Anywhere to Overfill Tanks Near the Keys?

[Addison Snyder]

Concerning the Z-factor, thanks for the ELI5.

 

[Johnoly]

So it seems to me I'm out of luck as far as a place that fills tanks worth a darn....

By "Darn" don't forget that on Nitrox fills your normal mainland fill card price of ~$7 / tank is now ~$12-15 per tank in the keys...AT 2500 PSI !!
Like everyone else said above, just borrow friends tanks & bring all your own tanks.

 

[rob.mwpropane]

1*2 does not always equal 2, sometimes it equals 1.9...

Basically the molecules can only be packed in there so tightly. Up to about 2500psi at room temperature we assume that everything is linear, i.e. 1*2=2. After that, it gets exponentially harder to compress, so even though 1*4 should equal 4, it only gets to about 3.9.
In "normal" open circuit diving it really doesn't matter because everything is gauged off of pressure.

Contrary to the point that @The Chairman made though about rebreathers, is that rebreathers are one of two pieces of dive gear that caused us to pay attention to it in the cave diving community. On normal open circuit diving, we dive to thirds. The Z-factor actually adds conservatism to the dive because the "1st third" of 3600psi down to 2400psi is actually smaller than the "2nd third" of 2400psi to 1200psi, which is slightly smaller than the "3rd third" or safety/reserve third of 1200psi to 0. Happy days.
When scooters are used though, there aren't any magic ratios that you can use because your "planned speed" is different than your "emergency speed" so everything now has to be done on gas volume. Rebreathers are the same. Now instead of the math being super easy and going "well I kicked in for 1200psi, so it will take me 1200psi to get out", it now becomes "well I kick 50fpm, and I breathe about .65cfm while I am kicking, and I am planning to dive at 100ft or 4ata's, so 4*.65=2.6cfm DAC, and at 50fpm it takes me 2 minutes to go 100ft, so I need 5.2cf/100ft. I'm planning a dive to 3000ft, so I need 30*5.2=156cf of gas to get out, plus a 1.5x reserve=234cf total. PHEW! So now the compressibility factor gets REALLY important. Someone will then say well I have a pair of LP85's and if I pump them up to 3650psi I'll have 235cf and enough gas to get out. The issue is the Z-factor at 300K/250bar is 1.067. 235/1.067 is only 220cf of gas and while they think they have enough, they really don't.

Does anyone else feel like this man needs / deserves a round from us?

I learn stuff that I didn't even know I needed to know...

And then you say, "yes, but I knew that too".... but HE took the time to post it.

Thanks!! I'm always learning something from this guy!

 

[deeper thoughts]

If the original poster is still here try scuba tec in key largo mm 99

 

[Divin'Papaw]

When I owned a couple of LP85s Force-E (Pompano, Boca, Riviera Beach) and ScubAdventures (Naples) consistently would fill them to 3000. But neither of those are anywhere close to Key Largo. I've since sold them and gone with HP100 exclusively. I get consistent full fills in them at the shops listed above.

 

[Degenerate]

Why do you guys not just use tanks rated for the amount of gas you want in them? Genuine question, because (systematic) overfills are not a thing on this side of the pond and I'm trying to wrap my head around why it is over there...

I'm also in Sweden and never heard of a shop that will overfill tanks for customers.
Lots of folks have their own compressors though, we always fill our 232 bar tanks to 270 bar, no one has ever died from having too much gas.

 

[Addison Snyder]

no one has ever died from having too much gas

<Inevitable fart joke>

Within reason, I try to give my money to those that take a pretty minimal extra risk and cost to pack tanks. From multiple principles. Pushing supply and demand. They care to help the diver beyond the normal. Stuff like that.