Introduction and History

In November 1990 the permanent system was installed by diamond drilling an inclined hole through the coastal bedrock to locate and protect a new seawater transducer. The 12.8 m long drill hole is lined with 32 mm ID low density polythene pipe with a stainless steel cage at the bottom which locates the pressure transducer 8.04 m below the Cape Roberts tide gauge bench mark (CRTGBM #1). At start up the system consisted of a Campbell CR10 Data logger (S/N 0004172) and Geokon vibrating wire transducer (25 psi, 4500 ALV, S/N 13131). On 27 November 1994 at 1700 NZST the replacement transducer (25 psi, 4500 ALV, S/N 14740) became operative with a different CR10 data logger and this equipment continued to function until refurbishment in 2002.

Refurbishment

The Geokon 4500 ALV vibrating wire transducer SN 14740 was melted out of the polythene casing using a specially designed miniature hot water drill. This drill melted out the transducer in less than 2 hours and now provides a quick and simple method to replace a failed frozen transducer in the future. The transducer was checked for signs of corrosion and leakage, and returned to position in the cage at the bottom of the casing. The vented cabling that transmits the signal about 40 metres to the data recording instrumentation was re terminated in a sealed connection box and wire wrapped to the catenary cable for long term security.

A new instrument package with Campbell CR10X data logger, 4 MB storage module, Vaisala digital barometer, batteries and solar panels were installed in a single blue polyethylene enclosure (Space Case 0944). The housing is mounted on an aluminium frame bolted to the local gneiss basement rock with the Solar panels inclined and facing northwards.

Programming

The Vibrating wire transducer is plucked and read every 10 seconds and an average value (from 60 readings) calculated every 5 minutes and recorded at the beginning of the next 5-minute interval. Six Barometric pressure readings are made between 2 and 3 minutes in the 5-minute interval and averaged and recorded with the tide value at the beginning of the next 5-minute interval. Time is recorded in UTC.

The transducer pressure value is converted to a seawater equivalent using a density value of 1.02825 for seawater with a salinity of 35 parts per thousand at a constant temperature of minus 1.8 degrees Celsius (freezing point).

Future work: 2003-2004

The instrumentation will be checked, the data recovered and the data logger clock adjusted if required. Other instrumentation and programming corrections will be made if required.

The position of the transducer in the submerged casing cage will be checked and measured by drilling a hole through the ice foot above the transducer and levelling on to the cage. A portable submarine video system will be used to align the level rod and casing cage.

Future instrumentation

The new data recording instrumentation has capacity for Meteorological sensors to be installed at a future date. This option will be assessed this season after a year of operation and with discussion with LINZ to determine future the requirements.

Data and Data Processing

Data from the Cape Roberts tide gauge for the period November 1990 to the time of refurbishment (27 November 2002) have been supplied to LINZ for archive.