SCUBA, "Sea Hunt," Dumb buoyancy question.

[230RN]

Caught a bit of the old TV show "Sea Hunt."  Didn't catch the rest of it but Mike Nelson was shown handling some air tanks underwater and remarked in the narrative that now that they were empty, they had become bouyant.

I know there are some variables involved, but is it possible to stuff enough air into those tanks to make them sink, then float when empty?  I don't think depth would matter  --it would seem the water  density would not change much at different depths.

I always thought the tech stuff on "Sea Hunt" was pretty accurate, but this sounded wierd.  It would seem that the tanks themselves would have to be pretty light in the first place, and enough air compressed in them to make them weigh more in total than an equal volume of (sea) water.



Terry, 230RN

[bedlamite]

https://www.quora.com/Can-you-measure-the-weight-difference-of-an-empty-vs-full-scuba-tank

[MikeB]

The TV show is partially wrong and you are hitting on it. The volume doesn’t change when you add air, but the weight does. The tanks should sink either full or empty, but should sink faster when full since the compressed air will add weight. It sounds like the show was mistaken or not doing a good job of differentiating between more and less buoyant.

[makattak]

https://www.trails.com/facts_22572_does-scuba-tank-not-float.html

I had a different snarky link.


However, from the article, a filled tank weighs 6 lbs more than an empty one. While it says tanks sink whether full or empty, it is possible that older tanks were made so they could float while empty. Any perspective from long-time divers on that?

[MikeB]

https://www.trails.com/facts_22572_does-scuba-tank-not-float.html

I had a different snarky link.


However, from the article, a filled tank weighs 6 lbs more than an empty one. While it says tanks sink whether full or empty, it is possible that older tanks were made so they could float while empty. Any perspective from long-time divers on that?

It’s been a long time since I was certified or went diving, but now that I think about it more the Aluminum tanks may float when empty. I think the steel sink, but have some buoyancy at least compared to full.

[Ben]

Most aluminum tanks start neutrally to slightly negatively buoyant when full, and end a few pounds positively buoyant when empty.  Compact, high pressure aluminum less so, standard aluminum more so. Even varies by manufacturer.

Steel tanks, especially high pressure steel tanks (not manufactured during the Sea Hunt era) will start several pounds (maybe up to 10-15) negatively buoyant and end a couple pounds negatively buoyant.

Aluminum, steel, high pressure tank, low pressure tank, size (e.g. 72 vs 80 vs 120 cuft) all make a difference and have varying results. I can't recall if aluminum tanks were around during Sea Hunt. If they were, the show was right. If they weren't and they were using 2200psi steel, the show was wrong. Though I've watched the whole series, and they got more stuff right than wrong.

[230RN]

From bedlamite's link:

Pardon my hand as I didn’t want the inside of the valve wet. Both times I’m only holding the tank by two fingers. The tank at 1000PSI on the left floats and is a bit touchy to keep upright. The tank on the right; if you look at my pinky for a water line reference; the tank would readily sink if I let it go.

So first of all, there's a significant weight to the compressed air in a full cylinder and the 3000 psi one would sink and the 1000 psi would apparently float. There was a 1.5 pound weight difference between 1000 psi and 3000 psi tanks.

But according to other remarks above, steel ones would sink.

I therefore suspect that the weight of the tank itself is a major factor in whether a given tank would sink versus float when full versus empty.

The weight of the tank is dependent on construction --steel versus aluminum.

So it would appear that the "Sea Hunt" folks were using lightweight (possibly aluminum) tanks.  I doubt that carbon fiber tanks existed back then, but I guess fiberglass ones might have.

The main thing here is that there is indeed a substantial weight to the air in a full tank... enough to make a difference in buoyancy, that is whether the tank would float when empty, depending on tank construction.  I note there are interesting facts salted throughout the series, so I reckon this is one of them.

So thanks,

Terry

[dogmush]

Not that it matters to the SeaHunt discussion, but they make Carbon Fiber air tanks now.  (Actually CF and fiberglass wrapped aluminium)  They go to 4500psi, which is higher then most SCUBA tanks, so it might change the buoyancy question.

That's what fire fighters use in their air packs, and I've seen them on divers as well.

[Ben]

BTW Terry - any info on what that episode was about? I have all the DVDs and now am curious to watch the episode.

[230RN]

I had to do something else so I missed most of it, and I could not capture a season/episode from the TV listings.  I came in just as Mike was handling the tanks and made the bouyancy remark in the narrative.  A group of them had to stay underwater while a storm raged above, and they were almost out of food (narrator remarked it may be their last meal) and one of the guys started uncontrollable shivering from the cold.  Scenes of high ocean waves.  That's it, I had to go somewhere.

Terry

[TommyGunn]

My father was U. D. T.  in Korea and brought back the steel (1950s era) scuba tank.  For years it was stowed in the attic of our house.  NO WAY would that tank float, empty, full, ornot.

When SEA HUNT was being filmed, actor Lloyd Bridges got tired of lugging around the double tank unit (a rare set-up in real life) which were pretty heavy.  The prop department made up a dummy scuba outfit.  They DID actually float --- all the time.  In some scenes when Bridges is on the surface you can actually see the tanks trying to float up.

With modern alloys and designs,  I think tanks are now lighter and stronger.  When I went on a Caribbean Cruise @ 20 years ago I did some scuba diving and used some modern stuff.  Lighter for sure than my father's old tank, but I rather doubt it would float on empty ....