posted 06-01-2006 08:20 AM ET (US)
I believe the Flo-Scan and the Navman are very similar. The type of error that could result in inaccurate tank level is a very simple one: the operator enters incorrect data when adding fuel to the tank. Perhaps an example will better show the problem.Assume you have a fuel flow device such as a Flo-Scan. You have it corrected calibrated and the current level of fuel in the tank (actual) matches the level computed by the Flo-Scan. Next, you add fuel to the tank. You add 40-gallons. You must inform the fuel management computer of this addition. Here we assume an error is made. Instead of entering 40-gallons as data for the computer, a incorrect button push causes 50-gallons to be entered. The fuel management computer now thinks the tank has ten gallons more fuel in it than the tank actually has.
Next, at some point the variance between the actual tank level and the computed tank level becomes noticeable. How can the tank be re-calibrated to proper level? You have two choices:
--empty the tank and add a known quantity of fuel
--fill the tank and assume the tanks contains its rated capacity
The option of emptying the tank can be awkward in some situations. As a general operating procedure, running a fuel tank until empty does pose some inconvenience. Typically you'd would not do this unless you had some alternate fuel source available to restart your engines after the main tank was out of fuel. There may also be some concern about the last gallon of gas in the tank--does it contain more impurities from sediment that is on the tank bottom?
Filling the tank can also be awkward--the expense of buying all that fuel! Calibrating the fuel computer this way can also build in errors, as the tank may not be able to be filled to its rated capacity. The angle of the tank and the location of the vent line may prevent the tank from being filled to completely full volume.
As I used my boat last season with a Navman fuel computer, by the end of the season it was apparent that I had more fuel in my tank than the fuel management computer had calculated. There could have been two causes for this error:
--the flow calibration was in error, and the computer was overstating the amount of fuel used. This eventually accumulated enough error to become noticeable. After consumption of 500 gallons of fuel, an error of only 1-percent in the flow measurement could lead to a 5-gallon error in the computed tank level.
--the vessel operator made an error in entering data when adding fuel to the tank, e.g., 50-gallons was added but the fuel flow computer was told that 60-gallons was added. This introduces a ten gallon error in the tank level.
My anecdotal experience is similar to Tom's, but the error was in the other direction. By the end of the season, my fuel tank clearly had more fuel in it than the computer was calculating. The computer calculated a very low level of fuel in the tank, yet the mechanical fuel level indicator (which directly measures the tank level) was showing a level near 1/4-tank.