Because most tanks are aluminum or steel, you can't see the liquid level, so you can't visually assess how full the tank is. There are a few companies that make fiberglass tanks where you can see the level, but they have been problematic with a number of recalls, and some refill stations won't fill them at all. I've also heard that some countries won't allow them to be filled.
A pressure gauge doesn't help assess how full the tank is either. Because of the whole vaporization process in the tank, the pressure remains pretty much constant right up until the tank is empty. So a pressure drop is kind of a sudden-death indication that the tank is empty, just like seeing your grill flame go out.
Home grills often indicate the tank level by means of a weight scale. The tank is held by a sprung mechanism with a pointer indicating the tank level as its weight changes. But on a boat, the tank is secured in one way or another, so a scale won't work. And for the same reason you can't easily pick them up to feel how heavy they are.
But recently, a few companies have started selling tanks with an actual float gauge built into the tank valve. U-Haul carries them, and I tracked one down at the local U-Haul in Seattle and decided to give one a try. It was a standard gas grill size tank, and I watched the gauge go from empty to full as the attendant filled it up. I'd say the gauge was not exactly a proportional representation of how full the tank was, but it was a whole lot better than sudden-death.
I figured I had this problem solved - at least until I got back to the boat. It turns out my tanks on the boat are a different shape than the standard grill tanks, standing a little taller and little narrower. I couldn't fit two of the new tanks in my propane locker, and they wouldn't work with the lock-down brackets in the boat. And so began my unexpected education in propane tank valves, valve manufacturers, dip tubes, and dip tube lengths.
What made the most sense was to replace the tank valves on my tanks with valves that included the float gauge.
For obvious reasons, tanks and valves are highly regulated with requirements to meet a variety of standards, be stamped with the approval numbers, inspection dates, etc. Valves are all equipped with a dip tube that is used to prevent over filling. There is a little bleeder screw on the side of the valve that opens the dip tube to the atmosphere. This is opened while the tank is being filled, and vapor will come out of the bleeder while the tank is being filled. But as soon as the liquid level reaches the bottom of the dip tube, liquid will start spitting out, and you know the tank is full. Tanks are all stamped with the required dip tube length. For example, a 4" dip tube will start to spit when the liquid is 4" below the valve neck.
To retrofit my tanks, I needed to find the gauge valve vendor, confirm I could get them with the correct dip tube length, then get a certified propane shop to order the valves, swap them onto my tanks, and re-certify the tanks. I had been working with Sure Marine in Seattle on some other projects, and asked them if they could do the work if I could find the valve. They said sure (no pun intended).
Rochester Gauges in Texas makes the tank valves with float gauges. But the more common gas grill tanks use 4.0" dip tubes, where my tanks require 4.6" dip tubes. That posed a complication. Rochester did indeed offer a 4.0" valve with gauge, but the only other one was 4.7" designed for a 30lb tank (mine are 20lb). A quick check with the folks at Sure Marine confirmed that using a LONGER dip tube is ok, but not a smaller one. With a longer tube, the tank will just read full a little bit sooner, so you lose a bit of capacity. But it's perfectly safe. If you used a shorter tube you would be over filling the tank, and that's not good.
So I decided to proceed with the 4.7" dip tube valve, and had Sure Marine order two for me. When they came in, I dropped my tanks off and they swapped the valves and re-certified and filled my tanks. And now, finally, I have gauges on my propane tanks that at least give an idea when they are starting to get low. I don't remember the exact cost, but I think it was about $50 per tank, much less than replacing the nice aluminum tanks that I had.
|Float gauge indicating actual (approximate) fuel level|
One interesting thing is that there is no mechanical coupling between the float mechanism in the tank, and the gauge on the outside. This makes sense from a fuel/leak containment perspective. The gauge just clips onto the valve body, and I expect uses some sort of magnetic coupling between the indicator needle and the float.
|Gauge just snaps onto valve body, presumably with magnetic coupling to float|