Holding Tank Level Monitors: Fix, Calibrate or Upgrade Your Tank Gauge

Look at the top of the tank for a sender flange or tube, a thin air line, or a cable going into the tank; look at the sides for stuck-on strips or pads. Follow the wire or tube to the display. Note the maker and model, and find its manual.

For electric senders:

• Is the display getting power? Check the breaker or fuse and measure 12 volts at the display's power terminals.
• Look for corroded or loose connections at the display and the sender, especially near the tank where it's damp.
• Check the ground (negative) wire; a poor ground makes gauges read high, low or jump around.
• On external or wireless sensors, replace batteries and check the app's pairing.

If the display is dead with good power, the display itself may have failed.

Right after a pump-out and rinse, the gauge should read empty. Then count flushes or know roughly how much goes in per flush and see if the gauge rises as expected. Typical findings:

• Reads full even when empty: a float stuck at the top, or a fouled probe.
• Never rises: a stuck float at the bottom, a broken wire, or no power.
• Jumps around: a loose connection, poor ground, or slosh while underway (normal on some types).
• Wrong in the middle of the range: the tank shape is irregular and the gauge needs calibrating.

A no-mess way to check the real level: Practical Sailor found that an inexpensive pinless moisture meter or a stud finder, slid down the outside of a plastic or fiberglass tank, reliably showed the liquid level to within about half an inch. Both use the same principle as external tank sensors. It doesn't work on metal tanks, which read as "full" everywhere. On a warm day you can sometimes feel the level with your palm too, since the empty plastic above the liquid warms faster than the contents.

Lanier explains that floats get coated when they dry out after each pump-out, and after many cycles they stick. Before opening anything, fill the tank with fresh water and a tank-safe cleaning solution and let it slosh for a few days (a run in choppy water helps), then pump out and rinse. The holding tank deep clean guide covers the method. Many owners find the gauge works again.

Pump out and rinse the tank twice first. Ventilate and gear up. Turn off the gauge power, label and disconnect the wires, then remove the sender flange screws or unscrew the sender from its port. Lift it straight out, carefully; don't bend the tubes. Rinse it with fresh water and scrub the floats and tubes gently with a soft brush and a mild cleaner until each float slides freely. Refit it with a new gasket or the sealant the maker specifies, reconnect, and test. If floats are cracked or the tubes are scaled solid, replace the sender.

If an air gauge drifts back to zero, check the small tubing and fittings for air leaks first, since a leak gives low or falling readings. If the tube tip is clogged with waste, some holding tank versions have a purge feature to blow the tube clear; use it as the maker directs. Never blow the line out with high-pressure air.

Make sure each strip or sensor is still firmly stuck to a clean, dry tank wall at the position the instructions specify. Peeling, damp or dirty surfaces cause errors. Confirm the tank wall material and thickness are within the sensor's limits, and that nothing stowed against the tank presses on the sensors. Gobius's support notes give two limits owners often miss: the tank wall should be at least about 68°F when the sensors are stuck on (so do it on a warm day or warm the area first), and each sensor needs some distance from the tank's edges and corners (Gobius gives about 70 mm, roughly 2 3/4 inches, from the sensor's center). A sensor squeezed into a corner may never read reliably.

Most electronic monitors have an empty and a full setting; some let you enter a tank shape or several points. Follow the maker's procedure. To do it by volume, as a Cruising World writer did for his irregular tanks:

• Start with the tank empty and rinsed, and set "empty."
• Add fresh water in measured amounts (for example a quarter of the tank's real capacity at a time) and note or set each level.
• Set "full" below the true top, leaving a safety margin so you never reach the vent.

Tanks against the hull are often narrower at the bottom, so "half way up" can be well under half the volume. Also check the sender is long enough: that writer found his old senders stopped well short of the tank bottom, so they could never read accurately.

• Plastic or fiberglass tank with access to a side: an external sensor avoids fouling and drilling.
• Metal tank: choose an external system rated for metal or a well-sealed internal sender.
• Tank with a spare cleanout or sender port: an internal sender that fits that port avoids drilling a new hole.
• Networked boat: a sender with NMEA 2000 output can show on your plotter and in a remote-monitoring app.
• Submersible sensor through a top or side fitting: some newer systems drop a sealed sensor into the tank instead of using floats. Raritan's TankAssure, for example, is sold for polyethylene, fiberglass or metal tanks, shows a percentage on a panel or phone app, can watch up to four tanks, and includes a blocked-vent warning, which is handy because a clogged vent causes many holding tank problems.

What long-term testing showed: Practical Sailor left seven internal sensors in a tank of homemade "sewage" for nine months in the Florida heat. Some float-type senders failed to read mid-levels because greasy sludge kept a float from sliding, and one had its float coating eaten away. The two air-pressure (pneumatic) gauges and an ultrasonic sender kept reading accurately; the pneumatic gauge just needed its "purge" setting to blow debris out of the tube. If floats keep sticking in your tank, a type with no moving parts in the waste is the usual fix.

Match the sender length to the tank depth (some are cut to length) and order any flange or fittings for your tank.

Lanier mounted his in the head compartment above the toilet; the galley, helm or near the tank are other choices. Pick a dry spot, plan the wire run from the sender and a fused 12-volt supply, check the space behind the panel, and be sure there are no hoses or wires where you drill. Use marine tinned wire and sealed connectors.

Useful options on many monitors:

• A "Do Not Flush" light in the head that comes on when the tank is nearly full
• An interlock that cuts power to an electric toilet at the full mark. Some toilet makers sell full-tank sensors made for their own flush controls: Raritan offers one that fits in the tank cap and an external one for plastic or fiberglass tanks, and says its control boards are designed for its own sensors, so check compatibility before mixing brands.
• An alarm or phone alert for remote monitoring

• Pump out before the gauge reaches full; treat "full" as "pump out today."
• Rinse the tank with fresh water every few pump-outs and deep clean it twice a year.
• Use little or no toilet paper; Lanier notes this reduces float fouling, though in-tank floats still foul eventually.
• Test the gauge right after each pump-out: it should drop to empty.
• Note sender and display models and calibration settings in BoatMend.