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How much fuel is in my tanks?

Oct 25, 2013

(page 2 of 2)

Senders
There are a huge array of technologies available to measure fuel tank levels from old school approaches such as the measuring stick or sight glasses, to sophisticated ultrasonic and capacitance based systems.

Courtesy Hardin Marine

Capacitance senders like this Hardin Marine unit provide high accuracy at low cost.

Resistance senders
The resistance based float sender is by far the most commonly used and least expensive. The float on the arm rides at the current fuel level, and the ohm reading is dependent on the angle of the elbow. It is a well-tested device, but over time floats may deteriorate, or come off leaving you with an empty tank reading all of the time. Problems can also occur if there is wear causing the wiper to encounter locations with no conductivity, and sloshing fuel in rough waters will cause bouncy gauge readings.

Another variation of a resistance style sender is Moeller’s reed switch based sender. A magnetized float slides on a tube actuating reed switches that create a specific ohm reading.

Capacitance senders
This type of sensor measures the capacitance between two probes. As the fuel rises, the capacitance between the tubes increases, and vice versa. In the case of the Hardin Marine sensor, the sensing is done inside the tube, which makes it nearly immune from fuel level changes caused by sloshing fuel. The electronics potted inside the sender read the capacitance values, and converts them to readings of 33 to 240 ohms used by standard marine fuel gauges. This style of sensor is about 99 percent accurate.

Ultrasonic senders
This technology, like the capacitance based sender, offers very precise level sensing. Fuel sloshing around inside the tank is digitally smoothed to produce a solid level reading. Units like BEP Marine’s TS1 are a good option for larger tanks. Most of these require some amount of advanced programming done by the manufacturer. In BEP’s case most users accomplish this by filling in a worksheet with the tank dimensions when they order. For more complex tank shapes, an optional programming interface kit is available. This technology is also ideal for scenarios where access above the sender is not possible without heroics since it does not protrude notably inside the tank.

Pressure sense senders
The last option is one that is more commonly used in industrial applications, but still works well for power voyager’s fuel tanks. An example of this unit is Maretron’s FPM100. The pressure transducer is mounted through the wall of the tank at the bottom, and it is suitable for many fluids, including diesel. The data is transferred via a NMEA 2000 network.

No matter what sender you’re using or its precision, if it’s attached to a standard analog fuel gauge, you will never have more than a general sense of how much fuel you actually have, and no sense of how fast you’re using it. To make things a bit worse, not all senders sold for marine use are exactly 33 to 240 ohms, and corroded sender connections can increase the resistance making it appear as if you have less fuel. Some standard fuel gauges are also more susceptible to voltage changes than others, creating reading errors.

A good capacitance or ultrasonic sender when combined with a device like the CruzPro digital gauge are your best options to getting reasonably accurate information you can really use from your fuel tank system. Fuel monitoring systems like Floscan weren’t discussed here because they can’t provide real-time tank level information, but these systems are also part of the bigger picture for the serious power voyager.

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Bill Bishop is a marine electronics installer based in Sarasota, Fla. He also writes an entertaining blog about his installation experiences called The Marine Installer’s Rant: themarineinstallersrant.blogpot.com.

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