AUTONOMOUS WATER LEVEL RECORDER

                                KEY FEATURES
 
                                  * Battery operated data logger
                                  * Data retrieval on laptop computer
                                  * Transmission by VHF radio modem (optional)
                                  * Visual readout

                                APPLICATIONS

                                  * Forecasting of tidal level and preparation of tide tables.
                                  * Hydro-meteorological and irrigation applications.
                                  * Hydrographic survey, dredging & reclamation,
                                    shipping and coastal & harbour developement.
                                  * To work out flushing rates of estuaries.
                                  * To monitor storm-surge events.
                                  * For detection of slow changes in mean sea level.


SPECIFICATIONS

Power     Measurement   Range  Accuracy

12 volts  Pressure      0-20   0.01% 
battery   transducer      &      &
                        0-70   0.2%

Water level detection is by pressure transducer. The recorder works on 12 volts battery power. Two types of pressure transducers are used namely; strain guage and quartz. Water level measurements in the range 0-20m and 0-70m are possible. The factory calibrated accuracies (i.e., under quiescent conditions) of strain guage and quatrz transducers are 0.2% and 0.01% respectively. These accuracies are valid under quiescent conditions only. The accuracy of any pressure transducer deployed in riverine, estuarine or oceanic conditions are deteriorated by site related factors such as flows, waves, and suspended sediments (commercial manufacturers quote the quiescent accuracy of the transducer only, and do not generally mention about the deterioration in accuracy due to the site related factors mentioned above). Detailed experiments by us in flow-channel, wave-flume, and suspended sediment-laden natural waters revealed that the accuracy obtained in the field can be improved by the use of a suitable front end mechanism at the pressure inlet. The attainable accuracy in the presence of laminar flow for a protuding pressure inlet is 0.34% of full-scale (F.S.). This can be enhanced to 0.12% of F.S. with the use of a large thin flat plate at the pressure inlet. For turbulent flows, the accuracy can be improved to 0.08% of F.S. with the use of perforated sheet or a pair of parallel horizontal plates. The latter is especially advantageous in the case of waves riding in flows than solely for flows or for waves. In turbid natural waters the use of a single pressure transducer gives rise to under-estimation of tidal range. This under-estimation depends on the suspended sediment concentration. The reason for this behaviour of pressure transducer based water level measurement is that in such waters the depth-mean effective density of fluid which contributes to the pressure sensed by a pressure transducer is less than the density of water free from suspended sediments. Automatic density compensation can be obtained with the use of two pressure transducers mounted at a known vertical distance between them.

In the autonomous water level recorders designed at NIO these fluid dynamic considerations have been taken into account. These are new findings, and are not available with commercial manufacturers at present.