An externally mounted sensor takes the guesswork out of the holding tank status.
Issue 143: March/April 2022
In 2014, I rebuilt the holding tank in our 1978 Bristol 29.9 using 3/8-inch polypropylene welded into a custom shape for the boat. This tank has worked well, but one shortcoming has persisted: the lack of a good way to gauge its fullness.
When I built the tank, I expected that I would be able to detect its level by looking into an open area under the V-berth, or by pulling back the V-berth mattress and looking through the clear access port on the tank. Neither method is convenient, and after a few weeks, the plastic became too opaque to detect the contents’ level.
Our solution has been to know that we have about three days of capacity and to make sure that we get pumped or discharge at sea at least every three days. This has worked reasonably well, but it does create some worry when we are anchored for several days in a harbor with no pump-out boat, especially if we have a guest or two on board.
Recently, though, I came upon a simple, low-cost means of creating a full-tank alarm. While researching another issue in a yachting forum, I saw a reference to a “non-contact liquid water level sensor induction switch.” A search of this phrase brings up dozens of eBay sellers offering these devices for about $6. As the name implies, it can detect the presence of liquid behind a non-metallic tank wall up to 20 millimeters (mm) thick (3/8 inch is about 10 mm). When liquid is present, a red LED comes on. When the liquid drops below the level of the device, the LED turns off.
I thought this could be useful as a warning that the holding tank was getting full. As a bonus, no parts would actually be in the tank, which means no worries about leaking, corrosion, or working inside the tank.
But wouldn’t it be even better if I could make the LED remote and put it somewhere easy to see and monitor? A little more searching brought up an item called “XKC-Y25-PNP liquid level detector sensor,” which was essentially the same item but with a switched lead that can be taken to a remote LED. I found it on Amazon for $15, plus free shipping and return.
The specifications for the detection switch stated that the output signal for the remote LED alarm needed to be below 100 milliamps (mA). Most single LED lights are under 100 mA, so this was not much of an issue. I found a light that could be mounted in a 1/4-inch (6 mm) hole and purchased a package of six for $10.99. Both parts arrived in a day. The sensor only draws 5 mA, so it wouldn’t put any significant load on the battery.
Before the boat came home for the winter, I set up the switch and LED and powered them with a small 12-volt battery. I used a quart plastic milk jug, half-filled with water, as my test bed. The thing worked! A sensitivity switch on the side of the device needed tweaking, but other than that, when water was present, the LED on the sensor and the remote LED illuminated. When water was not present, the LED stayed off.
Once the boat was home, I installed the sensor and the alarm LED. This was simple 12-volt DC wiring and did not present any challenges. Initially, the sensor had trouble detecting liquid through the tank’s 3/8-inch polypropylene plastic. I increased the sensitivity adjustment screw several full rotations before the device could detect liquid. After adjusting it, I checked its function by sliding the sensor up and down the side of the half-filled tank.
The last issue was where and how to attach the sensor to the tank. The tank holds 14 gallons, so I decided to set the alarm at the 10-gallon level. This would give me about one day of warning before the tank topped out.
The device has a mounting pad about 1/2 x 1-1/2 inches, which seemed like a sufficient area to securely attach it. I glued it on using Loctite’s two-part plastic bonding glue. Available in most home centers and hardware stores, this glue is specifically for use with polypropylene plastic. I ran tests, filling and draining the tank with water to confirm the sensor’s operation.
Though not a perfect solution, it works reliably, and it sufficiently warns of an impending full tank, providing a little more peace of mind for the onboard waste management engineers.
Homer Shannon and his wife, Denise, sail a 1978 Bristol 29.9 out of American Yacht Club in Newburyport, Massachusetts, where they enjoy sailing to Portsmouth, Gloucester, and Rockport as well as taking more extended cruises to Maine and Cape Cod Bay.
Thank you to Sailrite Enterprises, Inc., for providing free access to back issues of Good Old Boat through intellectual property rights. Sailrite.com
