PST 95's vs. 104's

[roakey]

Originally posted by CapeFearDiveGal
These divers probably don't have 5 sets of doubles to match tanks do they? I'm guessing this is an acceptable risk with diving different volumes, so long as you turn on 1/3 of the smallest volume. Although it looks like Roak makes a good argument for matching tanks.

No, they don't have 5 sets. You CAN dive easily with dissimilar cylinders; you just have to break out a calculator before the dive.

What we're trying to do is educate you on the issues of what cylinders to buy so you can make an informed decision on your own, we don’t want to just tell you what to buy!

My point is if the folks you're going to dive with (and this might boil down to a guess at this point) just have 95s, no need to go for 104s. If they have 104s or you just don't know whom you might dive with AND you can handle 104s, by all means go for them. 95s and 104s are probably the most ubiquitous sizes in cave diving, you won't go wrong with either. Especially when you overfill them, but that’s for another note.

Let’s go back and see how you and big guy could have lived through the previous example I gave. Since I made up cylinder sizes, I’ll now make up cylinder pressures.

Your 60 CF cylinder is an 1800 PSI cylinder and big guy’s 120 is a 3000 PSI cylinder.

First, how many CF is 1/3 of the smallest cylinder? That’d be your 60, so 20 CF. So turn pressure is easy for you, it’s 2/3rds of 1800 PSI or 1200 PSI. What’s big guy’s turn pressure?

Two ways to figure it out. The obvious way is how many PSI is 20 CF in his 120? Well, 20/120*3000 = 500 PSI. So his turn pressure is 3000-500 = 2500 PSI. So he’ll call the dive when his gauge reads 2500 PSI.

The shortcut is to directly figure out how many CF of gas you want left when big guy’s supposed to turn: (120-20)/120*3000 = 2500 PSI.

So now what happens in the same scenario? You both start the dive and at 2500 PSI big guy calls the dive (you don’t, you’re only half way to your turn pressure having only used 10cf of gas). You have 50 CF of gas left. Now big guy’s catastrophic gas failure means that he needs only 20 CF to get out and you only need 10 CF. Turns out you’ve got 20 CF to spare. You’ll both live. With 20 CF extra you could probably even pull the primary on the way out.

Please note this large amount of “spare” gas is only because I’m using wildly dissimilar cylinders and wildly dissimilar SACs. Dissimilar cylinder calculations do result in slightly more conservative gas management, but not nearly as conservative as this may appear.

Roak

 

[Cave Diver]

Originally posted by CapeFearDiveGal
Rick, John and Roak,


Thanks for the explainations, I understand it now. I'm sooo looking forward to cave instruction this fall.

I have another question: It looks like all the computations can be avoided if everyone uses the same tanks with the same volumes, but in practice, how make cave divers actually do this?
-christen

Christen,

Once you have had a little practice doing the math that I showed you in the PM you will learn that it is actually pretty simple. I understand (I have no first hand knowledge myself) that GUE uses something they call the "Battlefield method" that assigns a number to tank sizes that makes it easy to figure out dissimilar turn pressures while in the water. In the example I gave you with the LP95's with a .072 cft per psi, you would multiply that by 100 and round to the nearest whole number (.072 x 100 = 7.2, rounds to 7). You then know how many cft of air are in a tank per 100 psi and you can much more easily calculate tank volume.

(1000 psi x 7) / 100 = 70 cft of air @ 1000psi for a LP95.

Some people seem to prefer this method as it is somewhat easier to figure in the water, but it a little less accurate and I personally would rather spend an extra 5 minutes before the dive and make sure my number are dead on. Once you commit to a set of doubles, you will quickly learn (and remember) that derivative number that you use to calculate cylinder volume for a given psi and it takes a few steps out of the process. I wrote the 'magic number' on my tanks with a permanent marker (.084 for my double 112's) and now all I have to do is multiply the pressure in the tank by that number and I instantly know what the volume is.

Once you get into your full cave class you will become more familiar with these calculations and they will soon seem like second nature. I am in the habit of even doing this on sport dives just to stay in practice and make sure my dive buddy has about the same amount of air as I do.

Dive often, but dive safe

John

 

[CapeFearDiveGal]

Hi Guys,

My main 2 dive buddies will both have 95's. So, based on that I'm going to go ahead and get a 95 with an H valve. Thanks for all the info!!

-christen

 

[Cave Diver]

Christen,

You're welcome. Hope the info we provided was helpful.

Dive often, dive safe

John