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Author Topic:   Direct versus Indirect Fuel Level Measurement
jimh posted 06-01-2006 09:07 AM ET (US)   Profile for jimh   Send Email to jimh  
[This topic was introduced into another discussion but has been separated into its own thread.]
DjPro posted 05-31-2006 01:29 PM ET (US)     Profile for DjPro  Send Email to DjPro     
I had situation/frustrations last year on my 1979 OUTRAGE V-20 [with the direct fuel tank level indicator]. So this year I installed a Flo-scan gauge (5500) and love it. I know exactly how much fuel I have in the tank and can monitor my fuel consumption and make RPM/trim adjustments as necessary. Material costs under $300 and a weekend afternoon of labor to install. Now I use the old fuel gauge as a manual backup.
jimh posted 05-31-2006 08:22 PM ET (US)     Profile for jimh  Send Email to jimh     
Fuel management devices like the Flo-Scan or the very similar Navman 3100 Fuel gauge do not make direct measurement of the level of fuel in a tank. Devices such as this rely on the diligence of the vessel master to perpetually enter the correct data into the fuel management computer. Tank levels computed by these devices are only as accurate as the data entered by the vessel master.

Once an error has been made in the data entered for the fuel system computer, it can be expensive to correct. The only reliable method of re-calibration is to fill the tank to the brim and reset the level. With even a modest 77-gallon tank, the cost can be over $200 to correct such an error (at today's fuel prices of near $3/gallon).

Having used such a device for one full season of boating, I would never rely on it to tell me the tank level without some independent confirmation from a gauge that makes a direct measurement of the actual tank level. One erroneous button push on a Flo-Scan and you could very easily be fooled into thinking you had significantly more fuel in the tank than the actual level.

Tom W Clark posted 06-01-2006 12:49 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
I have a FloScan in my boat and I rely upon its highly accurate tabulation of fuel burned to let me know how much fuel I have left in my tank. The accuracy of the FloScan is amazing.

I am unfamiliar with the Navman instruments, but with the FloScan fuel totalizer, the only mistake you can make is to "zero" it unintentionally. I only zero the instrument when I fill the tank (as I did last Friday, 328 dollars ago.) The chances of both doing this inadvertently AND not noticing it is extremely slim.

How do I know what my own fuel tank gauge is telling me? I have learned the hard way. It's a part of getting to know your new boat. It was just last Memorial Day, 2005, 10:00 PM after crossing Puget Sound and knowing I has low on fuel that I lost one engine about a quarter mile outside of the Shilshole Marina breakwater. The other motor continued until I was just inside of the breakwater buy still 100 yards short of the ramp.

There was NOBODY around at that hour and the wind was pushing me past the ramp. No paddle on board but I quickly figured that the teak hatch over the stern fishwell would do as a substitute. It did. I paddled my 5000 pound boat those last few hundred feet and all was well.

I was VERY lucky as I had covered a lot of ground over that weekend. I could NOT have run out of fuel any sooner and avoided serious trouble.

Having a totally empty tank allowed me to very carefully note the true "useable capacity" of the tank. I have since kept a very close log of fuel added and fuel burned. I now know what my fuel gauge is telling me.

swist posted 06-01-2006 08:14 AM ET (US)     Profile for swist  Send Email to swist     
And to make things worse, your standard "variable resistor at the end of a float arm" tank pickup is not an ideal design for a boat - too many stresses. (I went through 3 of them in the 11 year life of my last boat). But I've never been able to find anything a little higher tech.
jimh posted 06-01-2006 08:20 AM ET (US)     Profile for jimh  Send Email to jimh     
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.

DjPro posted 06-01-2006 09:05 AM ET (US)     Profile for DjPro  Send Email to DjPro     
Thankfully my 5500 doesn't tell me how much fuel I have left but how much I have used, this all but eliminates the issue with captain error with the exception of as previously mentioned the gauge is zero'ed out inadvertently which is unlikely.
JMARTIN posted 06-01-2006 03:17 PM ET (US)     Profile for JMARTIN  Send Email to JMARTIN     
Well, I am not using all of my Navmans ability yet, but I like it. When I get to the fuel dock, I ask the Navman how many gallons it thinks is left in the tank. Let's say it says 20 galons left. The tank size is 77 gallons. I then fill the tank and it the pump says I put in 56 gallons. So I actually had a little more in the tank than the Navman thought, close enough for me. I then tell the Navman that the tank is "full" at 77 gallons and start back over. The one gallon error does not accumulate this way. This only works if you fill the tank to the fuel vent spitting level each time.

There is a way to calibrate the error out. I think you tell the Navman the precentage that it is off. I still have the manual guage. It will show almost full at the gas dock, and 1/2 full at cruise, and when you are about 1/2 full at stop, it will show 1/4 tank left. I also have a rough idea of how much is in the tank by my log. I write down how many hours are on the hour meter when I fill up. If I screw up the Navman, I can always do a guesstimate (my own word} by determining how many hours I have ran at about 11 gallons an hour consumption.


andygere posted 06-01-2006 04:41 PM ET (US)     Profile for andygere  Send Email to andygere     
The way I avoid making a mistake with my fuel flow meter (Standard Horizon) is that I always fill the tank when I gas up. Then I simply set the meter display to read 77 gallons. I have found that after the initial calibration, the meter was extremely accurate. The beauty of it is that you can set a low fuel alarm at whatever reserve level you want. This helps to avoid those unpleasant situations where one must improvise a paddle out of a fine piece of yacht hardware to avoid foundering in one's own marina. It is worth noting that I've been teased by more than one individual for keeping a proper paddle in my Outrage 22, but I still think it's a good idea, reliable fuel metering not withstanding.

I do not think topping the tank off with each fuel stop adds any real expense that would not ultimately be realized. The one exception to that would be someone with a disposable Whaler. Those are single-use boats, and when the fuel is gone you simply throw them away.

I am lucky to have a Whaler that has a pretty reliable and repeatable mechanical fuel gauge. I consider the mechanical gauge to be the back up device, since it is prone to some fluctuation with the attitude of the vessel, both afloat and on the trailer. I have found that since I began adding fuel based on the measured volume used from my fuel flow meter, I no longer spew gas out the vent when filling up, because I'm able to slow the pump down within a few gallons of the expected total fill up.

Buckda posted 06-01-2006 04:51 PM ET (US)     Profile for Buckda  Send Email to Buckda     
I'm with Andy.

Fill the boat up every time and don't worry about it.

I also do this with the truck...I fill it full every time and log the mileage, fuel, price, etc.

In the boat - it may sit all winter nearly empty or nearly full...but it gets fresh gas in the spring no matter what, along with a full tank.

I've towed the boat with an empty tank and a full one (63 gallons) and it makes little/no difference in my economy in the tow vehicle.

Buckda posted 06-01-2006 04:52 PM ET (US)     Profile for Buckda  Send Email to Buckda     
But then again, I have two engines AND I carry a paddle.


JMARTIN posted 06-01-2006 04:56 PM ET (US)     Profile for JMARTIN  Send Email to JMARTIN     
There is another variable in this equation. How accurate is the meter on the fuel dock? The weights and measure people hardly ever check fuel docks because you check the meter with a calibrated 5 gallon can. You do a high speed 5 gallon test then a slow speed 5 gallon test. It is usually a long ways to carry the full 40 lb. can back to the fuel tank on land to dump it back in the tank. It is mush easier to check gasoline stations.

If a meter is going to throw over, give you fuel, it will do it at slow pumping speeds. I try to fill my tank real slow if possible. John

jimh posted 06-01-2006 05:22 PM ET (US)     Profile for jimh  Send Email to jimh     
I guess I am the odd man out at the fuel dock. I don't normally top off the tank, unless I have a really long haul and want plenty of reserve. I try to estimate the fuel usage in advance so I can ultimately load the boat on the trailer with the tank at 3/8-th-full or lower (in order to hold the weight down on the trailer and to not have a lot of fuel sloshing around the tank while towing on the highway).

This abhorrent fuel management practice makes it harder for me to re-calibrate my fuel computer using the fill-to-the-brim method.

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