Bubbler Sea-Level Recorders

The idea of a bubbler was first devised by Royal Navy Hydrographer Admiral Mostyn-Field in 1908. The principle is to have a gas chamber which discharges a constant flow of bubbles into the water, with a regulator that adjusts the pressure so as to maintain a constant bubble-rate. The changing pressure is proportional to the changing water level. It's like blowing into a milkshake with a straw. To maintain the same bubble-rate, you must blow harder if the mikshake is full than you do if the milkshake is almost empty.

Back in Admiral Mostyn-Field's day in the early 20th century, bubblers were classified as impractical because the technology was not up it. These days, with modern technology, bubblers have become the instrument of choice for measuring sea levels, especially for recorders on the open-coast where a structure like a wharf is not available.

But what really makes bubblers the answer for almost all applications is that all the expensive equipment can be separated from the hostile marine environment. Indeed, the only part of the recorder that needs to be in contact with seawater is the gasline (the end of the "straw"). All the rest can be housed in an environmental chamber well above water level, like the installation at Frenchman Island shown below:

Frenchman Island, near Marsden Point, with the sea-level recorder atop.

The concrete structure near water's edge is an old acetylene gas holder, a relic from when the island was used as a lighthouse.

The bubbler's gasline comes through the scrub, past the concrete structure and down into the water to about 1 m below Lowest Astronomical Tide.