The gauge reads differently whether you're on the trailer, in the water, or on plane. After all these years, still feel the need to record the same tank fill at least on the water then on the trailer. Tried a few times in the past and just restarted. Not aware of the alarm, but 5 hours reading at 1/2 then 5 more sounds in the ballpark. Bottom line, once it reads empty on the water (idle or stationary) you've still got maybe two hours at 25 or so. I have run it out of gas once. That sucked.