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Fuel Gauge Accuracy

Discussion in 'Electronics' started by DOCKMASTER, Jun 22, 2021.

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  1. DOCKMASTER

    DOCKMASTER Senior Member

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    I'm curious what others are using for fuel tank gauges and how accurate they are? My gauges have always been horribly inaccurate. I have 3 fuel sources on my 54' Sportfish. The aft tank is a single tank under the cockpit that measures out just under 700 gals. Then I have Port and Stbd engine room tanks. Each engine room tank is actually three individual tanks connected in series that act as one tank at about 350 gallons. So my total fuel capacity is about 1,400 gallons.
    The issue is that my gauges all read very low compared to actual fuel level. For example, my aft tank reads empty after using only about 400 gallons. I have verified how much fuel is used by my digital trip fuel monitor as well as what it takes to fill the tank back up at the pump. My engine room tanks are similarly inaccurate and ready very low. All the sending units in the tanks are in the aft portion of the tank as obviously the fuel runs aft when the boat is on plane. When I drop down to idle or slow speed and the fuel levels out the gauges read even lower. But unfortunately they are really far off when I'm on plane or at slow speeds. When I fill the tanks all three gauges read full.
    I've just lived with this over the years as I'm never really pushing range. However, the engine room tanks are a bit of a PITA to fill as the fuel has to limber fwd in the tanks and unless I fill slowly it won't limber fwd and the fill hoses are in the aft end. So I would like to not have to go to the fuel dock as often. At the same time, running around with all my fuel gauges on empty just causes me stress. So, how can I get a more accurate readings? The sending units I currently have are the basic float style on the end of a rod.
  2. Pascal

    Pascal Senior Member

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    I never trust fuel gauges. Never. Ever. I don’t care how fancy they are even Headhunter pricey gauge system can be inaccurate.

    the most reliable way to know is to have a calibrated dipstick. Of course that only works of the top of the tank is accessible... Lazzara always delivered their boats with a nicely carved wooden dipstick. Ok my personal 53 Hatteras the tanks are under the companionway. I just Pull a small hatch, undo the mech gauges and put in a stick I calibrated. Accuracy guaranteed. Boats with tanks above the floors usually have sight gauges which are just as accurate.

    I also keep a fuel log in a spreadsheet on my phone. I record every trip including fuel totalizers from the Engine ECUs, distance covered (from the (GPS odometer), gen hours etc. it calculates fuel used, fuel left as well as NMPG. I m usually within 5 to 6% of actual fuel used.

    here is a screen shot

    8429DDFE-5544-4ECB-8A23-24B8B08410E0.jpeg
    TahoeJohn likes this.
  3. SplashFl

    SplashFl Member

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    I initially put one of these in the 400 gal. Bertram tank, and many years later one in the 165 gal. Contender tank. Been a long time but never had any problems with either but I think I may have had to add an extra wire to each with the install. http://www.centroidproducts.com/
  4. DOCKMASTER

    DOCKMASTER Senior Member

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    I can't get to my tanks easily enough for a dipstick gauge. Actually, not sure I could do this at all. I only have a few inches above each tank so no way to get a dipstick in there.
  5. DOCKMASTER

    DOCKMASTER Senior Member

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    Those look very interesting!
  6. SplashFl

    SplashFl Member

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    Obviously the length is critical, and then there's the relationship of the size of the top half of the tank vs. the lower half. From recollection (sold the Bertram last month) when the gauge read 1/2 it was really 3/4 full and when it read 1/4 was actually 1/2. Below 1/4 I learned to gauge the fuel based on where the surface of the water was on the transom and when it wasn't there, she was down to the last approx. 25 gallons.
    My Contender's original sending unit stuck 1/2 way down on an internal tank wall soon after her purchase leaving us floating 8 miles off Laud on one of the most beautiful days just around the time I was bragging to myself how great she was on fuel. :( I got a Centroid sender a little extra shorter then the tank depth so as to provide a larger reserve. When the Yamaha fuel gauge starts blinking there is still 30 gallons so good for approx. 50 miles in the little 25 footer. Still embarrassed from the only ever offshore stranding, I borrowed a small gasoline rated 12v. pump and confirmed the reserve.
  7. MBevins

    MBevins Senior Member

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    Look at a gauge from CruzPro(com).
    I've had their fuel gauge for many years. Use a reed style sending unit with their gauge. After installing get the tank down as low as you dare, then refill tank as per instructions. Did mine in 5 gal increments, it's easy, 5 gal push button, rinse and repeat till full.
    The wonderful thing , I'm accurate to a half gallon over 400 gallons.
    What I don't know is if one gauge will support 3 sending units via switch.
    They're based out of Australia.
  8. mapism

    mapism Senior Member

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    Sight gauges. 100% accurate, I'd never trust anything else.
    Not the most convenient system admittedly, since you must go down the e/r (or wherever your tanks are) to check, but well worth the hassle, imho.
  9. Capt Ralph

    Capt Ralph Senior Member

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    I would hope your new Cats show a fuel use log.

    One issue your having is bow angle affecting the reading. The best senders will always have this issue.
    With your lack of comments, I can guess you have never worked on these senders during your ownership.
    Without records, you may never have any service history but it sounds nill or poor at best.

    I have tried the Centroid products, What a PIA. Never again.
    All of these have been replaced with KUS / WEMA products and all are working very well years later on ours and many customers boats.
    https://kus-usa.com/products/liquid-level-sender/

    Centroid, Wema and some others do not have a swing arm and float twisting a rheostat. Probably what you have and installed poorly.
    A single vertical rod just off the tank bottom to the tank top is all there is with this newer stuff. You do need vertical clearance above the tank to install these senders.

    The best level check is with a dip and gallons per inch log starting from the bottom of the tank.
    Sadly, Not able to do so on my boat.

    Our forward tank is T shaped. When full, the fuel gauge takes for ever to make 1/2 tank, then the lower half of the tank disappears 3 times the top half rate.
    Our aft tank is long (for/aft) and skinny. Forward end is lower in the hull several inches. Fuel pickups are in the after end. Sender in the middle.
    You get nervous quickly when you slow down (lowering the bow) and suddenly one of the engines starts to stumble.
    We never run both engines from the same tank for several reasons, this is one example.
    The best fuel gauge in the world will not help this mess.
    Hours on the gen-set(s) and mains is my only unit of measure. 18 years (thousands of hours) later, just starting to get any good at this.

    Yep, I hope your fuel consumption logs in those new Cats work. :):)
  10. Capt Ralph

    Capt Ralph Senior Member

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    Another idea on many multi tank boats, Sight tubes on the tank sides.
    You can also remote mount these sight tubes but have to be at the tanks top & bottom levels.

    One more idea, You have 3 tanks. Use one as a day tank and meter what you transfer from the other tanks to your day tank.

    Sadly, Without spending some quality time and rum on your boat, These are just out loud ideas. :D:D
  11. Pascal

    Pascal Senior Member

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    That s why you have to calibrate the stick. I have a similar issue with my Hatteras keel tanks as they taper at the bottom. If a gauge or linear stick shows 1/4 you re actually at 1/8.

    first refueling, I paused every 50 gallons and marked the stick

    as I explained earlier, with flow meters and totalizers which most modern engines have, you can just keep a log. Generator burn vary with load which you can easily can an average that’s close enough. Getting within 5 to 10% accuracy is easy.

    I ve never seen a boat where the limber holes where so small that it took a long time to refuel. What you may to do is fuel the problematic tank first, then the other one(s) and top off the tank after.

    nowadays it s difficult to know when a tank is full as spilling a couple of drops will land you in jail... when I redid my old hatt I moved the fillers and vents high above deck in the house side which allows me to catch any spill. Much better than having the vent down on the hull side. Not always easy to set up though.
  12. DOCKMASTER

    DOCKMASTER Senior Member

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    Yes, the new CATS have fuel logs. Both total fuel burned and an easily resettable trip log. So far these have been spot on except obviously the additional fuel used by the gens but that is pretty minimal. Sight tubes and dipsticks just aren't an option for me, especially for the cockpit tank. The only way to access the area is to lift out the fish boxes. They are big boxes and take two people to lift out when empty. And mine are usually full of halibut and salmon :):);)

    I'm really looking to find as accurate of a gauge/sender as I can and use the CAT fuel logs as validation.

    What was the issue with the Centroid units?
  13. DOCKMASTER

    DOCKMASTER Senior Member

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    Yes, I take fuel the same way. I start with the engine room tanks. Once they start to fill the after tank in the series I move to filling the cockpit tank. After the engine room tanks limber fwd and equalize I add more fuel. I repeat this about 3-4 times and I'm full. I've managed to get this process down pretty well and don't spill any full. Partly because I've learned it is not critical to fill to the brim. As I mentioned earlier, I'm not at all pushing my limits of range so not being topped off isn't an issue for me. I would simply like to not go to the fuel dock so often. Most times I'm taking around 500 gallons. I would prefer to at least get comfortable using 700-800 gallons before feeling like I need to fuel and have gauges on empty.
  14. Pascal

    Pascal Senior Member

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    Generator are pretty consistent. I used 1.75 GPH on the 27 KWs. On my 15KW I average 1.25. So far on the “new” boat I m going to try 2.5 GPH average on the 45s

    don’t over think it. Keep a log and you ll be fine
  15. MBevins

    MBevins Senior Member

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    If you don't look at these, your making a mistake.

    http://www.cruzpro.com/f60.html
  16. DOCKMASTER

    DOCKMASTER Senior Member

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    These look fantastic but appear to be just the gauge. My issue is more related to the senders in the tank. I don’t see how these help with my issue unless I can find a sender to reasonably measure the tank level. Am I missing something with these?
  17. Metatron

    Metatron New Member

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    Have you considered ultrasonic? Basically you mount an ultrasonic transducer on the top of the tank and it measures the distance to the oil. I have used them before for measuring tank levels in a stationary environment but you should be OK on a boat if it isn't too rough.

    https://www.flowline.com/product/echopod-ug06-ug12-reflective-ultrasonic-liquid-level-transmitter/

    There are many types and manufacturers available. The above is just an example. All it requires is a 2 inch hole in the top of the tank and some wiring. I am sure there are other form factors available. They should be pretty reliable and accurate particularly if the sea is calm.
  18. Capt Ralph

    Capt Ralph Senior Member

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    You stated the your gauges read full when full, then always seem to be reading low.
    Reading into this http://www.cruzpro.com/fu60.html , the FU60 has a learning mode.
    It's more than just a display meter.
  19. Capt Ralph

    Capt Ralph Senior Member

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    Reliability and repeatable accuracy.

    Kus/Wema also has a capacitance sender if you like this technology.
  20. DOCKMASTER

    DOCKMASTER Senior Member

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    Thanks but I’m not a fan of ultrasonic. I’ve used these commercially as draft and tank level indicators on large stationary floating dry docks. Never had any luck with reliability and accuracy. I can’t imagine them in a moving boat. Plus, we’ve all been stuck in heavy weather for extended periods. Certainly can’t hope for calm water to be able to get a fuel tank reading.