Originally Posted by Art
........... We also discussed utilizing transducer from depth sounder and he seemed reticent to comment positive or negative on that item, ...........
In my experience, a traditional depth sounder does not work through air and if it did, it would read the depth of the tank, not the level of the contents.
It's probably best to use products for what they were designed to be used for.
Looking at some of these products on the Internet, it seems they are overkill. Do you really want to spend $1K to measure the level in your poop tank?
Want to think outside the box? Mount the tank on an electronic scale and use the weight to determine the level of the contents.