Question: I’m wondering if you can help me figure out why the level gauge for the holding tank on my boat is inaccurate. It seems to consistently read lower than the actual amount in the tank, and it's getting to be a real annoyance since we cruise in an area where overboard discharge is not allowed and pump-out stations are few and far between.
I sent in a photo of the sending unit on the side of the tank so that you can see the type of set-up I have on my boat. Any insight you can provide would be greatly appreciated.
Answer: The photo you sent in shows what is known as a “capacitive sensor." Not all capacitive sensors look quite like this; many look more like a simple cylindrical device with a wire coming out of one end that is ultimately connected to a gauge somewhere.
Simply put, these sensors use capacitance to make measurements. Capacitance describes a property that exists between any two conductive surfaces within fairly close proximity. Changes in the distance between the surfaces effectively changes the capacitance, which in turn gets interpolated into a level reading on a gauge. In the case of your holding tank, the metallic tape you see represents one of the two conductive surfaces, and the fluid inside the tank represents the other.
In general, this type of sensor has had a fairly bad run in the marine sector; I hear of failures and inaccuracy issues quite frequently, and it doesn’t seem to be isolated to any one manufacturer.
One thing I've learned from a manufacturer supplying this sensor type is that the position of the tape on the outside of the tank is quite critical to the accuracy of the readings it will provide. This issue, as described to me, has to do with the rigidity of the tank in proximity to the tape. I’ve been advised that to ensure accuracy, the tape needs to be located no closer than about 3-4 inches from a tank corner or internal tank baffle, where the tank is more rigid. This apparently affects the capacitance reading. Look at the positioning of the metallic tape on your set up and see how it compares to that 3-4 inch rule. That could be the answer.