Electronic – Cheap sensor for measureing air dissolved in water

Given the working parameters, the Ultrasonic Ranging solution indicates the best maintenance-free option. Depending on the desired MTTF, it need not be an expensive solution either.

The budget approach involves modules such as this one:

_{(From eBay, for under $2)}

Rather than deploying the module as-is, the module can be potted using epoxy potting compounds, covering everything but the tops of the ultrasound transmitter and receiver and the connector pins. Better yet, suitable weatherproof cabling can be connected, and the junction also potted. This promises reasonable longevity.

With a larger budget, other ultrasonic ranging modules can be found, which are IP67 rated, designed from the ground up for extreme environments.

Actually, if you just need discrete high/low levels, float switches would work fine, are super reliable, and not expensive. Look for a sump pump type float switch. Basically a mercury switch in a sealed floating ball.

If you need actual level, one of the best systems is a 'bubbler', which uses a controlled air pressure to literally blow bubbles out of a tube, which has the endpoint at the bottom of the liquid stack. The level is measured by monitoring the air pressure and air flow supplied to the bubbler tube. Bubblers are good as they are inherently self cleaning, and give instant indication if there is a severe clog or fault in the system.

Another option for actual level would be ultrasonic. Ultrasonic rangers aren't expensive, and using ceramic transducers, the corrosion problem is eliminated.