Gas Reserve when using doubles

[dscuba]

In open water class, etc. people are always told to "surface with at least 500 psi", what about when using a two-tank setup? 500 psi is, in reality, twice as much gas, right?

So, for example on a SINGLE HP120 (with 3442psi working pressure) 500 psi remaining would be ~17 cu ft [(500psi / 3442psi) * 120cuft], on doubles (the same HP 120's) there would be ~34 cu ft remaining at 500psi. If at the end of a RECREATIONAL DIVE after completing the dive plan including safety stop, might any folks out there consider going below 500 psi while puttering around in 10-15 fsw? Or is this utter stupidity? (perhaps if gauges started to be less-accurate around some threshold around this level?)

[loanwolf]

You will hear some different opinions on this. I most often will draw them down below 500 psi on solo dives or with certain buddies, depends of what we do as a dive. But always making sure that their is enough gas for surface safety. You are talking about recreational no deco diving so gas planning is not as critical as it is on a tech dive. In the not so old days some of the high preference deep regs you did not want to let the supply get below 500psi or they would free flow, never could figure that one out. I still know a few extremely deep 400+ OC-tech divers that are still using their regs that do this. But for the most part it is up to what you are comfortable with. At 500psi on T120's you are right their is a lot of gas left, 250psi would be the same amount of gas in cubic feet. Baring you do not have a reg that does not like low pressures do what you are comfortable with. But keep in mind that not all regulators scavenge all the gas from a tank so you may not get all of what is left. Some quit at just over 100psi. For training safety and insurance purposes if you are doing a class it is alway have more than 500psi when you surface. In some team diving instances they run the same standard at 500psi when you surface.

[Nwbrewer]

The reason for the 500 psi isn't as much about gas planning as it is about SPG unreliability in that range and you're reg IP. In theory in dubs you've got 34 cubic feet at 500 psi, but remember your reg needs ~150 of that just to operate, so now you're talking about 350psi. Analog SPG's are least accurate at both ends of the range. That 350 psi you think you've got might be closer to 200psi.

[Sockmonkey]

A few links written by local nw folks for your surfing pleasure:

• Lamont's Rockbottom and Gas Managment
• Nwgratefuldiver's Understanding Gas Managment (PDF)
• Edge and Bdub's Rock Bottom for Recreational Dives

Aloha hard,

-Eric

[Nwbrewer]

A few links written by local nw folks for your surfing pleasure:

• Lamont's Rockbottom and Gas Managment
• Nwgratefuldiver's Understanding Gas Managment (PDF)
• Edge and Bdub's Rock Bottom for Recreational Dives

Aloha hard,

-Eric

Eric, those all address good gas planning, but I think his question was more about when you're doing recreational shore dives, maybe 2 dives on one set of doubles, and you're poking around in the shallows (~10-15fsw) at the end of a dive. RB is a couple of cubes at most, but I still try not to let me tank go below 500psi for the reasons mentioned above.

Maybe I misinterpreted though....

[CaptnJack]

I have a hard time getting below 8-900 since I usually end up cold. If I routinely got to 4-500psi I'd probably add a couple pounds just to avoid shivering (which like Lynne and others doesn't take much for me). In any case, assuming I had enough reserve to surface myself and a buddy (who's spg might be the wildly inaccurate one just as much as mine) I'd be willing to go to 400psi in doubles. I really don't like doubles for recreational dives if I can help it.

[Mattleycrue76]

Maybe we've finally discovered a use for this

[LCF]

I'm assuming you're talking about using doubles for multiple recreational dives . . . as already stated, the issue is SPG accuracy. If your gauge is off by 500 psi at 500 psi, it's pretty bad, so I think you're probably safe to go a little below that, but I'm paranoid so I don't. If you want to use doubles on a recreational charter, for example, for two dives, you really either ought to have big enough tanks to know you'll have an adequate reserve, or you should bring something like an HP130 to transfill with.

[Burntchef]

i have been able to get 2 long dives out of a set of lp95s with 30% in them and still not come close to 500 psi. 3 dives would be pushing it.

[mz53480]

... might any folks out there consider going below 500 psi while puttering around in 10-15 fsw?....

I have taken mine down to around 200psi at the end of a dive before (as a bouyancy check at ~10fsw) and was comfortable doing so. YMMV....

[loanwolf]

If you have a newer SPG or a gas integrated computer it will be built to CE and EN250 standards witch is +-1%. If your guage is off much more than that you should be thinking about replacing it. Because it is probably full of corrosion making it susceptible to failure. Their are a few dealer that are selling cheap SPG's that are not to the standards but the main stream companies are all to standards so they can sell there products in the EU.

[Raydar]

You mean that people don't check their SPG's against a test gauge to verify the accuracy prior to use?

Huh, learn something new every day.

[ljjames]

make sure to leave enough gas in your tanks to inflate your bcd and/or suit at the surface and breath for a few minutes even if you get a bit of a freeflow, so when that rogue wave comes and knocks you over whilst getting your fins off, you have a bit of time to get un-turtled. You can drown in a bathtub, so they say...<name that band> I mention the freeflow, because some first stages actually deliver gas more readily to your second stage as your tank gets lower, making some systems more prone to free-flow at the end of a dive

that said, taking into account the wild inaccuracies of the cheaper (and some expensive if they get salt water in em) pressure gauges, i will rarely if ever take mine below 4 or 500 psi, if only because i sure as heck don't want to screw up and drain them by accident and have to VIP them in case some salt water got in.

[Joshua Smith]

You mean that people don't check their SPG's against a test gauge to verify the accuracy prior to use?

Huh, learn something new every day.

I used to do that, but the "test guage test guage" that I used to verify the accuracy of my test guage broke, so I couldn't be sure my test guage was still accurate. So, I started using the ol' "Puget Sound SPG check", where you whack it against the side of your van before you dive to make sure the little needle isn't stuck on something.

[Raydar]

I used to do that, but the "test guage test guage" that I used to verify the accuracy of my test guage broke, so I couldn't be sure my test guage was still accurate. So, I started using the ol' "Puget Sound SPG check", where you whack it against the side of your van before you dive to make sure the little needle isn't stuck on something.

:roll: :roll: :roll:

[Sockmonkey]

So, I started using the ol' "Puget Sound SPG check", where you whack it against the side of your van before you dive to make sure the little needle isn't stuck on something.

Mother F... that means I need to buy a van to dive in puget sound? Thank heavens I moved away before that safety rule went into effect.

-Eric

[mz53480]

You mean that people don't check their SPG's against a test gauge to verify the accuracy prior to use?
Huh, learn something new every day.

I used to do that, but the "test guage test guage" that I used to verify the accuracy of my test guage broke, so I couldn't be sure my test guage was still accurate. So, I started using the ol' "Puget Sound SPG check", where you whack it against the side of your van before you dive.....

Strange, after doing the 'van-bang' test method, my spg always reads ~5000psi now. Sweet! I'll never run out of gas!

1

[loanwolf]

You mean that people don't check their SPG's against a test gauge to verify the accuracy prior to use?
Huh, learn something new every day.

I used to do that, but the "test guage test guage" that I used to verify the accuracy of my test guage broke, so I couldn't be sure my test guage was still accurate. So, I started using the ol' "Puget Sound SPG check", where you whack it against the side of your van before you dive.....

Strange, after doing the 'van-bang' test method, my spg always reads ~5000psi now. Sweet! I'll never run out of gas!

1.JPG

I would say that one needs retiring .