Using an SCR range for gas planning

[rjack321]

I think Sloth's point was that people who are using 0.68725 cf/min * 3.7582 ATA need to understand the big picture

 

[MXGratefulDiver]

Two words ... scuba math ...

... if I can't do the math in my head, it's too complicated.

... Bob (Grateful Diver)

 

[Sloth]

I think Sloth's point was that people who are using 0.68725 cf/min * 3.7582 ATA need to understand the big picture

Two words ... scuba math ...

... if I can't do the math in my head, it's too complicated.

... Bob (Grateful Diver)

Yup, thats why i took it a step further and even did the scuba math ahead of time and added simple tables to my wet notes. If someone took away my wet notes id be lost. Probably not good. I hate math in any form hehe.

 

[Kevrumbo]

Here we go my Imperial Brethren . . .take Sloth's volume SCR of 0.5 cf/min*ATA and divide it by the double AL80's tank factor of 5 cf per 100psi. You get a pressure SCR of: 10 psi/min*ATA.

At 100' depth, same as 4 ATA: 10psi/min*ATA mulitplied by 4 ATA equals 40 psi/min.
At 110' depth, same as 4.3ATA: 10psi/min*ATA multiplied by 4.3ATA equals 43 psi/min.
At 120' depth, same as 4.6ATA: 10psi/min*ATA multiplied by 4.6ATA equals 46 psi/min.
At 130' depth, same as 4.9ATA: 10psi/min*ATA multiplied by 4.9ATA equals 49 psi/min.
At 140' depth, same as 5.2ATA: 10psi/min*ATA multiplied by 5.2ATA equals 52 psi/min.
At 150' depth, same as 5.5ATA: 10psi/min*ATA mulitplied by 5.5ATA equals 55 psi/min.

Now all you have to do is multiply the particular DCR above by the time you spend at that depth:
At 100' depth for 10min: 40 psi/min multiplied by 10min equals 400 psi of gas used.
At 110' depth for 10min: 43 psi/min multiplied by 10min equals 430 psi of gas used.
At 120' depth for 10min: 46 psi/min multiplied by 10min equals 460 psi of gas used.
(And so on. . .)

Does this help guys?
Can you see why I prefer doing it in metric?
(Doing arithmetic even with that extra order of magnitude in psi quickly saturates my Deep Air faculties ). . .

 

[Sloth]

I think you are losing sight of the forest from all the trees.

 

[Kevrumbo]

I think you are losing sight of the forest from all the trees.

[C'mon Sloth, bear with me . . .]

From 100' to the surface (the ATA values multiplied by ten):
40
37
34
31
28
25
22
19
16
13

Sum Total: 265 --this also happens to be the pressure in psi needed for one person to reach the surface.

[Note: Remember that magic unity conversion pressure SCR from above of 10 psi/min*ATA? As you can see, to simplify the tally, the time at each depth ascending to the surface is for only 1 minute; in other words after multiplying 10 psi/min*ATA by 1 minute and by the depth in ATA, the depth in ATA now directly transforms and is equivalent to pressure in psi. So tallying up the ATA's above is equal to totaling-up the pressure in psi]

For two divers: 265 psi multiplied by 2 equals 530 psi.

For two divers stressed: 530 psi plus 30% of 530 psi equals 690 psi (approx).

For two novice divers stressed: 530 psi plus 100% of 530 psi equals 1060 psi.

And so your SPG Rock Bottom Ranges are 530 psi to 1060 psi.


From the post above, a nominal pressure SCR of 10 psi/min*ATA translates to 400 psi of gas consumed in 10min at a 100' depth; therefore in 20min you will have consumed 800 psi of backgas from your double AL80 tanks. A full twinset is 3000 psi, so you now have 2200 psi left in backgas after 20min at 30m (and 20min is the NDL for 30 meters on Air mix) --enough left over should you have to gas share at the highest value of your Rock Bottom Range (1060 psi).

 

[Sloth]

lol, the whole point of this thread is how to simplify gas management and gas planning. This notion of exact math no matter what you are calculating is worthless in the real world of scuba diving. Deco, gas consumption, and bottom time cannot be calculated to an exact number. Our bodies are to complex and our pressure gauges to inaccurate to do so.

No one has a SCR of .635 every dive and for that matter no one has a SCR of .5 every dive. So why try and calculate the rock bottom, total bottom time or your consumption at a given ATA to a tenth of a thousand PSI based on a non-static number?

I mean really 1060 PSI rock bottom?!? If someone signaled me a psi value that had tens in it I would look at them like they are crazy. How about we call it 1100 PSI?

Your math is very pretty but totally meaningless underwater.

Dont look for a digital answer for a analog problem.

 

[AndyNZ]

Does this help guys?
Can you see why I prefer doing it in metric?

But why not use bar rather than psi??? I'd almost think you'd be Canadian with that mixing of Imperial and Metric!

No offence intended to Canadians - I've just never figured how a country that has road distances in kms can still use feet to measure depth! not that my home country (UK) is any better, mind you.... measuring depth in metres but road distances in miles).

 

[Kevrumbo]

lol, the whole point of this thread is how to simplify gas management and gas planning. This notion of exact math no matter what you are calculating is worthless in the real world of scuba diving. Deco, gas consumption, and bottom time cannot be calculated to an exact number. Our bodies are to complex and our pressure gauges to inaccurate to do so.

No one has a SCR of .635 every dive and for that matter no one has a SCR of .5 every dive. So why try and calculate the rock bottom, total bottom time or your consumption at a given ATA to a tenth of a thousand PSI based on a non-static number?

I mean really 1060 PSI rock bottom?!? If someone signaled me a psi value that had tens in it I would look at them like they are crazy. How about we call it 1100 PSI?

Your math is very pretty but totally meaningless underwater.

Dont look for a digital answer for a analog problem.

You miss my point Sloth and unfortunately don't follow despite both of us having similar SCR's: accuracy & precision is not the aim, but rather a simple arithmetic process demonstrated to estimate gas consumption that you can do both pre-dive as well as on-the-fly, in your head, and after some practice --without whipping out your wetnotes anymore looking for a tabular reference. (And it's easier to do in metric!)

 

[Kevrumbo]

But why not use bar rather than psi??? I'd almost think you'd be Canadian with that mixing of Imperial and Metric!

No offence intended to Canadians - I've just never figured how a country that has road distances in kms can still use feet to measure depth! not that my home country (UK) is any better, mind you.... measuring depth in metres but road distances in miles).

Actually AndyNZ . . .I do use both feet and meters here in the States if I have dive buddy that's only familiar with Imperial Units. Overseas and abroad of course, I always use Metric Units. . .