Strain gauges and quartz gauges
The first pressure sensor tide gauges were introduced with pneumatic gauges. This technological advance resulted in the development of small pressure sensors, in particular strain gauges and piezoelectric quartz gauges. This made it possible to design bottom gauges with sufficient battery life to record data for up to one year or more.
These units are widely used in hydrography, either to measure the tide height near sounding areas, or to determine the conditions at the limits of numerical tide or current models in a specific area of the continental shelf (to a depth of less than one hundred meters).
How it works
In strain gauges, their electrical resistance changes as a function of both pressure and temperature, but their biggest advantage is their low price. Because of their sensitivity to temperature and the supply voltage for measuring their electrical resistance, determining the pressure with this sensor requires careful calibration and control, and a well regulated power supply. These units can provide:
- Either very good measurements if they are maintained by capable organizations
- Or measurements useful for some applications, but unusable for hydrography or oceanography.
The most accurate gauges use a quartz crystal sensor whose resonant frequency varies with the pressure applied. These sensors are used in deep-sea gauges because they are capable of one millimetre resolution at depths of several hundred meters. They are also sensitive to temperature, but to a lesser degree than strain gauges, since the calibration coefficients as a function of temperature can be considered constant over several years. Because this correction is necessary, the unit should be able to record pressure directly. This is achieved by adding a temperature sensor and internal software to eliminate the effect of this parameter on the frequency of the signal.
These sensors do have a small calibration drift related to the mean use pressure, and this drift is especially noticeable at great depths. This fault cannot be modelled, because it differs from one sensor to another and depends on their history. It is a slow drift that does not affect the measurement of amplitude fluctuations, but it does make it impossible to study long-term mean sea level trends. Calibration before and after the measurements can detect this drift, but cannot correct it, because it is not linear.
This type of unit is relatively easy to install. The instrument is integrated in a structure (figure) comprising a ballast for submerging it, an acoustic transponder/release system and a buoy large enough to provide buoyancy without the ballast. The buoy, non-deformable under the effect of pressure, is equipped with a light beacon and optionally a radar reflector, making it easy to find on the surface. The transponder is used to find the tide gauge at sea. The ballast is left on the seabed after release of the buoy supporting the cage (tide gauge and release) by acoustic command from a ship.
Because modern computing systems do not allow much freedom in the acquisition, recording and processing of measurements, the systems submerged far from the coast can easily be completely autonomous for several months or even years.
To find out more:
- Simon B. (2007). La Marée - La marée océanique et côtière. Edition Institut océanographique, 434pp.
Dernière mise à jour de la page : 18/08/2012