Fuel Gauge Calibration

[knitchie]

Returned home yesterday with the fuel gauge on E, so we milked it for the last few hours, sailing as much as the fickle wind allowed (after all, it is a sailboat, right). At the fuel dock, I expected to easily add 15 gallons to the 17 gallon tank, but at 14 gallons the tank was full and the gauge showed 3/4.

A quick search here didn't find any threads related to poorly calibrated fuel gauges. Of course, this isn't the worst problem to have, much better than showing 1/4 full on the gauge when the tank is empty, but is there a way to test and recalibrate the sending unit and gauge so they read more accurately? Or, am I in the live with it or replace it arena?

Fair winds,
Ken - Obsession
C30 TRBS - #0973
Lake Champlain, VT

[David Masury]

I have never had a fuel guage read correctly... The length of the swing arm usually cannot be set to the right length because there may be baffles in the tank, or you cannot get the unit into the tank. I try to make the height of the sending unit mid depth so that half a tank indicated is a good reading. Thus like your guage, when showing empty you may have a gallon or so left...so of like reserve. Full is full and may show up like that until you use a bit of fuel.

David

[Kurt]

I've got Fuel Gauge problems too

Since I've owned my boat, my fuel gauge appears accurate until it gets to the half tank mark, then it suddenly plunges to empty. This freaked me out the first couple times it happened, but now I know I've got about 9 gallons left when it does that. However, as 9 gallons lasts so long, it's hard to keep track of exactly how much fuel I have left a couple weeks into looking at a gauge that reads empty. Any ideas on a possible fix for this situation? Maybe I need a new tank sensor?

[David Masury]

Kurt, try taking your sensor out of the tank, and measure the depth of your tank and see where the arm is located on the staff. it should be half the depth of the tank when installed....

You can try moving the arm when you have it out and see how your guage responds.... do have something over the sending unit hole.

If your are going to replace the sensor, make sure it is the same brand as the guage... or replace both. They are not expensive units. The ones from teleflex are the most common, but usually need a bit of trimming to make fit in some tanks.

David

[msauntry]

From what I understand, the gauges operate on resistance to tell the sender the level. If you look at sending units, they have a little rheostat that the float arm usually slides on. When shopping for them, you will see things like
"240-33" ohm senders. When full, it reads 33 ohms and when empty it reads 240 ohms. As the float arm slides down the rheostat, resistance increases.
If the wires or connections get corroded, it will also increase resistance and make the sender think the tank is more empty than it really is.

My gauge does the same thing now. Worked great when new, but over time it starts to read short. I'm gonna try to clean up the sending wire and see if that gets it back on track.
Hope that helps.

[knitchie]

Fuel Gauge Calibration

Thanks everyone. I don't know how to calibrate anything yet, but with this info, I think I have the ideas for diagnosing whether this is a sender, wiring and/or gauge problem.

While tank is at a reasonably well known fuel level:
1) Test resistance at fuel tank
2) Test resistance at gauge
3) View gauge output

If 1 is significantly different than expected, looks like a sender problem
If 1 differs significantly from 2, then looks like a wiring problem
If the reading in 2 doesn't match the gauge output, then may be time for a new gauge

Of course, all three conditions may exist.

Messing with boats sure is fun, as long as it doesn't get in the way of sailing!

[Jesse Delanoy]

I've never had a fuel gauge on a sailboat that has been accurate enough to be particularly useful. I find the best method of keeping track of my fuel supply is to carefully monitor my engine hours, and my fuel consumption per hour (it's running about three quarters of a gallon per hour on average, depending on how hard I run the engine).

I installed an hour meter a couple of years ago when I replaced my A4 with a rebuilt engine - one of the best modifications I've made. I log my hours everytime I gas up (keeping track of dates, gallons of gas and ounces of Mystery Oil) and every time I change the oil.

[High Hopes]

I dip a stick into the fill tube and measure wetness. This used to be standard operating procedure for truckers years ago. You can even score the stick to callibrate it.

[Chris T]

Poly propolene Tank

I also have a inoperable gas guage. However I monitor my gas consumption by placing a bright flashlight up aganst the side of the Propolene tank. The bright light is enough to show the level of fuel in the tank. Measuring the hight of the tank 8.5 inches times 2" (equals about 2.1 gallons) gives me about 17.85 gallons of fuel in an 18 gallon tank. I mark the side of the tank
into incriments and this is the end of the story. Works well. But if your have a metal fuel tank it won't work. Chris

[wrapper]

Hobbs Meter

I have been depending on my Hobbs meter to measure hours since last fill up. I typically burn 3 liters per hour and get concerned when I hit 12 hours.

I have been looking for one of those temperature sensitive tape for use on BBQ propane tank to try this on my gas tank.