NAUI Master Diver Course: understanding theory regarding amount of air needed in lift bag

[Zef]

Follow-on Question:

Since we typically use a pressure gauge and bar as a unit of measure for air (or psi depending on what side of the pond you are from), for practical planning purposes Liters of air needs to be converted to Bar

So lets say the diver has a 12 Liter tank with 200 bar in it. How many bar of air would be needed in the lift bag in the above example?

Would the following formula be correct?
Surface Equivalent / 12 Liters (tank volume) = Surface Equivalent in Bar
Surface Equivalent in Bar x ATA for depth of object = Bar needed at depth

84.37 Liters /12 Liters = 7.03 Bar
7.03 Bar * 2.5 ATA = 17.6 Bar
Rounded up for ease of reading the SPG: 18 Bar of air needs to be used from the tank at 15 meters to float the object.

Is the logic and formula correct here?

-Zef

 

[dmaziuk]

(Disclaimer: I am only on my 2nd coffee this morning)

I think it's 12l @ 200 bar = 2400 l. 2400/84.37 = 200/x => 7.03 bar is what you'll need. To put it another way, 18 bar out of 200 is almost 10% and 10% of a fully-charged Al80 should be 240l, not 84. Unless my brain's completely off.

 

[CptTightPants21]

Follow-on Question:

Since we typically use a pressure gauge and bar as a unit of measure for air (or psi depending on what side of the pond you are from), for practical planning purposes Liters of air needs to be converted to Bar

So lets say the diver has a 12 Liter tank with 200 bar in it. How many bar of air would be needed in the lift bag in the above example?

Would the following formula be correct?
Surface Equivalent / 12 Liters (tank volume) = Surface Equivalent in Bar
Surface Equivalent in Bar x ATA for depth of object = Bar needed at depth

84.37 Liters /12 Liters = 7.03 Bar
7.03 Bar * 2.5 ATA = 17.6 Bar
Rounded up for ease of reading the SPG: 18 Bar of air needs to be used from the tank at 15 meters to float the object.

Is the logic and formula correct here?

-Zef

The 84.37 has already taken into account the depth/pressure factor (see previous post). The answer is 7.03 Bar.

Also, the formulas, both your initial formula and dmaziuk, take into account different size tanks and a larger tank will use "less" Bar to get the same amount of air, just as a smaller tank will use "more" Bar to get the same amount of air---just something to remember and think about when you are playing the scenario in your head.

 

[dmaziuk]

The 84.37 has already taken into account the depth/pressure factor (see previous post). The answer is 7.03 Bar.

Also, the formulas, both your initial formula and dmaziuk, take into account different size tanks and a larger tank will use "less" Bar to get the same amount of air, just as a smaller tank will use "more" Bar to get the same amount of air---just something to remember and think about when you are playing the scenario in your head.

Well, that's why they invented proper units. Not only 1 litre of water is 1 kilo for practical purposes, a 12-litre tank contains 12 litres of gas at 1 bar. No conversion factor required, you just take the size and multiply by pressure.

 

[CptTightPants21]

Well, that's why they invented proper units. Not only 1 litre of water is 1 kilo for practical purposes, a 12-litre tank contains 12 litres of gas at 1 bar. No conversion factor required, you just take the size and multiply by pressure.

I'm talking about tank factors. The answer is 7.03 Bar for a 12 liter tank and 5.6 Bar for a 15 liter tank

Edit: But yes, I see we are talking about the same thing..

 

[Zef]

Thanks for the help.

-Zef

 

[Rich Keller]

I mentioned the "in the real world..." because I read through some other posts on lift bags and there were a bunch of comments from folks with varying experience that insinuated to just fill the bag with air until the object starts moving...this comment was an effort to placate them so that I would not get a bunch of those types of responses. I think your assessment of our instructors is a quite a bit off though, as they never mentioned doing that at all, and it was based on classroom discussion that encouraged me to have a deeper understanding of what is at play here. I could have just left it at understanding the computation and memorizing Boyle's Law (P1xV1=P2xV2)

-Zef

I am not trying to put your instructors down, this is just an area they know little or nothing about. The problem with just filling the bag until it starts to move is the suction between the object and the bottom. This will vary depending on the type of bottom and how long the object has been there. You need more lift to break the suction then you do to lift the object to the surface so once you break suction the object tends to take off for the surface far faster then you would expect. This is the most common and most dangerous problem for someone who has never done this before. Again if you are caught up it the rigging you will be taking an uncontrolled assent to the surface alone with the item.

 

[dmaziuk]

While that is a useful knowledge to have, it's a bit like complaining to your Algebra 101 teacher that only a really dumb builder would build a swimming pool with two pipes and one drain.

 

[kelemvor]

How do you go about determining the volume of a complex object like a boat motor?

 

[CptTightPants21]

While that is a useful knowledge to have, it's a bit like complaining to your Algebra 101 teacher that only a really dumb builder would build a swimming pool with two pipes and one drain.

Before this starts going back and forth. Rich has been very polite and provided useful information about the differences between theory and real world experience. His posts don't warrant a response that does not provide new information or continue the discussion.