Gobius Pro Tank Sensor
The black waste tank on my Sealine F33 had a traditional tank full sensor. Unfortunately, this device is prone to getting clogged with unmentionables and the only solution is to remove and clean it - eww!

I felt that there must be a more reliable way of monitoring the tank level but initial ideas for using inductive or capacitive water level sensors weren't that practical as they only operate through plastic tanks and my waste tank is stainless steel. It would have been possible to use these in an intrusive fitting similar to the original, but these might just suffer the same issue.

Eventually I found an external sensor that will operate through up to 3mm of steel - the Gobius Pro

In order to preserve the original sensor as an option, I cut the original cable between it and its molex plug and added a male and female automotive connector. 



The wires to the original sensor were coloured as follows:
Red 12V+
Black -ve
Red/Yellow Stripe Signal (~12V) from control panel waste full diode.

The panel diode lights when the signal wire is connected to a -ve ground.

The Gobius Pro has four connection wires:
Red 12V supply
Black -ve
Green - signal connection
Yellow - signal connection.

For the tank full sensor, I only want the panel light to show when the tank is full, so I did not use the supplied red/green led display; I just connected the green wire on the Gobius Pro to the Red/Yellow Stripe signal wire. It doesn't matter whether you use the green or yellow wire because the operation is configurable in the Gobius Pro App.
However, when I made the connection, I added an additional socket so that I can daisy chain an additional low level tank monitor in future:-



Installation was very straightforward, though you need to ensure the sensor is not too close to the tank edges and that the surface is clean and above 20 deg C before you attach it. I warmed the tank with a hairdryer to about 30 deg. C before cleaning it with the supplied wipe and attaching the sensor using its double-sided tape. I placed it slightly lower than the original intrusive sensor, also ensuring that it was not close to any internal braces (see vertical heat marks halfway across the tank side). I'd have preferred to place it midway across the forward tank wall, underneath the vent pipe, but I felt the tank would be too rigid here with the welding around the outlet and original resistive sensor.


 
As instructed by the manufacturer, I left it for over 30 minutes before switch-on.

Device setup is via an Android or iPhone App that connects via Bluetooth. The App walks you through the procedure which is very straightforward. You can even test the panel light by switching the way it works in the App.
I set mine up to check the tank level every hour but you can set it to much more frequent intervals (down to 10s) if you want.

Stage 2 : 
I felt it would be useful to install a 2nd sensor around the 40% full level.

I made up the lead as follows :-



All four cables from the Gobius Pro were connected to the matching colour cables on the Gobius LED. The +12V red and -ve black wires were also connected to the red and black wires on the 3-way plug, leaving the blue wire unconnected (nothing to connect this one to on the boat control panel !) 

As I had left a daisy-chained socket on the first sensor lead, installation was just a case of sticking the sensor to the tank, waiting the required 30 minutes before plugging in, and then calibrating it:-
 


I estimate that the two sensors together indicate the 40% and 80% full levels.
I attached the supplied LED indicator underneath the storage hatch (it's cute but not smart enough to display in the cabin!) :-



It's not that convenient there but it's really just to get an idea of how quickly the tank is filling up so that I can plan the pump-out visits into our trips. In any case, I can just as easily check the levels using the Gobius Pro App on my phone.

I'm pleased with the upgrade; the non-intrusive sensors don't suffer from fouling and I get an interim measurement on the LED or my phone as well as a tank-full signal on the boat control panel.