The feed back I've collected here and elsewhere is as follows:
- The seem to work OK for fuel and water - basically clean, clear liquids.
- The focus tubes seem to help with the TLM100 (the model that ranges up to 40").
- You cannot use the focus tubes with the TLM200 (the model that ranges up to 105", and the ones I'm using). I don't know why.
- They are very sensitive to installation. The hole needs to be at least 1.25". There can be no contact between the hole edges and the sounder, You can't use sealant. I don't know why.
- They seem marginal at best for black water. As surface scum develops, they lose their ability to get a reading. So far that appears to be why mine have stopped working. I'm going to flush my holding tank and see if that improves things.
- Gray water is marginal too. Suds from soap will cause it to lose it's reading.
- Condensation on the face of the sounder will cause it to lose it's reading. Hot shower and laundry water going into a gray water tank seems to cause exactly this. Coating the face of the sender with petroleum jelly helps.
- It needs to have an unobstructed view straight down to the bottom of the tank with nothing coming closer that about 6". No pickup pipes, baffles, etc.
- If when the tank is empty, there is no longer a remaining fluid surface to reflect back the signal, it will lose it.
My summary take so far is:
1) Don't use them fro black or gray water, only for fuel or fresh water.
2) Carefully check all the installation requirements before buying them. If you can't 100% comply, don't buy them.
3) They are finicky at best.
Oh, and last but most important. Holding tanks REALLY smell bad.