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Author Topic:   Potential for Error in Indirect Fuel Tank Level Measurements
jimh posted 04-05-2013 09:14 AM ET (US)   Profile for jimh   Send Email to jimh  
In a prior article

I described the indirect fuel tank level measurement method. This method is in common usage on many small boats. The accuracy of the method depends upon the diligence of the operator in entering data to the system to report fuel added to the fuel tank. It occurs to me that there is an opportunity for a systematic error in this method.

When fuel is added to a boat fuel tank which is being monitored by the indirect fuel tank level method, the operator must enter the amount of fuel added to the FUEL MANAGER function. Generally there are two ways to enter an amount of fuel added: the tank can be set to "FULL", or the volume of fuel added can be entered by the operator with a precision to typically 0.01-gallon.

If the set-to-FULL method is used, there is potential for a systematic error to occur. Let us say a boat has a fuel tank with a capacity of 77-gallons. If fuel is added to the tank until the tank appears to be full, that is, no more fuel can be added, this addition of fuel may be entered as a "full" tank. The FUEL MANAGER sets the tank level to the specified capacity, in this case 77-gallons. However, it may not be possible to actually instill 77-gallons in the tank, and the fuel volume in the tank might be lower. Let us say the fuel volume in the tank after a filling operation to the maximum possible fuel fill only produces 72-gallons of fuel in the tank. Each time the operator enters the fuel added amount by using the set-to-FULL option there will be an error of five gallons in the amount of fuel added.

Next, let us say the boat is used on a week of cruising and about half a tank of fuel is consumed each day, or about 35-gallons, and this goes on for six days. The operator refills the tank each day, and tells the FUEL MANAGER the tank has been refilled to full each time. At the end of the week, the FUEL MANAGER has been informed of six refills, but each time an error was made. Each refill only put refilled the tank to 72-gallons, not 75-gallons. The 5-gallon error was repeated six times. By the end of the week the FUEL MANAGER will be showing 30-gallons of fuel in the tank that were never added.

Because of the possibility for this error to occur, I recommend that the refill-to-FULL option for entering fuel added to the tank not be used. If the actual amount of fuel added as measured by the retail fuel pump is used for the volume of fuel added, there will not be a systematic or repetitive error. Of course, there might be an error in the measured fuel from the retail pump, but these errors ought to be small and random.

Commander Coo1 posted 04-05-2013 11:53 AM ET (US)     Profile for Commander Coo1  Send Email to Commander Coo1     
Im not quite sure I follow you that you will end up with 30 gallons less than you think you have. Each time you fill your tank and are missing 5 gallons, that's all you are missing if you truly fill the tank up until you can fit no more fuel. I dont understand how you could have a different error whether it is the first time or the 100th time. you are still referencing a full tank to the 72 gallons in your situation. Am i missing something? Maybe i use my fuel manager different, i only use the fuel used count instead of fuel remaining.
DVollrath posted 04-05-2013 01:46 PM ET (US)     Profile for DVollrath  Send Email to DVollrath     
I too believe that the largest error you would have is the delta between your stated full tank capacity and the actual amount you put in.

One way to minimize this is (at least with portable tanks) is to fill the completely empty tank to some level you can reasonably repeat each time. With my "12 gal" Mirax tanks, this is about 11.6gal. I've set my tank full level to 11.5 gal.

Jim's reasoning seems to apply more to the situation where you try to calculate aggregate fuel consumed after a number of fills by using the "full" value each cycle, and a measured tank level at the end.

contender posted 04-05-2013 04:32 PM ET (US)     Profile for contender  Send Email to contender     
This is why I like to fill the tank when you are done for the day, and have a manual gauge on the tank. I always know how much fuel I have...
jimh posted 04-05-2013 08:52 PM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the comments. I am going to review the procedure to see if the error accumulates. I'll be back later.
jimh posted 04-06-2013 08:26 AM ET (US)     Profile for jimh  Send Email to jimh     
After a fresh pot of coffee and a second analysis, I agree with the comments about the error not accumulating. I was wrong in my thinking about how the FUEL MANAGER tracks the fuel used volume. The FUEL MANAGER only tracks the fuel used volume as it flows out of the tank. It doesn't track the fuel volume as it flows into the tank. If the tank capacity is listed at 77-gallons and a "full" fill-up only puts 72-gallons in the tank, that error does not accumulate with each re-fill. It just repeats each time.

The error that occurs is the tank level indicated by the FUEL MANAGER is going to be wrong. In my example of burning 35-gallons, the FUEL MANAGER will compute the remaining fuel volume in the tank as 77-gallons minus 35-gallons or 42-gallons. The FUEL MANAGER will show this level as being 42/77 or 0.545-FULL. The tank level indicator will read slightly above 1/2-FULL.

The actual tank level will be 72-gallons minus 35-gallons, or 37-gallons. A 77-gallon tank with 37-gallons would be 37/77 or 0.48-FULL. This would be shown on a tank level indicator as slightly below 1/2-FULL.

Over a week's time the FUEL MANAGER will accumulate the fuel used value by measuring it as the fuel flows out of the tank. This volume will be as accurate as the flow measuring devices is accurate. The volume of fuel used shown on the FUEL MANAGER will not be in error due to the short re-fill volume, because those volumes are not used in calculating the fuel used volume.

The FUEL MANAGER will compute the volume remaining in the tank (Gallons Remaining) based on the tank having held a known volume at some point. From that starting point, the FUEL MANAGER will decrement the volume remaining as the fuel flows out. The effect of the error in the short re-fill is that at any time the fuel remaining value is going to be off by five gallons (the amount of the short fill).

This error can be avoided if the tank capacity is just understated in the FUEL MANAGER. For example, if a tank is said to hold 77-gallons, the FUEL MANAGER can be told the tank only holds 72-gallons. This will help make the tank level indication to more accurately reflect how much fuel remains in the tank, and it will also improve the accuracy of the Gallons-Remaining value.

The accuracy of the indirect fuel tank level method always depends on having started from a known fuel tank volume, and having an accurate flow measurement as fuel is withdrawn. I see now that there is not an accumulating error if you use the refill-to-full option. You just keep repeating the same error. The outcome would be the same if you entered the amount of fuel added to the totalizer by entering 30-gallons.

Thanks for the help with this problem.

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