Do you double or single..

[SeaJay]

Not to hijack the thread but, nicely written article SeaJay!

Jason

Thanks, guys...

 

[Scubaroo]

so by the end of diving the doubles you're going to have the 500psi reserve split between your two tanks, basically 250 in each?

No.

500psi means 500psi. You don't "add" pressures in each tank to find the total pressure remaining. 500psi in a set of manifolded doubles means that both tanks are equalised at 500psi. If you closed the manifold and measured each tank independantly, they would both be reading 500psi. You would just have more gas volume available.

Lets pick a mythical single tank - 80cf at 3000psi. 500psi would be 500/3000*80 = 13.3cf. In a set of doubles, 500psi = 26.7cf.

250psi doesn't even come into the picture.

As SeaJay pointed out, 500psi is such an arbitrary figure. In countries where metric units are taught in scuba classes - the figure is 50 bar - which is about 725psi. Big difference. Such planning is designed to keep OW students out of trouble, not plan double tank dives.

 

[mempilot]

Added note. If you have 500psi left in a single 98cf, you're wasting a lot of gass(volume). Point is, you can't really use PSI as a standard, unless everyone is diving the same AL80 tanks.

The 500psi rule engaged by many boat captains is really just an arbitrary number used for people diving standard AL80's. Many standard rental regs start breathing hard at around 250psi on an AL80 and 500psi on this tank is a good margin to shoot for.

 

[SeaJay]

No.

500psi means 500psi. You don't "add" pressures in each tank to find the total pressure remaining. 500psi in a set of manifolded doubles means that both tanks are equalised at 500psi. If you closed the manifold and measured each tank independantly, they would both be reading 500psi. You would just have more gas volume available.

Lets pick a mythical single tank - 80cf at 3000psi. 500psi would be 500/3000*80 = 13.3cf. In a set of doubles, 500psi = 26.7cf.

250psi doesn't even come into the picture.

I think what he meant was that he was observing that in a set of doubles, the 12.95 cuft of reserve gas would mean that there's 250 psi in a set of AL80 doubles.

...In other words, if he had 13 cuft of reserve gas in a single (which made for 500 psi), then 13 cuft of reserve gas in a set of doubles would be 250 psi.

Another way to say it would be that 500 psi in a set of doubles is twice the amount of gas than it is in a single. If you want the same amount of gas in a set of doubles, you'd halve the psi.

...So 500 psi in a single would be roughly equivalent to 250 psi in a set of doubles.

...Or - like you showed him in your calculation, 'Roo - the same psi means twice the amount of gas in doubles than a single.

...Which is why "Rock Bottom" needs to be calculated in cuft (or liters, or some volume measurement) and then translated into pressure (psi or bar) so that you can actually use the figure.

 

[mossym]

No.

500psi means 500psi. You don't "add" pressures in each tank to find the total pressure remaining. 500psi in a set of manifolded doubles means that both tanks are equalised at 500psi. If you closed the manifold and measured each tank independantly, they would both be reading 500psi. You would just have more gas volume available.

.

not what i meant,

...In other words, if he had 13 cuft of reserve gas in a single (which made for 500 psi), then 13 cuft of reserve gas in a set of doubles would be 250 psi.

exactly..so with doubles to have the same amount of air left you can bleed them to a lower pressure.....thanks again seajay!!

 

[AndrewJD]

Quick question. Rockbottom as I was taught is never less then 500psi.

Example. A diver with twin E8-130s is doing a 100ft NDL dive. Rockbottom requirements are 24cuft of gas. Tank Factor is 7.4 cuft/100psi (260cuft capcity @ 3500psi), which translates to a rockbottom of 323psi rounded up to 350psi. However, for these tanks 500psi would be used for rockbottom.

Why should rock bottom never be less then 500psi?


Note: This is purely to illustrate the never less then 500psi rule rather then the probablity of a diver using twin E8-130s on a NDL recreational dive.

 

[SeaJay]

Well, the truth is that:

1. Most gauges simply aren't 100% accurate, and it would really suck to find out that you're off by 100 psi when you most need it.

2. Not sure how to describe this - but the reality is that pressure in a tank isn't linear. That is, the last 100 psi in a tank is a much smaller volume of gas than the amount needed to push pressures from 2900 to 3000 psi.

...So "rounding up" is a good thing. Practically speaking, it just makes sense to never have a rock bottom below 500 psi... But the 150 psi difference between 350 and 500 is really starting to split hairs.

If that's the way you were taught to do it, then by all means, stick to your training. 500 is a good minimum number, although it also makes sense that you wouldn't need that much if you were doing a 13 foot dive in zero visibility.

 

[boomx5]

Quick question. Rockbottom as I was taught is never less then 500psi.

Example. A diver with twin E8-130s is doing a 100ft NDL dive. Rockbottom requirements are 24cuft of gas. Tank Factor is 7.4 cuft/100psi (260cuft capcity @ 3500psi), which translates to a rockbottom of 323psi rounded up to 350psi. However, for these tanks 500psi would be used for rockbottom.

Why should rock bottom never be less then 500psi?


Note: This is purely to illustrate the never less then 500psi rule rather then the probablity of a diver using twin E8-130s on a NDL recreational dive.

Nice scooter Andrew. I hope she worked better for you than when I rode her.

 

[PatH]

Not sure how to describe this - but the reality is that pressure in a tank isn't linear. That is, the last 100 psi in a tank is a much smaller volume of gas than the amount needed to push pressures from 2900 to 3000 psi.

I know I need to pull out my old Chemistry text to verify the exact limits, but the ideal gas law works fairly well for most of pressure range in scuba tanks... thus can be approximated as linear. Sure it deviates slightly at the very top end... but by less than the SPG is inaccurate. Makes a small difference when blending, but for volumes calculations at pressures less than 3000 psi I think you are safe assuming linear variation.

Off to check my text...

 

[AndrewJD]

SeaJay,

I agree with all of your points and would add that our regulators are pneumatically driven. The difference between high pressure (tank) and intermediate pressure is ~120psi. Without pressure in the tanks we wouldn't be able to breath no matter how much volume there was. Minimum of 500psi helps to ensure this.

Thanks,

Jonathan