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How much fuel is in my tanks?

Oct 25, 2013
Schematic of a fuel tank with a float-type fuel level sender.

Schematic of a fuel tank with a float-type fuel level sender.

Bill Bishop

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You’ve been watching the storm front creeping ever closer and you have 45 nm to go. You wonder if there is enough fuel to go the distance. There are two things you need to know before you can be sure: How accurate is your gauge and tank sender?

You’ve got your power voyaging boat set at its best cruise speed and are getting 0.7 nm to a gallon. You want to speed up to beat the approaching front. The bouncing fuel gauge appears to say there is slightly more than a quarter tank, and you do the calculations. At full speed you get 0.5 nm to a gallon. Your tank capacity is 400 gallons so you should have about 100 gallons of fuel. You’ll need 90 gallons. Do you actually have that much fuel?

The gauge
To understand how all of it works, we have to know the magic numbers of 33 ohms (your tank is full) to 240 ohms (your tank is empty). This resistance range is read by analog marine fuel gauges, and the gauge needle theoretically points to the corresponding fuel level. For example, if the circuit resistance is 103 ohms the needle points to half full. More sophisticated sender technologies emulate the 33 to 240 ohm output for standard fuel gauge use.

Fuel gauges come in two forms. The typical analog versions, and the digital forms. We’re not going to dwell on the analog fuel gauges with the pointing needle since most are familiar with them. They work reliably, but only provide generalized information on how much fuel you actually have.

Digital fuel gauges can do much more with the same 33- to 240-ohm input. Instead of pushing a needle around a dial, a 100-gallon tank with a 103-ohm reading now says you have 50 gallons. Since we now know how many gallons we have, gauges like the CruzPro FU60 can manipulate the information and calculate current consumption rate, time to empty, and fuel used on a trip. The CruzPro gauge also has tools that allow calibration of irregular tank shapes, and the ability to transmit the data with a NMEA 0183 output.

Your chartplotter in many cases can also be a fuel gauge by using a NMEA 2000 converter like Maretron’s TLA100 attached to the sender. The data will appear on most systems as a digital simulation of an analog gauge. Since this unit allows calibration, it’s capable of dealing with irregular tank shapes.

The tank
Before we deal with the abundance of available fuel sender technologies, we have to take a few minutes to consider the tank it’s attached to. The first item we can’t use is all of the fuel we have in the tank, and we don’t want to. The fuel pick up is usually mounted about an inch above the tank bottom. The bottom of the tank is where water from condensation has settled, along with any solids in the fuel. But this fuel needs to be accounted for, and it’s easy to do.

Measure the tank’s width and length in inches, and multiply them together. Then divide this number by 231 (number of cubic inches in a gallon) to get the not usable gallons. If you’re going to install a new sender, you might as well get the math finished. In inches, length by width by height gives you the total cubic inches in the tank. Divide this number again by 231, and you now have total gallon capacity.

The typical sender is attached to the top of the tank using a Society of Automotive Engineers (SAE) specified five-hole bolt pattern. Despite the appearance of symmetry, the hole pattern only allows the sender to mount in one orientation. There is also a variation used by some tank manufacturers that has a four-hole pattern, so check first and don’t be caught by surprise.

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