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These devices track your blackwater tank levels, helping you avoid potentially ‘explosive’ situations.
Every holding tank should have a monitoring system. This one is readily visible every time you step into the head compartment. Photo, Frank Lanier
I learned a few things when I moved aboard my first boat in Hawaii. One was never talking politics with a dock neighbor who wears a black trash bag (complete with head and arm holes) for clothing … and nothing else. The other was the explosive results of overfilling a holding tank on a hot Hawaiian day.
In the first case, I was thankful the bag was black, rather than clear. In the second, I was glad no one on the dock was in the line of fire when a viscous discharge of effluent erupted from the tank vent – that is, until the vent hose popped off the discharge fitting and continued projectile spewing about the hanging locker like Linda Blair in “The Exorcist.”
If your boat has a holding tank, chances are it doesn’t have a capacity monitor or level gauge. This is something I’ve always found puzzling as a marine surveyor, particularly for something I believe should be standard equipment for any holding tank installation. Here’s the lowdown on holding tank sensors, from selection to installation, as well as useful options to consider.
Internal sensors are physically mounted inside the tank, meaning you’ll have to use an existing hole or drill a new hole for the sensor. The sensor shown here has been mounted in the cleanout access plate. Photo, Frank Lanier
What’s Your Type?
Holding tank sensors can be divided into two broad types: those with internal sensors and those with external sensors. Both types have different installation requirements, and the type you choose will be based on a number of factors, from the material the holding tank is made from to accessibility for the installation. Here’s a look at both, along with the pros and cons of each.
Internal Sensors
Internal sensors are physically mounted inside the tank, meaning you’ll have to use an existing hole or drill a new hole for the sensor. On the plus side, internal sensors can work with any tank material. They’re also typically less expensive than external sensors.
As to downsides, having to drill a hole in your holding tank (if required) is a step that increases the complexity of the installation. Another potential issue is that the sensor is exposed to the contents of the holding tank, which can affect operation (due to crud buildup) and increase maintenance requirements (such as having to remove crud-encrusted components for cleaning).
One example, the Dometic DTM04 TankWatch 4-Level Tank Monitor system, features a sending unit that threads into a standard 3-inch NPT (national pipe taper) cleanout port flange. While most holding tanks are equipped with such a cleanout port already in place, the manufacturer can also provide flanges for installation in tanks that don’t. The sending unit also has a fitting that will accept a 5/8-inch vent hose. This means if your holding tank isn’t vented or vents using the existing cleanout port, you can use the vent provided by the sending unit.
The sending units themselves come with either 22-inch- or 45-inch-long probes. The probes are polyethylene tubes that carry the float switches and are cut to match the depth of the tank. Each sender has three tubes, one for full, half-full, and empty.
While the problems associated with internal, float-type sensors are obvious, there are a number of other internal sensor types that promise reliability.
The Maretron FPM100 system, for example, operates by measuring the current transmitted by pressure transducers mounted to a fitting on the tank bottom (or side as close to the bottom as possible). Data is transmitted over a NMEA2000 network and allows you to monitor the fluid levels in tanks, even those that are deep or have internal baffles. The system can also be calibrated to monitor tanks with irregular shapes.
External sensors detect fluid level from outside the tank. The system typically involves mounting some type of sensor or sensor strip to the holding tank. Photo, Frank Lanier
Unsticking A Stuck Sensor
For internal sensors that operate via a float switch, the float switch is immersed in waste when the tank is full. However, when pumped out, it’s no longer immersed, meaning any residue sticking to the switch dries. After repeated fill and pumpout cycles, the sensor and switch can become covered in crud, causing the sliding part to stick. When this happens, you’ll have to crack open the holding tank to remove and clean the sensor — an unpleasant prospect under any circumstances. It’s also possible to clean a stuck sensor of debris by filling the holding tank with water and some type of cleaning solution and letting it slosh around for a few days. (Some opt for not putting any toilet paper into the tank. This helps with several issues, including float switch fouling, but even so, in-tank switches will foul eventually.) — F.L.
External Sensors
External sensors are mounted to and sense fluid levels from outside the tank. This typically involves mounting some type of sensor or sensor strip to the outside of the tank. The benefits of an external sensor include simpler installations. Maintenance requirements are also easier and, as the sensors cannot become fouled, they don’t involve opening the holding tank or removing a sensor for cleaning.
As for cons, the cost for an external sensor system is typically higher than for an internal sensor. Tank material is also a consideration, as many will only work with holding tanks made of plastic or fiberglass. Tank accessibility is also important, as you’ll need access to the tank side or end to mount the sensors.
The SCAD TM1 Tank Monitor with External Sensor Strip uses external stick-on level sensors. Basically, you stick two metal strips a measured distance apart on the outside of any plastic or fiberglass holding tank (up to 30 inches tall), and it can gauge the percent of fill in 1/8 increments. An optional internal sensor (for metal tanks) is also available.
The Gobius Pro tank monitoring system comes with three stick-on sensors, which work with almost any tank material (steel, stainless steel, polyethylene, and fiberglass), although there are maximum thicknesses for each material. The Gobius sensor consists of two active parts: a shaker and an accelerator. It works on the “knock and listen” principle by tapping on the tank side and listening for the echo, which “tells” the sensors if there is liquid inside and at what level. The shaker creates a vibration in the tank wall; the accelerator measures the size of the vibration and passes the data on to the control unit. The control unit then calculates whether the liquid level has passed the sensor or not.
Gobius Pro tank monitors are Bluetooth-enabled and can be monitored on your phone or tablet. The control box is connected to the display with a Cat 5 computer cable, so the installation is “plug-and-play.”
Tank level indicators range from simple tank level LEDs to graphic displays provided to your smart device via Bluetooth. Photo, Frank Lanier
Monitor Panels
All holding tank monitors, regardless of the type of sensor or sending unit selected, will have some type of display. The most basic may be a single “Tank Full” LED, while more complex systems may actively display tank levels in real time, or as with the Gobius Pro, provide information via Bluetooth to your smart device.
Other features to look for include an option to disable the power to electric heads when the tank reaches the full mark. Another would be triggering a “Do Not Flush” indicator light at the head when the holding tank is full. Most holding tank monitoring systems also offer dual-purpose options, a great deal if you also want to monitor your freshwater tanks.
Monitor panel installation consists of selecting a suitable location and running the applicable cables and wires for sensor data and power. Most system instructions will include an installation template to assist with cutting a mounting hole for the panel. When deciding where to mount the monitor panel, keep the following in mind:
- Select a dry location that is readily visible. I mounted mine in the head compartment over the toilet, however, other popular options include the galley, helm, or near the holding tank itself.
- Ensure there’s a path for wiring from both the sending unit and 12-volt DC power.
- Confirm clearances for the panel behind the walls, hull liner, or bulkhead.
- Be doubly sure of what’s on the other side of the selected mounting location. Drilling into equipment, hoses, or electrical wiring is never conducive to a good install!