Why Surface with 500 PSI?

[John C. Ratliff]

I look at these calculations and smile a bit. I'm from a diving era before the SPG, and we used doubles with a J-valve on one cylinder which was set at about 500 psig. When we pulled it for our reserve, our twin 72s or twin 90s equalized and were at 250 psig; we survived that era.

Looking at the current divers, and situations, I agree on the returning with 500 psig. BUT, and this is a big "BUT," that 500 psig is for use in emergencies. If there is an emergency, this air pressure will be used. If there is an emergency, and the air is used, there should be no repercussions on the diver who did so. It needs only be explained what happened.

SeaRat

 

[scubadada]

I look at these calculations and smile a bit. I'm from a diving era before the SPG, and we used doubles with a J-valve on one cylinder which was set at about 500 psig. When we pulled it for our reserve, our twin 72s or twin 90s equalized and were at 250 psig; we survived that era.

Looking at the current divers, and situations, I agree on the returning with 500 psig. BUT, and this is a big "BUT," that 500 psig is for use in emergencies. If there is an emergency, this air pressure will be used. If there is an emergency, and the air is used, there should be no repercussions on the diver who did so. It needs only be explained what happened.

SeaRat

I dived a J-valve on a single steel 72 for two years, 1970-1972, obviously, I lived too. I think my J-valve trigger was more like 400 psi. You pulled the lever and you went up, ascent was 60 ft/min and there was no safety stop. NDL planning was Navy tables.

I bought a SPG in 1972, I liked knowing how much gas I had . It probably did not change my surfacing pressure. This was mostly shore diving off LA, Orange, and San Diego Counties in Southern California, along with periodic trips to Catalina. The good old days, Blue Sharks and occasional Makos off Catalina, kelp everywhere off the coast.

 

[rsingler]

I look at these calculations and smile a bit.

I dived a J-valve on a single steel 72 for two years, 1970-1972, obviously, I lived too.

Since I'm over 70, I'll not deny what you both are saying. And since I count you both my friends, I hope you'll let me chide you a little. This thread isn't for you guys; it's for those that read along.
Let's all admit that diving has gotten a bit more adventuresome for a much larger group of people than it was decades ago. So "surviving" only gives us modest bragging rights. Diving doubles gives you lots of gas that that the recreational diver doesn't have (or at least would have if they weren't both at 250psi ).
The only reason I started this thread was because "500 psi" as an arbitrary ending pressure is stupid. If you don't gas plan and you do 50ft dives, yeah. It all works out with what the Divemaster requests.
But lots of "occasional divers" go much deeper than that in Cozumel, Florida and elsewhere.
And 500 psi will get them home only if they're solo and don't panic and remember everything they've been taught. If their instabuddy has trouble at the end of a deepish dive with them at 700 psi, they're now BOTH in trouble.
Hopefully, those numbers might get someone to think. Maybe.

Thus endeth the sermon.

 

[John C. Ratliff]

Since I'm over 70, I'll not deny what you both are saying. And since I count you both my friends, I hope you'll let me chide you a little. This thread isn't for you guys; it's for those that read along.
Let's all admit that diving has gotten a bit more adventuresome for a much larger group of people than it was decades ago. So "surviving" only gives us modest bragging rights. Diving doubles gives you lots of gas that that the recreational diver doesn't have (or at least would have if they weren't both at 250psi ).
The only reason I started this thread was because "500 psi" as an arbitrary ending pressure is stupid. If you don't gas plan and you do 50ft dives, yeah. It all works out with what the Divemaster requests.
But lots of "occasional divers" go much deeper than that in Cozumel, Florida and elsewhere.
And 500 psi will get them home only if they're solo and don't panic and remember everything they've been taught. If their instabuddy has trouble at the end of a deepish dive with them at 700 psi, they're now BOTH in trouble.
Hopefully, those numbers might get someone to think. Maybe.

Thus endeth the sermon.

I think, if you'll read my entire post, I was supporting you and these calculations. I agree, a lot of people are diving places I've never been, and my river dives to 22 feet are not real thrilling to people who regularly dive in Cozumel, or the Keys in Florida. Some people won't get excited about taking a photo of a small sculpin next to a Western Pearl Mussel too. But that's not pertinent to this discussion.

You calculations and spreadsheet should be used for anyone diving deeper than about 33 feet (10 meters). Planning dives is a somewhat lost art for modern divers who are dependent upon their dive computers, but that's what this is. Know the depth of your dive, and use the calculations spreadsheet that rsingler put together to know when to terminate the dive and begin an ascent, so as to have that reserve available in case of an emergency. I'd like to thank rsingler for putting this together so we'd have a tool to make dive planning easier.

SeaRat

 

[rsingler]

I think, if you'll read my entire post, I was supporting you and these calculations.

That you were, John. That you were. My apologies.

 

[John C. Ratliff]

That you were, John. That you were. My apologies.

Thanks, I just did a few calculations, as I now dive mostly my steel 72 tanks. But steel 72 tanks at 2250 psig (their stamped pressure) are not 71.2 cubic feet. That is only true if they are rated with a "+" sign, and filled to 2475 psig. So I worked your scenario using my tank type, after determining the actual amount of air in the tank at 2250 psig:

71.2 cubic feet / 2475 psig = X cubic feet /2250 psig
X cubic feet = (71.2 cubic feet * 2250 psig) / 2475 psig
X cubic feet = 64.7 cubic feet

So I set up the spreadsheet based on 2250 psi and 64.7 cubic feet of air. My RBP (1 diver) was 805 psig. For two divers, it was 1460 psig (I use "psig" to stand for "psi gauge"), as the absolute pressure would need to add the sea level pressure to that (14.7 psi).

Taking that information, and asuming that the 24.3 psi/min figure is correct (based upon my experience diving) I can find the approximate time I would have for the dive:

2250 psig - 1460 psig = 790 psig used to get to my RBP
790 psig / 24.3 psig/min = 32.5 minutes for my planned dive time at 80 feet.

Doing the same for a solo dive, with a turnaround RBP of 805 psig, I get:

2250 psig - 805 psig = 1445 psig used to get to my RBP
1445 psig / 24.3 psig/min = 59.5 minutes for my planned dive time at 80 feet.

BUT, I must use the actual air consumption rate, which at 80 feet is more like 82.6 cubic feet per minute (surface air consumption rate times the number of atmospheres depth). Looking at actual CFM/minute, the calculation looks like:

2250 psig - 805 psig = 1445 psig used to get to my RBP
1445 psig / 82.6 psig/min = 17.5 minutes for my planned dive time at 80 feet.


By the way, using my old dive tables from Dacor (1971), the no decompression limit for this dive is 40 minutes. So using the solo dive calculation on the spreadsheet, without thinking about decompression limits and the NDL of 40 minutes would potentially put the diver at risk of DCI.

BUT, I figured out that the actual air consumption rate at depth needs to be factored into these calculations, and using the 82.6 psig/min at 80 feet, the dive time for a solo dive would be 17.5minutes (9.6 minutes for a buddy dive).

Dive planning must consult both these issues (RBP and No Decompression Limits).

SeaRat

PS, it's getting late, and I may need to modify the rate of air consumption to absolute atmospheres instead of atmospheres of depth; I'll work on that later today (it's past midnight).
PS2, I've made the changes in the calcs.

 

[Germie]

Cmas changed the 50bar rule as it is also dependend from the depth you come and the size of the tank.
On some dives you can drain every tank till 20 bars as it are shallow dives at the end (reef goes to surface). They have a minimum gas rule now, in liters. Not in bars.

On other dives like cavedives you have the rule of 3rd that you always follow (or rule of 1/4th or other conservative rules, 1/3rd is most agressive).

When I do a dive in my houselake with my twin12, I can empty a dive with 15 bars left. No problem. I only don't want to drain tanks for reasons as moisture in the tanks. The dive profile goes to 12m and we end by swimming at 1.5m depth the way back. So snorkling depth. Then for sure no 500psi or 50 bar rule needed. 1 kick and you are at surface, no safety stop needed.

 

[60plus]

Why surface with 500 PSI - because to surface with more than 500 PSI is a waste of valuable dive time

 

[John C. Ratliff]

This is the Basic Forum. Please resist the temptation to start the "pony versus doubles tanks" wars. Thank you.

Ahhh! And I was going to ask, what about me using my doubles, and returning to the boat with 350 psig? If the DM then questions me, and I trip my J-valve, presto…500 psig.

SeaRat

 

[Blackcrusader]

Ahhh! And I was going to ask, what about me using my doubles, and returning to the boat with 350 psig? If the DM then questions me, and I trip my J-valve, presto…500 psig.

SeaRat

Well imagine when your DM tells you to come to the safety stop on a shallow dive as some of the vacation divers are down to 45 bar. Some back on the boat with 40 bar after a 53 minute dive time. Some of those same divers came over to look at my gauge thinking it must have got stuck and wasn't working. Doubles would be a waste of dive gear lol.