Last summer the WEMA holding tank gauge and sensor stopped working. It is not a comfortable feeling not to know how much space is left in the holding tank, especially when you have guests on board. But there are few boat jobs that are worse than opening a holding tank in the summer heat. So this project was postponed until the winter.
Now that the temperature is in the low 30s (20s at night) I thought it would be a good time to tackle this project. When I was on board in January I tried to disassemble the old sensor unit. I removed the flush-hose and the six machine screws that attach the sensor to the top of the tank. Then I pulled up the sensor unit. The entire assembly is made of plastic, and although the float assembly was easy to extract, the tube in which the float moves up and down stayed stuck inside the tank. It also appeared that the tube had a thread and that the sensor float was screwed down into the tube. I assume the intention was that the float unit could be removed for cleaning without having to remove the tube. You would still have to remove the six machine screws before you could remove it, though. Anyhow, the thread and the top of the tube appeared to be cracked.
With the assembly outside the tank I checked the movement of the float and although dirty (what else do you expect!) it appeared to move freely up and down. But no reading on the gauge, until the float was in its uppermost position. So the fault was in the float, and not in the gauge. I measured the length of the float; it was about 12". A quick check on the internet showed that WEMA no longer manufactures this particular type of sensor. The current model ("SHS") is made of stainless steel, but has the same dimensions, and should be an easy drop-in replacement. I ordered a unit from Sailboatowners.com (the only outlet that seems to carry these units). I put the old sender back to plug the hole in the tank and went home.
When I came back on board a few days ago I tackled the project of replacing the sensor. I am not an optimist by nature, and having some experience with boat projects, I did not believe that this was going to be easy. But, to my surprise, it was not too bad. (The smell was not too bad either. The tank was "empty", but there is always some liquid left over. When in use I use Odorless holding tank additive, perhaps that stuff really works!)
After removing the old float assembly again, I tried to pull the plastic tube out of the tank. Of course it was stuck, but after some wiggling it started to move. When I got about 4 inches of it out, something broke, and the bottom of the tube fell in the tank. It appeared that the plastic tube was made of segments and threaded together. As the wall thickness was very thin, and the outside of the tube probably full of crud, the wiggling must have pulled the tube segments apart. I decided that I could live with this piece of plastic settling on the bottom of the tank, as the alternative would have been to open at least some of the inspection plates (there are four) and digging around the bottom of the tank to get it out.
The new mounting ring and the position of the machine screws were identical to the old unit. Next I connected the wires to the gauge and checked that everything worked. Finally I wrapped teflon tape around the thread and screwed the float assembly in place. Done!
The old sensor had a flush port that was connected to the fresh water system (separated from it by a ball valve and a check valve), the intention being that you could flush the crud off the float to prevent it from sticking, without having to remove it from the tank. How effective this was remains an open question. I flushed it a few times a year, and the float was fairly clean when I pulled it. The new assembly does not have this option. Instead you can easily unscrew and remove the float assembly and tube without removing the mounting ring. Anyhow, this new model is made of better materials and is easier to remove. How stainless steel will hold up in this hostile environment remains to be seen.
This is a picture of the old sensor. You can see the flush port with the clear hose going to the check valve.
The old sensor removed, with the section of the tube that I was able to get out of the tank.
Here you can see the thread and the crack in the top of the tube.
The hole in the tank with the sensor removed.
The new mounting ring in place.
The new float assembly.
We have a reading!
The entire assembly ready to go.