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Fuel Tank Level Measurement

by James W. Hebert

An overview is presented of methods to measure and display the level of a fuel tank on a small boat.

Direct and Indirect Measurement

There are two general techniques for fuel tank level measurement: direct and indirect. In the direct method, the tank level is observed or indicated directly by visual, mechanical, electrical, or electronic means. In the indirect method, the fuel tank level is deduced from monitoring the flow of fuel from the tank and subtracting that volume from an initial tank volume.

To measure a tank level directly there are many possible methods. The simplest is perhaps to have a ullage pipe and a sounding rod which has been calibrated for the tank. A sight-glass is also possible, although on boat fuel tanks this method is usually limited to on-deck tanks made of plastic with a transparent tank segment. More common on boats is the use of a float level sender.

Float level senders can take many forms. Some float level senders are mechanically or magnetically coupled to a dial. The most common is a float linked to a variable resistor arranged as a two-terminal resistor or rheostat. This method is so common that a standard resistance range has been established and is in very wide use. An electrical current is passed through the resistor; the current varies in proportion to tank level and is typically indicated on a remote milliammeter whose scale has been calibrated. More sophisticated level sensors have been invented, using a variety of electrical and electronic properties to indicate the tank level.

To measure a tank level indirectly the general approach is to measure the flow of fuel from the tank. The fuel outflow is accumulated and the volume is deducted from the tank volume. The measurement of the fuel outflow can itself be measured either directly or indirectly. Direct measurement of fuel outflow can be accomplished with a fuel flow sensor inserted into the fuel hose connecting the engine to the tank. Indirect measurement of fuel flow can be accomplished by the engine consuming the fuel; modern engines tend to know precisely how much fuel they are using due to their sophisticated electronic control of their fuel injectors.

Fuel Remaining

Fuel tank level is only an approximation to the actual volume of fuel remaining in a tank. In many V-hull boats the fuel tank geometry is not uniform, and the lower portion of the fuel tank typically has a v-shape. Also, when a boat is underway it is typical for the hull to have some trim other than perfectly level, causing the distribution of fuel in the tank to be non-uniform. These factors cause a fuel tank level measurement to become less accurate as a fuel remaining measurement. Fuel remaining in a tank as computed by the indirect method is not affected by variation in tank geometry or trim.

Calibration

Fuel tank level sensors and their associated gauges require some sort of calibration to establish an accurate fuel tank level. For the more common resistive sender and remote meter system, there is seldom any sort of individualized calibration possible. The calibration is typically fixed by the marking on the meter face of the remote indicator. The more sophisticated level sensors may include control software that permits a user to make a unique calibration for their individual tank by which the sensor signal is calibrated directly in terms of fuel volume remaining. During the calibration process the fuel volume in the tank must be accurately known from an independent method. Usually the tank is made as empty as possible, and fuel is added using a calibrated flow sensor, typically the flow sensor on the retail fuel pump that is dispensing the fuel. Once the calibration process had been accomplished, no further calibration is needed. The tank level sensor directly monitors fuel level and the calibration process converts this to a tank level and a fuel volume remaining.

In the indirect method an on-going calibration is needed. The indirect method depends on the fuel level in the tank being known accurately at some starting point for the fuel flow accumulation to begin subtracting volume from the tank. Typically this is accomplished by filling the tank to its rated capacity and informing the fuel volume instrumentation that the tank has a certain fuel volume. From this initial value the indirect method begins to subtract fuel volume as it flows to the engine. When any fuel is subsequently added to the fuel tank, the fuel volume instrument must be adjusted to incorporate the added fuel into the fuel volume remaining calculation. If an error is made by the operator, the accuracy of the computed fuel volume remaining is lost. Fortunately, the most common error made is to completely forget to inform the fuel manager instrument of the addition of new fuel to the tank. This produces a non-harmful outcome: the tank level is actually higher than indicated. If an error is made in the other direction, that is, if the fuel manager instrument is told that more fuel has been added than actually was, the outcome is harmful: the tank will have less fuel than indicated, leading to a risk of running out of fuel.

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Copyright © 2012 by James W. Hebert. Unauthorized reproduction prohibited!

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Last modified: Wednesday, 16-Apr-2014 10:59:50 EDT
Author: James W. Hebert
This article first appeared January 22, 2012.