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

Sea Level Measurement and Interpretationremovable orifice of bronze and the orifice can beadjusted with one or more nipples. Some of the tidalstations in Norway are exposed to ice and low temperatures,and 220 V AC heating cables are installed insidethese wells.Each sea level gauge has at least one level switchused for quality control. It is installed inside the stillingwell approximately at mean sea level (MSL). The levelswitch is a tiny float, which switches a current loopwhen the sea level passes the level at which the switchis mounted. The computer registers the time and sealevel when the switch turns on or off, and these datacan be compared with the level at which the switch ismounted. The level switch has been very important inthe detection of several problems, such as drift (trend)in the observations.An encoder with an SDI-12 output is installed abovethe tube, mounted on a concrete block (Figure 2). Theencoder has a sprocket (wheel) on the shaft and is programmedto give an output with a resolution of 0.1cm. The encoder has internal battery backup whichremembers the angular position, in case of powerfailure. A chain with a float and a counterweight runsover the sprocket and the sea level is given by theangular position of the sprocket. It is important thatthe sprocket and chain fit well together. If not, therecould be very small tangential movements betweenthe sprocket and chain and this in turn presents itselfas very slow drift (trend) in the sea level observations.NHS is using a US manufacturer of the chainand sprocket (W.M.Berg Inc. (www.wmberg.com)).Leading marks on the chain and sprocket make it easyto control the system.The data logger is a Sutron 8210 and it collects andstores sea level, barometric pressure and switch-leveldata. The memory of the Sutron 8210 has batterybackup and can hold several months of data.Normally the data are retrieved twice a day from thedata logger, sometimes more often. The data loggerhas a serial port, which is used for communication,either through the ISDN-network or with a GPRS-router.We are now in the process of converting the datatransmission from the ISDN-network to GPRS-routerfor all the tidal stations. This work will be completedduring the next 2–3 years (depending on allocatedbudget) and sea level data will be sent to the office inStavanger every 10–20 min and will be immediately(an automatic quality control is applied to the data)available on our website.The data logger and the communication unit have separatebattery backups. The data logger has less powerconsumption than the communication unit, so, in caseof power failure, the gauge will continue to store dataeven if the communication is broken.ChainContact PointEncoderFigure 2. Encoder, chain and contact point.Levelling is done from the TGBM and one or twoadditional benchmarks to a contact point at thesame bracket as the encoder. The TGBM is in solidrock as close to the sea level gauge as possible.After modernization of the gauges between 1985and 1991, levelling has been done every year. Sincemost of the gauges are located on stable ground,the levelling interval now is three years, except for afew gauges which are sinking. The levelling followsthe procedures outlined in the UNESCO <strong>Manual</strong> onSea-Level Measurement and Interpretation, Volume I,and the accuracy is millimetric. It is more difficult tocalibrate the sea level gauge to the same accuracy.Calibration is done by measuring the distance fromthe sea surface to the contact point with a levellingstaff inside the well, and by taking simultaneouslyreadings on the sea level gauge. This is repeatedseveral times, and the sea level gauge is calibrated bymaking its readings equal to the observations on thestaff. Taking many readings will reduce the problemwith a moving sea surface. There might, however, besome individual differences in the way the calibrationis performed and this may introduce systematicerrors of a few millimetres. We use a sea level gaugezero that is below the lowest observed sea level. Toavoid any confusion, we never use levels like ChartDatum or Land Survey Datum as the gauge zero.The sea level gauges are inspected at intervals of 18months. We should like to carry out levelling andinspections more frequently, but we have to reducethe operational costs as much as possible.For all stations, a close co-operation with localoperators is essential. They look after the equipmentand assist us in various situations, i.e. whenthere are power or communication problems withthe gauge.IOC <strong>Manual</strong>s and Guides No 14 vol IV59