Bottom Line of the Bottom Time

[zboss]

Folks -

Given that all variations on equipment, diver, dive type, depth, ET AL remain constant, what tank gives the longest bottom times based upon air consumption (with AIR) at 130 feet?

 

[Scubaroo]

The biggest one.

 

[Scubaroo]

All things being equal, bottom time is a function of SAC, ATA and gas supply. So the most gas produces the most bottom time. Tank characteristics such as LP, HP, steel, aluminum, will have no bearing. Maybe HP tanks will give you more of your air than a LP tank would, but we are talking about the last 150psi in the tank. Which at 130', becomes a moot point

 

[zboss]

The question really comes down to this:

Does a low pressure tank but large tank have more or less available gas than a high pressure smaller tank at 100 feet and does someone have a graph... give me the executive overview here

 

[metridium]

Filled to their capacities, but not overfilled, a HP 120 = a LP 120. Go by the rated capacity, not the PSI (unless you're overfilling), to compare.

 

[Scubaroo]

duh

The question really comes down to this:

Does a low pressure tank but large tank have more or less available gas than a high pressure smaller tank at 100 feet and does someone have a graph... give me the executive overview here

It's going to depend somewhat on your first stage regulator's performance, but the answer will be the HP tank. Barely.

Let's compare two (fictitious) tanks - both hold 80cf at their respective working pressures, 2400psi and 3500psi.

Your (fictitious) first stage has an intermediate pressure of 140psi (at the surface).

At zero fsw:

Let's say the bottom line is when the tank's remaining pressure matches the IP of the regulator, that is, 140psi. Below that, and the regulator can be considered to fail.

2400psi tank - at 140psi, will have 4.66cf remaining
3500psi tank - at 140psi, will have 3.2cf remaining

Assuming you have a conservative SAC of 1, you're breathing 1cf a minute. So the 1.46cf difference will give you about 88 seconds more "bottom" time. This is on the surface, assuming the regulator doesn't supply air below it's rated IP. This is the conservative approach.

At 100 fsw:

At 100fsw, ambient pressure is 4ATA (about 59psi). Let's say you have a regulator that continues to deliver air until the ambient pressure of the surrounding seawater is reached (59psi). So the bottom line is now 59psi instead of 140psi.

2400psi tank - at 59psi, will have 1.966cf remaining
3500psi tank - at 59psi, will have 1.349cf remaining

Assuming you have a conservative SAC of 1, at 100fsw, you're breathing 4cf a minute. So the 0.618 cf difference will give you about 9 seconds more bottom time. Maybe two breaths? This is hypothetical, assuming your regulator keeps working down to 59psi.

Now here's a "duh" question for you - are you going to be using this information as criteria for a tank purchase?

 

[MikeS]

Ben,

The problem with your example is that you assume the tank will only continue to deliver air until the ambient pressure is reached. In fact, I believe that you could continue to draw some air creating a partial vacuum. To see what I mean, draw air on a second stage with the first stage not connected to a tank (make sure the sealing device is not attached). You should be able to get air.

Although Omar has revoked my PADI Diving Science Guy Certification, I propose that you would be able to draw air until the tank pressure equaled the pressure of the partial vacuum created by your lungs. Since the actual volume of the LP tank is greater you would get more air before that pressure was reached.

Of course I might just be dizzy from trying to breath through my regulator which is not connected to a tank.

Mike

 

[Uncle Pug]

zboss once bubbled...
give me the executive overview here

Dive what the charter boat mate gives you. Stick with the DM. Don't party too late if you dive the next day.

 

[Scubaroo]

Dunno if that's true or not, but the point of my post was, why should zboss even be concerned about which tank will deliver the most air at 100' or 130' - sucking a HP or LP tank dry at that depth is going to have the same net effect. Unless it was a hypothetical question, which has no intended real world application.

Anyone buying a HP tank after reading this thread, please be sure to deploy a lift bag as your tank approaches 59psi, and tie 100' of lift bag line to your BC. It sure makes it easier to sell someone's gear on eBay if the deceased leaves a surface marker for the recovery team. Oh yeah, as an added bonus, your relatives get the body back

 

[MikeS]

bengiddins once bubbled...
sucking a HP or LP tank dry at that depth is going to have the same net effect ... easier to sell someone's gear on eBay if the deceased leaves a surface marker for the recovery team. Oh yeah, as an added bonus, your relatives get the body back

Well if you’re going to get all practical and take all the fun out of it, even with a high SAC rate, you’ll hit the NDL before you run out of air. So despite the fact that upon realizing your our of air, you efficiently deploy your spare air, you’ll probably get bent.

Uncle Pug,

You crack me up! I think I just spit beer on the keyboard.

Mike