Is That Holding Tank Full Already?

Monitoring tank usage is important on LUX. We often are traveling in no discharge areas and have to get a holding tank pumpout done before it is full. The float sensors on the holding tanks aren't great. Calcium buildup quickly makes them ineffective. We needed a better way to monitor holding tank capacity and decided that it would also nice to be able to monitor water tank usage, though it is relatively easy to just open the forward lockers and look at the water levels.

There are a number of tank monitoring systems around. Getting one that works for a combination of water, waste, and fuel is rather interesting. It needs to work with potable water, corrosive waste, and flammable liquids. Two systems seem to get the best marks: capacitive sensors or pressure sensors. 

The capacitive sensors work only on non-metalic tanks. Two strips of conductive tape are applied to the side of the tank and the sensing electronics is calibrated to determine the range of values that correspond to the volume of liquid in the tank.

Pressure sensors work by using a dip tube that extends from the top to nearly the bottom of the tank. Either a manual or automatic pump is used to pressurize the tube until bubbles come out the bottom. The pressure is then read as an indication of the amount of liquid that is covering the tube.

Float switch-based systems typically don't work well, particularly in waste systems. Bits of gunk (that's a technical term ;-) ) and calcium buildup will quickly inhibit the float switch operation.

After looking around at some systems, we decided on the Profile Series 8-tank monitoring system from Ferriello Sales (now New Providence Marine Tank Monitor). <http://www.newprovidencemarine.com/> It can monitor up to 8 tanks and can accept input from a variety of sensors, including our existing fuel sensors, a propane tank sensor (for tanks equipped with a sensor), as well as capacitive sensors. The system can handle weird sized tanks, which is a plus on boats. The display can show a summary of all 8 tanks or a higher-resolution display of each tank's capacity. Alarms can be set for full or empty levels. Alarms can be disabled at night, as determined by a photo sensor. Dennis, the owner, has received a number of accolades from people regarding the superior level of service that he provides. Our experience was similar. The system cost, including wire from West Marine, was about $450.

We intend to monitor six tanks: two each of water, waste, and fuel, even though the fuel monitoring is redundant with the fuel gauges at the helm. We don't have propane tanks with sensors, but that's a possibility for the future.

The display is mounted at the top of the navigation station and provides good visibility. The display is in the process of being mounted and is sitting on the counter in front of the open DC electrical panel. The blue tape at the top of the nav station is used to mark the cutout and to protect the gelcoat during the cutting operation. We neglected to get a photo of the completed installation - something to do on the next trip to LUX.

We have reserved space above the Shore Power panel for the genset control panel, which will be installed this summer, 2014. This will put all the AC equipment panels together for ease of monitoring.

The holding tanks were the first to get sensors. The process is easy to follow and works well. We used nearly 100ft of wire to connect both holding tanks and expect to use that amount for the water tanks. The sensors require three connections: Ground, Power, and Sense. The Power and Sense connections have to come from the display. We were able to use the ground wire from the old tank "full" sensors at each tank, so we only needed to run two wires (Red=Power, Blue=Sensor) from the tanks to the display. We used quick-disconnect electrical connectors so that the tanks can be easily removed in the future.

Calibration required emptying the tanks, then filling them, which we did with water at the next pumpout. The system works well, allowing us to monitor all 8 tanks with one glance as well as more detailed views if we're filling water and fuel tanks.

We ran out of wire after doing the holding tank sensors, so we'll add water and fuel monitoring when we return to LUX.

Update 8/5/2021: Yes, we added monitors to the two fresh water tanks and all four have been valuable tools for knowing when to pump out or add water or switch tanks.

-Terry