Hi, did you ever manage to figure this out. I would like to fit 24 litre wheel arch water tank but have the same overfilling concern
It has been a long and winding road and probably gave me more problems than any other part of the build in trying to find information. I find it very frustrating that the manufacturers of these tanks don't seem to have considered all the issues of actually making the installation work properly. They need to position the drain outlet in the side and the access hatch needs to be situated further along the top of the tank to allow a sender to be positioned in the deepest part of the tank.
I ended up with a long CBE sender fitted into the access hatch lid. I had to seal the recesses in the lid with copious amounts of black bonding (Puraflex) and fitted a couple of bits of 6 mm dowel in the top of the sender to allow me to unscrew the lid and sender by hand. The sender is wired to my CBE control panel which shows the level in the tank. I had a devils own job getting the correct wiring which took months to sort out but that is another story. (It should come as part of the CBE kit but if you have similar problems let me know as I have the part number and supplier info)
I ordered the sender from Leisure Lines who advised that it can be used to monitor up to 500 mm deep tanks. When it came it was much shorter, about 350 mm from memory, but Leisure Lines assured me that it would send a signal down to the full depth. I am still waiting for that signal to arrive!
So far I have only carried tests by filling a measured amount of 10 litres and emptying and seeing how this affects the reading on the control. It seems that it is only marginally accurate as the initial reading showed with varying amounts each time. I believe that it will not show a progressive indication of the water level in specific levels (say in 1/3rds) but at least it will give enough of an indication for me. This is fine as we tend to use bottled water for drinking and I shall usually only have the tank half full anyway. I shall probably monitor the input by decanting from a jerry can anyway but my concern was always not knowing how much would have been left in the tank when filling.
I fitted a vent to the top corner with spider protection by covering the end with a fabric which allows the passage of air that I used when veneering. It has occurred to me though that I could easily extend this tube so that it could be temporarily placed outside the van or into a bowl when filling if I wanted to be absolutely sure that I didn't overfill.
I didn't want to leave water in the tank over the winter as I have had past experience of black algae forming so I ran the tank "dry" with the pump and siphoned and mopped out the remaining 0.75 litre by removing the access hatch. This was quite successful and took very little time to do.
Just for the record, the sink that I used had a tap which didn't have a micro-switch to operate the pump and I got over that problem by using a Whale pressure flow switch. (Another issue that I fussed over for many weeks)
I have waffled on as I wanted to describe my experience and give as much information to help anyone considering a similar installation. It was hard going at times but I think that the overall result has been a success as it frees up so much space. (I have posted pictures of the (nearly) finished conversion on the site if you are interested and looking for any other ideas.