Trompe

A trompe makes compressed air out of falling water. Water dropping into the top of a vertical shaft drags air bubbles down with it, and the deeper they go the harder the water above squeezes them. At the bottom the air gathers in a chamber and is piped away under pressure, while the water carries on downhill. There is no piston, no motor and no electricity; the water is the only moving part.

Two things decide whether one is worth building. The first is head - the height the water drops - because the machine stores nothing and makes air only while water is falling through it. A fall that dries up for part of the year is a machine that stops for part of the year, so the drop must be permanent: a dam spillway, the fall below an earthwork dam, or a grade change in a streambank. The second is sizing: matching shaft depth and air inlet to the head available is hydraulic design rather than assembly, and a first attempt rarely lands it.

The air is piped to a workshop, or to a diffuser that releases it as bubbles in a pond or tank to add oxygen to the water. That second use is where a trompe is over-estimated. It delivers a modest fraction of what a powered aerator moves at the same site, and its output follows stream flow, weakest in hot, low-water weather - exactly when a pond's oxygen is most at risk. It is a steady background trickle, valuable because it works unattended through the small hours when oxygen bottoms out, but no rescue for a pond already in trouble. How much oxygen arrives depends more on the diffuser than the machine: a fouled or coarse one delivers about a third of what a clean fine-bubble diffuser does with the same air. Since it cannot be switched off it needs a valve, or it keeps aerating water that is already over-full of oxygen at midday. Upkeep is clearing the air intake holes, the separation chamber and the diffuser.