5615mike once bubbled...
Up through my AOW specifics such as this were never discussed. So, have others had such specifics discussed up through including AOW? Size of tanks, PSI vs. cft have never been brought up. Shed some light on this Mike. Are you saying you teach this info at this level? Is it a requirement or are you.....if you are.......doing it b/c you believe it is valuable info for students? I'm interested to know if I was short changed on my education to that point.
Unfortunately it isn't a requirement in a PADI (and others) OW class. Also, it isn't a requirement for an AOW class which includes a deep dive. Given that so many OW and AOW do go on deep dives, especially on charters, without knowing anything about gas management, I think this is borderline criminal. I teach it because I feel I have to.
The math is pretty simple because the relationship between volume and pressure is linear.
The math in the opening post is correct except that a LP steel 95 contains about 95 cu ft at 2640 (10% overfill) and most al 80's are rated at 3000 psi.
An easy way to quicly convert presure to volume for a specific tank is to know hoe many cu ft/100 psi you have for different tank sizes. I have that memorized for the tanks I frequently use but it's a good thing to keep on a page in your wet notes.
For example an al 80...77 cu ft/3000 psi * 100 = 2.56 cu ft/100 psi. i use 2.5 in my calculations.
So...500 psi is this much volume...5 * 2.5 cu ft/100 psi = about 12.5 cu ft.
When planning a dive given the depth we simply look at how much gas we need to get the buddy team back. Once we know what volume we want to reserve we just devide by 2.5 to find out how many hundred psi we need to reserve. I wouldn't use a presure much below 500 psi because gauges can be off and/or hard to read for small incriments.
How much gas do you use at depth? You know that at 33 ft (2 ATA) that it takes twice the gas to fill a flexable container to the same volume right.
[(depth/33) + 1] * surface consumption/minute will tell you how much gas you can expect to use per minute at a given depth. This BTW is in the PADI AOW text but they don't tell you what to do with it.
So, lets say we're doing a 100 ft dive where we can surface anyplace (like a quarry). We just need to reserve enough to get both divers to the surface (the easiest case).
How long to ascend if we want to ascend at 30 ft/minute?
100 ft/33 ft. minute = 3.33 (I use 4 minutes)
If the ascent is at a constant rate we can use the average depth to calculate consumption right?...50 ft
For this example lets use 1 cu ft/minute surface consumption (RMV). It keeps the math easy and it's more than most people actually use. 50/33 + 1 * 1 cu ft/minute * 4 minutes tells how much gas 1 diver will use during the ascent. In this case 10 cu ft. But we have 2 people so we need 20 cu ft right? That's because the worst case is that one of you suffer a total gas lost right before you would be ending the dive.
So...what presure do we need to reserve? 20 cu ft/2.5 cu ft/100 psi = 800 psi
For the sake of keeping it simple I didn't add in any safety stops. You can figure your 3 min at 15 ft or 1 @ 32, 20 and 10 or whatever you like.
You might also want to give yourself an extra minute at depth to get set up sharing air and actually begine the ascent also which BTW is another 4 cu ft per person so we're up to reserving 28 cu ft which is 1100 psi
If you figure a 3 min stop at 15 ft thats another 4 cu ft per diver so now we're up to 36 cu ft in reserve or 1400 psi.
What if you had to get back to you entry point before ascending?
This kind of makes an AL 80 look too small for going to 100 ft doesn't it?
For very shallow dives 500 psi might work but it's easy to see that for deeper dives that isn't even close to enough to get both divers up (in a healthy way) in a worst case situation.
