Manual on sea level measurement and ... - unesdoc - Unesco

Sea Level Measurement and InterpretationPressure Transducers in Stilling WellsEquipment Pressure transducers are often placed in stilling wells, where these are available. This provides a protected and secureenvironment for the sensors and can augment measurements made by a float gauge. The comments above on pressuresensors are equally valid for this type of installation.Accuracy Approximately 1 cm of sea level. The absolute accuracy may be limited by the characteristics of the stilling well.Cost Band 2.Comments Problems associated with the use of stilling wells are well documented. (see Float Gauges).Bubbler Pressure GaugesEquipment Complete systems are available commercially, but considerable assembly work is required to construct an operational tidegauge. The equipment comprises an air supply (normally from a compressor), a gas control system, a connecting pipe, asub-sea pressure outlet, a pressure transducer at the landward end and the various data logging and support electronics.Operation The instrument supplies air from the high pressure supply at a reduced pressure and at a constant rate through the system.The pressure required to bubble the air through the sub-sea outlet at this rate is a measure of the sea level above theoutlet. A differential pressure transducer vented to the atmosphere alleviates the need to measure atmospheric pressureseparately, thereby producing a pressure reading proportional to sea level height. The sub-sea outlet is open at the base,has a large surface area relative to its volume and has a small exit port approximately half way from the base. This designreduces the effect of wave action and provides a very stable datum.Installation The outlet and part of the connecting tube are the only components in the sea. Such a configuration increases the reliabilityof the system and makes replacement relatively simple. All other components of the system are housed nearby.RequirementsThe system requires external power for continuous operation, and backup operation is relatively limited, owing to thelimited air supply.Location Bubbler systems can be used at virtually any location, even on shelving beaches. Connecting tubes can be several hundredmetres in length. As with most pressure measuring systems, regions with large variations in seawater density may cause significanterrors.Calibration Calibration is concerned with the pressure sensor accuracy and may need to be repeated at intervals. Calibrations suppliedby leading pressure transducer manufacturers are acceptable provided occasional means of fixing the datum valueare used.Accuracy In general, an average accuracy of 1 cm of sea level is achievable, but this may degrade under large-wave conditions.Cost Band 2.Record of Use Used extensively in the United States and the United Kingdom for their national tide gauge networks.CommentsAt a few locations, a secondary bubbler system has been installed at the mid-tide level as part of the United Kingdomnetwork. This can be used to fix the datum of measurements in the same fashion as the ‘B’ gauges discussed above.26IOC <strong>Manual</strong>s and Guides No 14 vol IV