Compressed air solder

The compressed air plumb bob is a measuring device for determining the water depth by measuring the pressure on a hose that is drained into the water, open at the bottom and through which air flows.

technology

The plumbing with a wire or rope and a heavy plumb weight practically required the ship to stand still so that the plumb bob and its suspension would not deviate too much from the plumb line due to the flow of water and the resulting drag force. The echo sounder, which was developed independently in different countries, initially only functioned through discrete sound impulses, for example through blasting capsules as a sounder , but not yet as a continuous measurement.

The compressed air plumb bob was described in 1925 as an invention by the American Haynes and for the first time made it possible to continuously measure the depth of a body of water under a moving ship . A heavy hose with high tensile strength and a typical length of 150 m was lowered into the water as far as necessary from a reel on the ship. A compressor pumped air into the hose through the roller axis until air bubbles rise from its lower end. At low flow rates, the flow resistance in the hose is negligible, and up to a depth of 160 m the average density of the air in the hose is less than 1% of that of the surrounding water, so it can be neglected. Then the air pressure at the end of the hose on the ship is as high as the water pressure at the lower end of the hose.

From this pressure, the depth can be calculated linearly using the density of water. Rule of thumb: 1 bar corresponds to 10 m.

The pressure in the hose is transmitted via a line to the ship's chart house and can use a writing device to record the measured depth on a strip of paper, giving the navigator an overview of the water depth along the route traveled in the form of a depth profile. Falling below an adjustable depth triggered a warning signal.

If the hose lies on the bottom of the water or is dragging, the actual water depth is measured quite accurately. If the end of the hose is raised accordingly while sailing over a ship lying on the ground, a correspondingly lower pressure is displayed.

This plumb bob was used in particular by ore motor ships on rivers and the large inland lakes of the USA.

Working principle

The compressed air plumb bob is based on a pressure measurement at the bottom. The measured pressure can be translated into a water depth . After solving the equation to determine the hydrostatic pressure ${\displaystyle p}$ ${\displaystyle h}$

${\displaystyle p(h)=\rho \,g\,h}$

after the water depth h results

${\displaystyle h(p)={\frac {p}{\rho \,g}}}$.

With the water density (about 1000 kg / m ³ for fresh water ), the acceleration due to gravity (about 9.81 m / s ² ) and the measured pressure , the water depth can be calculated. A pressure increase of one bar corresponds to an increase in water depth of ten meters. ${\displaystyle \rho }$ ${\displaystyle g}$ ${\displaystyle p}$

See also

• Lot (shipping)
• Dive computer

literature

• Max Fischer: The compressed air solder. In: Polytechnisches Journal . 340, 1925, p. 126.