Selection Guide
80 GHz radar or ultrasonic in a foaming wet well
A wet well with foam on the surface is the classic case where a cheaper instrument fails on the second visit, not the first. The installation is correct, the echo is strong on the day of commissioning, and three weeks later the reading sticks.
What foam does to each principle
Ultrasound at 40–200 kHz reflects from the top of the foam, not the water underneath, so a 200 mm foam layer reads as 200 mm of level that is not there. It also absorbs energy, and once the return echo falls below the gain threshold the meter holds its last value or jumps to the tank floor.
Radar at 80 GHz penetrates most sewage foam and reflects from the liquid surface. Its narrower beam also helps in a wet well with pipes and a ladder in the way. Two exceptions are worth stating: thick chemical froth can absorb even 80 GHz, and a very calm liquid with a low dielectric constant returns a weak signal whatever the frequency.
Where the foam is persistent and thick, no non-contact instrument is reliable. Measure the outflow instead, or use a pressure transmitter in a stilling well and accept the density assumption.