Just built and tested my grey water tank sensor prototype. It works great. I am using 1.25 in pvc which drops down through my 2 inch vent pipe which also holds this sensor vertical. As you can see (second pic) , air also escapes around it for venting. I may build one similar for the Backwater tank; however, I will field test this for a while and see how it goes. Given the place the sensor drops into the tank is on the "shelf", 1/3 = 1/2 and 2/3 = 3/4. Most concerned with the 1/2 and full anyhow (just need to know when to dump), but I will get a heads up at 1/2 and then a finer measurement until full as is. I could put in another tank grommet and make the sensor work on the full depth of the tank, but really not needed. This is good enough and the hole is already there. Will adjust some on final as I may be able to get 1/3 = 1/3 if I can get the first two sensors across from each-other and as close as possible to the bottom of this pipe (still need to cap however) and then equally space the other two from the bottom. It won't be exact... but close.
My plan is to put another cap on the end and just have a small hole for the wire feed thru which I can seal with silicone. Easy to pull out and clean. I just could not find what I needed on the market at a good price... so I built it and it works good enough. Now that I have a working prototype and I have lots of PVC... I'll dial it in.
This pic is looking down into 2in grey water vent pipe. The sensor pipe just rests on the bottom and is kept in place by larger vent pipe. Both sensor and vent stack will get capped when I am done. T in vent pipe goes to BW vent pipe. This should still breathe well. Plumbing has been working flawlessly.
This is what I used to build in addition to the PVC. It is just your standard sensor probe kit.