Pneumatic or bubbler gauges
Pneumatic or bubbler tide gauges were among the first to use the relationship between the water level and the corresponding hydrostatic pressure. This type of tide gauge is widely used in the United Kingdom, the US and Australia.
How it works
A low flow of air or nitrogen is fed down an air tube to the top of the pressure point, which must be located above the lowest low water level. At this level, the tube is connected to the upper face of a cylinder sealed on the top and open at the bottom, with a nozzle at mid-height. The compressed air or gas blows a stream of bubbles out the nozzle. Providing the flow rate is low enough, the pressure at the land end of the tube is the same as the pressure at the underwater open end; and that is proportional to the height of the water column above it.
The pressure is measured at the tide station and the water height is calculated and recorded on a graph.
This system has an number of advantages. No stilling well is needed, and the bubbler gauge requires virtually no maintenance. The main problem is caused by the presence of waves. which create overpressure causing the water to rise up into the cylinder and preventing the release of air bubbles. And as with tide staffs, the asymmetric profile of the wave peaks and troughs generates errors in the water height calculation. One solution is to increase the air flow but with the risk of causing a turbulent flow in the tube. This causes pressure losses in the tube, which increases the measurement error. A compromise must be found between the two types of errors induced by surface agitation or pressure loss. The longer the tube, the more difficult it is to regulate the flow through the tube. It is recommended to avoid using tubes longer than 200 m.
Due to their ease of installation, these devices were often used in estuaries. However, because the flow of rivers, there are often large vertical variations in density; without simultaneous measurements of the vertical distribution of the density, these measurements obviously cannot be used for applications requiring high accuracy.
Apart from these usage restrictions (estuaries and sites with rough seas), bubbler gauges can provide good quality measurements when properly controlled.
To find out more:
- Simon B. (2007). La Marée - La marée océanique et côtière. Edition Institut océanographique, 434pp.
- Woodworth P. L., D. E. Smith (2003). A One Year Comparison of Radar and Bubble Tide Gauges at Liverpool. International Hydrographic Review, vol 4, n°3, pp.2-9.
- Pugh, D. T. (1972). The physics of pneumatic tide gauges. International Hydrographic Review, 49 (2), pp.71-97.
Last updated: 12/12/2012