The digital dipstick

Knowing how much fuel remains in the tank is as useful as knowing what’s left in a checking account.

In both cases, too much optimism is far worse than an underestimate, and recent breakthroughs in sender and processor design make fuel gauge technology worth a new look. Every trawler owner and died-in-the-wool sailor has heard the workboat skipper’s rant about why they favor the utter simplicity of a dipstick, top tank sight glass, or the bad old days when tank regulations allowed for bottom fittings and a clear sight tube was set up along side the tank. Today, more sophisticated sensors and NMEA 2000 linked readouts are more accurate than ever. However, unlike measuring water in a well, tank shape, vessel motion and the idiosyncrasies of different sampling equipment can influence gauge readings.

Regardless of the type of measuring equipment used, tank shape compensation is an issue. In essence, unless the sides are parallel and the bottom flat, the relationship between fuel volume and surface height will not retain a linear relationship. This non-linearity means that at the fuel level halfway point, there may be more or less than half of the total volume of fuel remaining in the tank. The more triangular a tank shape happens to be, the less fuel is left at the half-height point. So, when calibrating a dipstick, a set of volume marks for (F, 1/4, 1/2, 3/4, E) can be set up by incrementally filling an empty tank and correlating hash marks on the dipstick with gallons added to the tank. The same principle holds true for gauge level tweaking, and though far from high tech, this is a still, accurate and reliable approach to correlating gallons remaining to level measurements indicated by a gauge.

The original electric fuel gauge technology was based on resistance type senders that incorporated a float connected to a wand-like arm that rose and fell actuating a variable resistor. Changes in ohms (33-240 or 10-180) induced changes in the circuit voltage. The meter reporting fuel in the tank was scaled from empty to full, but actually measured voltage change. The lever arms in these systems rose and fell like a railway traffic stop. Problems with this technology ranged from deterioration and the loss of buoyancy of the float, to broken linkage due to fuel slosh and fatigue. Another issue was the voltage shift from 12 VDC to 14 VDC (when the batteries are being charged) which could shift the fuel gauge reading by 10 or 15 percent. Better structural longevity was found in vertical tube and float collar assemblies that used reed switches to sense fuel level. Groco, Plastimo, Teleflex, Vetus, Wema and others now offer a wide range of electrical gauges and senders. New Zealand’s CruzPro equipment can even learn the shape of the tank and calculate volume influenced by shape changes.

Air pressure

Pneumatic sensing, like that done by the well proven Hart Tank Tender, measures the effect a column of liquid places on a tube of air by using a sensitive pressure gauge. In order to take a reading, a tiny, manual air pump is used to send air through a tube that runs all the way to within an inch from the bottom of a tank. Excess air bubbles rise to the surface, but the air in the tube is affected by the weight of the fuel column which increases the pressure. A sensitive gauge reveals the pressure changes and variations in fluid volume can be easily tracked. The direct relationship between air pressure and fuel volume reflects the same non-linearity that causes a skipper to calibrate a dipstick. Once a table has been developed that relates pressure readings to fuel volume, a quick pressure check and look at the offset table reveals the remaining amount of fuel. This isn’t the only accurate remote sampling measuring process, but it does so without the need for electricity nor any moving parts in the tank.

Fireboy-Xintex produces a pneumatic tank gauge that’s even more automated, and eliminates the pump-up process by placing a tube, with a built in pressure sensing device, directly in the tank. Time will tell as to whether or not it’s as reliable as the venerable Tank Tender.

Perhaps the most dramatic departure from traditional “stick the tank” monitoring is BEP Marine’s ultrasonic depth sounder-like tank monitor, the TS1. Its low profile transducer is placed on top of the tank, and when connected to a 12 or 24 VDC power source and fuel gauge, an ultrasonic pulse is generated to measure the fuel surface height. The processor is fed data to correlate fuel volume and fluid height information. Not only are there no moving parts, but there’s no need for another hole in the tank and a software program allows a user to input tank shape parameters and select output to display on a network. The meter’s accuracy is in the plus or minus 5 percent range and the electrical current draw is minimal.

Other options

Measuring fuel flow is a useful, albeit a round about way to calculate fuel left in the tank. With equipment such as FloScan’s CruiseMaster, the diesel actually burned in the engine can be recorded. This approach requires two impeller-like devices to be plumbed into the fuel system. One is installed downstream of the primary filter, and the other is placed in the fuel return line. The latter is needed because diesel fuel is used to cool the high pressure fuel pump and much more goes through the pump than is burned in the engine. Quantifying this volume and automatically subtracting it from the total flow is one of the measurements done by FloScan systems. The result yields data such as the rate of fuel being consumed (GPH), total gallons consumed and even the ability to view gallons remaining in the tank and the distance to “empty” at a given speed. This is more than a fuel gauge feature, it’s an opportunity to fine-tune performance and maximize range.

Because one impeller is directly downstream of the primary fuel filter, any slight air leak or restriction can induce air bubbling and cause cavitation resulting in misleading flow readings. This can result from the use of too fine a primary filter (2 or 5 micron). The manufacturer recommends a 30-micron filter, but also endorses the use of 10- or 20-micron filters. Greasing filter O rings before reassembly will also lessen the chance of unwanted air entry into the suction side of the fuel system.

Last but not least is the value of an engine hour meter and regular entries in an engine log. Simply by listing each fuel fill up alongside its date, and the gallons taken aboard gives a skipper a chance to compare this data with the time the engine has been running. Naturally, the power voyaging boat or motorsailer that is run at a consistent cruising RPM for protracted periods is best suited for such simplified fuel consumption calculations. Theoretically, the RPM remain consistent and a gallons per hour number is well known from prior trial and error. This approach will provide a ballpark idea of the volume of fuel being used, and if the gauge sender has given up the ghost, and the tank does not incorporate a dipstick, the hours times gallons per hour estimate may be all that’s left.

Keep in mind that all of these measurement approaches are based upon a level trim perspective, and many power cruises, when underway, assume at a bow up angle that’s considerably different. Tanks are set up in a static position and as the waterline trim changes so does the fuel surface in the tank. For example, a long, narrow, shallow tank, most prone to such anomalies, may read half full at rest, but switch to three fourths full as soon as the vessel gets underway. Knowing when a gauge reads most accurately is very important and from that point it’s quite easy to develop a simple table of offsets to compensate for the inaccuracies caused by trim change.

Ralph Naranjo is a freelance writer and photographer living in Annapolis, Md.

By Ocean Navigator