Fuel Gauge Madness

L

lnj

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Joined
Feb 10, 2021
RO Number
2465
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4
My "new" '04 Bayliner 245 Ciera has a 65 gal fuel tank. I pulled it around to fuel up for the first time a few days ago when the gauge read 1/4 tank left...16 gals. I was only able to pump in 35 gal before the gauge read full for a total of only 51 gals. I know boat gas gauges are horribly inaccurate, so is this just what I should expect?
 
In my experience, the labeling on the gauge is generally useless, BUT the needle position tends to be "repeatable". A careful testing and logging program can produce a "translation chart" which should be reasonable accurate.

There are exceptions. Boat tanks ( incl fuel ) are not necessarily "regularly shaped" and the sensor is not always well positioned. The result is that the reading you will get may vary with list and/or "angle of attack". The result is that you may show ( for example ) a half tank at rest, this may vary widely indicating empty or full when you speed up and the angle of attack changes. Or when someone my size ( large ) moves from side to side. These variations can occur with any sensor. Mostly, what you want is a well positioned sensor ( any type ), then you create a calibration chart to correct for an oddly shaped tank.
 
FWIW I never trust my fuel gauges. I go based on number of hours used and my experience with hours per gallon on my boat. When the boat was new, I filled it more frequently until I get a sense of what it would use. Regardless, I use plenty of margin for error.
 
What Radioactive said. The needle position is repeatable. Always take your readings stopped and settled.(which is what you are when you are fueling) If at half you add 50 gallons, now what does it read ? Well next time you get to the half reading you know you burned 50 gallons. That's over simplified, but you know what we mean.
If you have two tanks, you can run one of them dry and read the gauge, did it show 1/4 tank or ??
 
Needle is not so repeatable. Don't rely soley on that. Needles can get stuck, move erratic from cell phones places near them. Whatever, keep a log book and go by hours. Also never fill a tank full. Always go 3/4 to 7/8. Boat next to me on the hard guy obviosuly filled for winter storage, all winter long he was leaking gas out the vent on hot days. Don't underestimate how much fuel will expand when heated, or maybe it's the tank that shrinks, I am not sure, but it will leak.

On my boat I have my guages and the EVC. I use the guages as a guide and the EVC which is connected to motor is alot more accurate.
 
My old school senders were calibrated by me when I had to replace the gaskets. They are a reasonable representation of what is remaining in the tanks.
When they say half tank, that is pretty damned close to what is remaining.(75 Gallon remaining) When they hit the big E, there is still some fuel. How much ? well how far is the dip tube from the bottom of the tank, how big is the tank, on and on. Don't care.
Seen more than one fuel tank dripping from the vent when gas expanded in the spring.
FlowScan not in my budget, too many other hobbies to put all my eggs . . . .
 
It is always possible that the gauge has bad connections or bad sender. If you feel like removing the sender Youcan test it.
 
The gauges operate like your car. Not very accurate but very repeatable that is they will show the same for the same punt of gas every time
 
Centroid's work great, I have one and love it. However, all gauge systems use resistance or capacitance to measure the level and the circuit can change values. Everything from corrosion on the terminals and in the wires to temperature can change the circuit. Even ethanol versus non ethanol gas effects these units even Centroids. A better solution is to install a fuel flow gauge and use it as primary. I've had both Standard Horizon and Garmin fuel flow units and they are accurate at sub 1% levels.
 
quote:
Originally posted by GeeBee

Centroid's work great, I have one and love it. However, all gauge systems use resistance or capacitance to measure the level and the circuit can change values. Everything from corrosion on the terminals and in the wires to temperature can change the circuit. Even ethanol versus non ethanol gas effects these units even Centroids. A better solution is to install a fuel flow gauge and use it as primary. I've had both Standard Horizon and Garmin fuel flow units and they are accurate at sub 1% levels.
Click to expand...


Agreed. I might add that my primary "guage" is, although discontinued but works great, my Navman GPS with built in fuel sensors.
 
While I would have enjoyed having flow sensors, and agree that they are a fine solution, I did not have them.

Nevertheless, the fuel gauge was always my "backup" to actually knowing what my fuel consumption was and "dead reckoning". I "knew' what fuel I expected in the tank, and used the gauge as a check of my math. My DR numbers were typically within 5%, often better. the gauge less so. Still, it was nice to have a "back-up" Redundancy is your friend.

And, yes, a system in poor condition can be expected to perform "out of spec", just as happens with most systems.
 
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