Fitting a New Holding Tank Sensor
If your boat has a holding tank, chances are it doesn’t have a level gauge. This is odd, as you’d think they would be standard on all new boats, but in fact, just the opposite is true. After experiencing firsthand what happens when a holding tank is overfilled last summer—not at all a pleasant experience—we decided to fit a gauge to the holding tank on Poppy, our 60-foot canal barge, and set about researching the options.
Sensors for tank gauges are positioned either inside or outside the tank. If you opt for sensors inside the tank, it clearly will be a more complex job than fitting sensors on the outside. However, inside sensors are less expensive. For example a simple “almost full” alarm that fits into the top of a tank costs relatively little, while a system that measures the waste actual level will cost a bit more. The downside in either case is that you have to drill into the tank to fit the sensor and presumably have to clean it periodically if it gets gummed up—a maintenance task I would not relish, for sure.
For simplicity’s sake—both in terms of fitting and future servicing—we opted for a system that senses the level of a tank’s contents from outside the tank. The initial cost is higher, but the savings in time and effort made this approach worth it.
The Gobius tank gauge, from Sweden, comes with three sensors, which reportedly work with almost any tank material, including steel, stainless steel, polyethylene and fiberglass. Poppy’s holding tank is a 60-gallon polyethylene type, so we didn’t envision any problems.
The Gobius sensors work on the “knock and listen” principle, tapping on the side of the tank and listening for the echo, which tells the sensors whether there is liquid inside the tank and at what level. The face of each sensor is covered in 3M self-adhesive tape, so you literally just stick it onto the side of a tank. The three sensors plug into a control box connected to a separate display unit that can be fitted where it can easily be monitored—in our case next to the electric toilet control panel in Poppy’s head. The display features a series of LED lights to illustrate tank level. The control box is connected to the display with a Cat 5 computer cable, so the installation is “plug-and-play.”
The Gobius system draws 40mA, and we wired ours to the toilet circuit, so it would be switched off when the toilet was off. Overall, we found the system was simple to fit, with clear instructions. One thing we learned the hard way was that you need to heed the warning that the sensors won’t stick to a tank if applied when the temperature is below 68F. The cabin on Poppy was just 64F when we started work, and the first sensor wouldn’t stick, so I turned on a heater and left the boat to warm up for a couple of hours. Later we fitted all three sensors without a problem. It really does pay to read–and follow–the instructions when doing jobs like this.
Photos by James Turner; Illustration courtesy of Gobius