992)]. 2. Trapping of lower density water in the protective well, which surrounds the sounding tube of the air-acoustic gauge [as described by A. Joseph in "Modern Techniques of Sea Level Measurement" in Encyclopedia of Microcomputers, Vol. 23 (Supplement 2), pp. 319-344 (1999)]. An air-acoustic gauge estimates sea level by measuring the time elapsed between transmission and reception of a series of acoustic pulses that travel from the transducer head, through an air column, in a vertically mounted sounding tube down to the surface of water (as in Fig. 1). The elapsed time is converted to distance using the velocity of sound (C) in the air column within the sounding tube. The sea level height (H) is obtained by subtracting the measured air-column height (D) from the fixed elevation (K) of the acoustic transducer head above the chart datum (CD). It is well known that C is a strongly dependent function of the air temperature profile within the sounding tube. The said self-calibrating air-acoustic gauge computes an estimate for the value of C from the first echo partially reflected from a small (~ 4mm) air¬hole (known as calibration reference) drilled on the sounding tube at a distance of approximately 1 meter below the acoustic transducer head. The computed C is assumed to be valid for the entire length of the air column within the sounding tube, and is then used in the final estimation of the value of H. This leaves a large portion of the air column within the sounding tube uncompensated for the temperature gradient effect and thereby constitutes a major drawback, namely a temperature-gradient-induced error associated with the sea level measurement using an air-acoustic gauge. Various approaches have been adopted to overcome this drawback as in 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)], wherein a chain of temperature probes has been used to measure the in-situ air-temperature profile within the protective well of the air-acoustic gauge, thereby deriving the sound velocity profile. However, this approach was able to correct for the error in sea level measurement to within 32% only, and might vary in an unknown manner from site to site. In another approach, A.G. Pathak and G.A. Ramadass have claimed in their patent [patent pending No. 1443 / MAS / 97], a modified sounding tube having two or more spatially separated side-branches, of differing lengths fixed to the wall of the sounding tube, where each of the properly tuned resonating branches responds to a specific frequency such that the sound pulse with the appropriate center frequency is predominantly reflected by the said resonating branch, to estimate the effective velocity of sound for different portions of the sounding tube. However, because the spatial separation between the said resonant cavities progressively increases from the acoustic head, a complete correction to the temperature gradient effect in the sounding tube becomes practically difficult. The sounding tube and the protective well of the hitherto known air-acoustic gauges are basically pressure devices wherein the water pressures at the orifice level of the well, exerted by the water heads inside and outside of the well, must balance. Thus, if p1and p2 are the depth-mean densities of the water columns, bounded between the orifice level and the sea surfaces inside and outside of the well respectively, and h1and h2 are the corresponding heights of these water columns above the orifice level, (1) so that h, = h2(p2/p1) (2) Thus, the generally reduced value of p, (i.e., p, The main object of the present invention is to provide an augmented error-correcting sea level recorder that obviates the drawback of temperature-gradient-induced anomalies in the sounding tube of an air-acoustic gauge. Another object of the present invention is to provide an augmented error-correcting sea level recorder that minimizes the error in sea level measurements arising from trapping of lower density water in the sounding tube of an air-acoustic gauge. Yet another object of the present invention is to provide an augmented error-correcting sea level recorder that minimizes the error in sea level measurements, which arises from non-availability of in-situ depth-mean density of the entire water column above a given datum during a given sampling period. A still another object of the present invention is to provide a means to obtain a time-series of water density values, over a given vertical length of water column, which is useful for several studies related to oceanography and meteorology. The novelty and inventive steps of the present invention provides for an augmented error-correcting sea level recorder by improving a conventional air-acoustic gauge by incorporating a dual-pressure sensor based gauge, which yields an improvement in accuracy in relation to the accuracy that can be achieved from either of these two devices, thereby providing a better means for measurement of sea level which is hitherto not available with existing sea level measurement systems. This augmentation takes care of the drawbacks, i.e., temperature gradient and the lower density water trapping effects in a guided-air-acoustic gauge, by making use of the fact that the vertical separations (namely S1 and S2) between the acoustic transducer head in air and the pressure inlet of the submerged upper and lower transducers (as shown in Fig. 2) are invariant quantities for all times and seasons, for a given measurement site. The use of the invariant quantities S, and S2 provides an additional means to characterize the density stratification in the water column as described in our previously filed patent by A. Joseph, E.S. Desa, V. Kumar, V.B. Peshwe, and E. Desa (1999) on " A self-correcting pressure based water level gauge useful in clear and turbid waters [488/DEL/99]." In the drawings accompanying this specification: Fig.1 represents a self-calibrating guided-air-acoustic gauge (i.e., the acoustic pulse is guided by means of a sounding), which consists of an acoustic transducer [1] located at the top a sounding tube [2] having a calibration hole [3] at a known distance L from the acoustic transducer [1]. The said sounding tube is surrounded by a protective well [4] having an orifice [5], and a parallel plate assembly [6] at its wet end. The transducer [1] is located at a fixed elevation K from the chart datum (CD), and a variable elevation D from the varying sea level (SL). The data is recorded in its shore unit [7]. Fig.2 represents the augmented error-correcting sea level recorder in which two pressure transducers [8] and [9] are located below the end of the sounding tube [2] and within the protective well [4], wherein the parallel plate assembly [6] of Fig.1 is replaced by an array of perforations [10] provided on the entire surface of the protective well. The vertical separations S, and S2, between the acoustic transducer head, which is located in air, and the pressure inlets [11] and [12] respectively of the submerged pressure transducers, are invariant quantities for all times and seasons for a given configuration. The pressure transducers [8] and [9] are each vented to an atmospheric pressure chamber [13] to compensate for the time-dependent atmospheric pressure variations. This chamber also houses the electrical connectors of the two pressure transducers. The output data streams from the acoustic transducer [1], and the pressure transducers [8, 9] are connected to an electronic interface module [14], which comprises of a micro-controller based data acquisition system [15] which has a display module [16] for visual inspection of data, and a laptop computer [17] for data retrieval. Fig. 3 represents the sea level difference obtained from a self-calibrating air acoustic gauge of Fig.1, and a pressure transducer at the same site. The indicated temperature-gradient along the protective well of the air-acoustic gauge was measured by us using a chain made up of five spatially separated temperature probes [as described 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)], where tL and tD represent the depth-mean air temperatures along the lengths L and D respectively of Fig.1, and AH and PH represent the simultaneous sea level measurements obtained from the air-acoustic gauge and a pressure gauge [as described by 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", Proceedings of the International Conference in Trends in Industrial Measurements and Automation, pp. 205-214 (1999)] respectively. The diagram depicted in Fig. 3 highlights the strong correlation between the temperature-gradient in the air-acoustic gauge and the error in sea level measurements made in the Zuari Estuary, (Goa), as described in detail 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)], which supports similar findings from another sea level station in a different continent, namely the Holyhead Sea Level Station in the United Kingdom, as reported by J.M. Vassie, P.L. Woodworth, D.E. Smith, and R. Spencer in "Comparison of NGWLMS, Bubbler and Float Gauges at Holyhead" in Joint IAPSO-IOC Workshop on Sea Level Measurements and Quality Control,(IOC Workshop Report #81), pp 40-51 (1992)]. Accordingly, the present invention provides an augmented error-correcting sea level recorder, as shown in figure 2 which comprises a self-calibrating air-acoustic gauge consisting of an acoustic transducer [1] located at the top of a sounding tube [2] and at a fixed elevation K from the chart datum (CD), characterized in that with two differential pressure transducers [8] and [9] being located below the end of the sounding tube [2] at vertical separations S1 and S2 respectively below the acoustic transducer head and each pressure transducer being vented to an atmospheric pressure chamber [13] to compensate for the atmospheric pressure variations and all the three transducers being accommodated within a protective well [4] provided with an array of perforations [10] on the entire surface of the protective well, the said sounding tube having a calibration hole [3] at a preset vertical distance L from the acoustic transducer [1]; the output data streams from all the above said transducers [1], [8],and [9] connected to known electronic interface module [14] consisting of a known microcontroller based computing and data storage [15] having a display module[16] for data display, and a computer [17] for data retrieval. In an embodiment of the present invention, the protective well of the air acoustic gauge is provided with an array of perforations on the entire surface of its wall wherein the said perforations on the submerged portion of the wall serves the dual purpose of minimizing (1) the Bernoulli dynamic pressure effects as in [B. Lebreton, H. Dolou, C. Batany, and J.C. Kerinec, in "Evaluation Des Maregraphes Suber En Presence De Courants Forts , Suite: Evaluation d'un Attenuateur De Depression" in Ann. Hydrogr., 17, 37- 43, (1991)] and [A. Joseph, P. Foden, and K. Taylor in "An Experimental Evaluation of Flow-Induced Errors in a Pressure Transducer and Some Design Solutions to Improve its Performance in a Flow Field", Internal Document No. 82, Proudman Oceanographic Laboratory, (1995)] and (2) removing the lower-density water trapping effect as in our patent pending No. 2378/DEL/98 entitled "An improved stilling-well device useful for enabling error-free sea level measurements"; the said perforations on the exposed portion of the said protective well serves the purpose of ventilating the air vertical distance L from the acoustic transducer [1]; the output data streams from all the above said transducers [1], [8],and [9] connected to known electronic interface module [14] consisting of a known microcontroller based computing and data storage [15] having a display module[16] for data display, and a computer [17] for data retrieval. In an embodiment of the present invention, the protective well of the air acoustic gauge is provided with an array of perforations on the entire surface of its wall wherein the said perforations on the submerged portion of the wall serves the dual purpose of minimizing (1) the Bernoulli dynamic pressure effects as in [B. Lebreton, H. Dolou, C. Batany, and J.C. Kerinec, in "Evaluation Des Maregraphes Suber En Presence De Courants Forts , Suite: Evaluation dun Attenuateur De Depression" in Ann. Hydrogr., 17, 37- 43, (1991)] and [A. Joseph, P. Foden, and K. Taylor in "An Experimental Evaluation of Flow-Induced Errors in a Pressure Transducer and Some Design Solutions to Improve its Performance in a Flow Field", Internal Document No. 82, Proudman Oceanographic Laboratory, (1995)] and (2) removing the lower-density water trapping effect as in our patent pending No. 2378/DEL/98 entitled "An improved stilling-well device useful for enabling error-free sea level measurements"; the said perforations on the exposed portion of the said protective well serves the purpose of ventilating the air column surrounding the sounding tube thereby assisting the reduction of the undesirable temperature-gradient effect within the sounding tube of the air-acoustic gauge. In another embodiment of the present invention, the perforated well may be selected either from copper or copper alloy, or from a non-metal wherein the perforations on the said protective well may be given a copper lining to prevent closure of the perforations by bio-fouling process during prolonged deployments. The conventional air acoustic gauge of Fig. 1 as described earlier, measures the total air column height (D) in the sounding tube by timed reflections of acoustic pulses, and by conversion of D to sea level H through the fixed elevation K using the relation :- H = K-D (3) The augmented sea level gauge represented in Fig. 2 uses the invariant properties of the separations S, and S2, and the corrected depths Hu and HI of the upper and the lower transducers respectively, with reference to the instantaneous sea level (SL), to compute two independent estimates of the total air column height (Da), and thereby two independent estimates of the sea levelH1 and H2, with reference to the chart datum (CD), given by :- H, = K-^-HJ (4) H2 = K-(S2-H, ) (5) The algorithm used in the micro-controller of the augmented sea level recorder computes three indepencent estimates of the sea level; namely, one value H from the acoustic transducer in air, and two values H1 and H2 from the two pressure transducers in water. The deviations of H1 and H2 from H are expressed respectively as :- E, = ( H - H, ) (6) E2 = (H - H2) (7) The deviations E, and E2 computed by the augmented gauge contain, in essence, the combined error contributed by the temperature-gradient in the acoustic tube, and the trapping of lower-density water in the sounding tube of the acoustic gauge. There are three examples where errors occur - namely : (1) If the difference of E1 and E2 is equal to zero i.e., E1 ~ E2 = 0, then all three estimates are equal, and the gauge needs no correction. Such a condition occurs in Fig. 3 where the sea level difference between a pressure sensor and an air acoustic gauge is minimum (~0 cm) at midnight, and again at noon time (~0 cm ) for the site at the Zuari Estuary in Goa. (2) If E1 ~ E2 > 0, then the pressure transducer measurements H1 and H2 imply that the dominant error is arising from the temperature non-uniformity in the air tube of the acoustic gauge, and hence the measured H needs a correction value equal to E1 or E2. Such an example is shown in Fig. 3 where the sea level difference shows an overshoot peak to approximately 3 cm at 3 A.M in the early morning, (3) If E1 ~ E2 under-shoot to approximately (-5 cm) by 9 A.M in the morning, reversing its trend to zero difference by noon. The algorithm used in the augmented sea level recorder examines the three conditions given above, so that the error associated with the air acoustic gauge measurement is computed and stored together with the raw uncorrected sea level estimate, and the time of day. The nature of the temperature non-uniformity and the lower-density water trapping in the protective well are very difficult to assess, to remove, or to predict as these effects may vary in intensity with the seasons, time of day, and the site characteristics. The approach adopted in the present invention operates within the operational deficiencies of present-day air-acoustic gauge installations around the world, and may, by this advantage, have widespread applicability. The main advantages of the present invention are: 1. It provides for an augmented sea level recorder having an air-acoustic gauge and a dual pressure transducer gauge system, the data from these systems together enable estimation of the combined errors associated with temperature gradient and trapping of lower-density water in the protective wells of these gauges. 2. The perforations on the wet end the exposed portions of the protective well together assist the mixing of waters of different density inside and outside of the protective well, and help ventilate the air column around the sounding tube so as to minimize the temperature-gradient effect in the well. 3. The augmented error-correcting sea level recorder described here is suited to the objectives and goals of all Tide Gauge Reference Stations in the GLOSS (Global Sea Level Observing System) network around the world — some 1000 stations where high quality sea level data with error reporting features is necessary to recover the weak climate-related signal in the mean sea level variation. 4. It provides for an additional means to characterize the density stratification in the water column, which is useful for studies related to many oceanographic processes. We Claim: 1. An augmented error-correcting sea level recorder, as shown in figure 2 which comprises a self-calibrating air-acoustic gauge consisting of an acoustic transducer [1] located at the top of a sounding tube [2] and at a fixed elevation K from the chart datum (CD), characterized in that with two differential pressure transducers [8] and [9] being located below the end of the sounding tube [2] at vertical separations S1 and S2 respectively below the acoustic transducer head and each pressure transducer being vented to an atmospheric pressure chamber [13] to compensate for the atmospheric pressure variations and all the three transducers being accommodated within a protective well [4] provided with an array of perforations [10] on the entire surface of the protective well, the said sounding tube having a calibration hole [3] at a preset vertical distance L from the acoustic transducer [1]; the output data streams from all the above said transducers [1], [8],and [9] connected to known electronic interface module [14] consisting of known microcontroller based computing and data storage [15] having a display module[16] for data display, and a computer [17] for data retrieval. 2. An augmented error-correcting sea level recorder substantially as herein described with reference to the examples and drawings accompanying this specification.

Documents:

489-del-1999-abstract.pdf

489-del-1999-claims.pdf

489-del-1999-correspondence-others.pdf

489-del-1999-correspondence-po.pdf

489-del-1999-description (complete).pdf

489-del-1999-drawings.pdf

489-del-1999-form-1.pdf

489-del-1999-form-19.pdf

489-del-1999-form-2.pdf

489-del-1999-form-4.pdf

489-del-1999-form-5.pdf