CMST Ocean Wave Recorders
In 1988 CMST developed a low cost, easy-to-deploy ocean wave recorder suitable for short duration measurements - typically 4hours. Over the years the instrument and processing software have been refined, and the device is in regular use for ships trials and research projects.The device consists of a pressure sensor connected to a datalogger. Deployment of the sensor is quite like deploying a cray pot. The device is placed in a protective cylinder approx 600mm high, 300mm diameter, which in turn is connected to a 700mm diameter orange surface float by 20m of rope. Total weight is less than 10Kg. Prior to deployment the logger is programmed via PC serial port using software provided. After recovery, the device is downloaded, again via the serial port using software provided.
The output is in the form of spectral ordinates and the usual statistical values (Hs, Tz etc.). Maximum logging time is 16 hours (depending on chosen sample rate). The recorder can be deployed, recovered, downloaded and redeployed as often as is required. It is free floating, and requires sufficient water depth to avoid touching the sea bed. The rope length can be reduced for shallow water deployments, but the submersion depth of the device influences the low frequency cut off for the spectrum - the deeper it is the lower the low frequency cutoff. It is suggested that the GPS coordinates of the deployment are recorded, and wind/wave drift estimates be made for deployments of more than a couple of hours, in order to help locate the device at the end of trials. Drift rate is strongly dependent on wind and wave conditions (and current if applicable). We have deployed them scores of times and never lost one (yet!).
At low wave frequencies the sensor signal is severely attenuated, so any signal noise gets amplified which results in overestimation of the low frequency spectral ordinates. To minimise this effect we recommend the sensor be deployed at a depth of at least 10m and the spectrum low frequency cut off be at least 0.1Hz. Whilst it is possible to gain accurate Hs estimates for lower frequency cutoffs, it requires careful processing and quality checking of the data.
We have used the sensors regularly over the last decade for ship trials and other fieldwork, including surface truthing of the ERS-1 satellite synthetic aperture radar spectral estimates for DSTO. The software used for programming and downloading the device is not full commercial-level user friendly, but has been used successfully by several researchers. A manual is provided, and it is suggested that a dummy deployment be conducted (off a jetty or a swimming pool) prior to trials, to ensure that the user is familiar with the process.
The instruments are available for hire, with or without staff support, and we can quote for the supply of one or more.
For further information, contact:
The Director, Centre for Marine Science & Technology.
Email: Director
Phone: +61 8 9266 7380
Fax: +61 8 9266 4799
Related Information:
Marine Science Products
