PST 95's vs. 104's

[CapeFearDiveGal]

My first post. I hope it's a good one!

I have a question: I'm wanting to get a steel tank and I can't decide wether to get a pst 95 or 104. I'm wanting to get the 1st tank of an eventual set of doubles. This 1st tank will get me through cavern and intro to cave, with an H or Y valve. I'm 5'4" and female, btw. Looking at the stats, the main difference is about 3lbs wt., and 1.5 inches or so in height. So what's the cons against getting the 104 over the 95? I figure more gas is always a plus, right?

 

[Uncle Pug]

When thinking weight difference be sure to include filled weight and then double it because that is where you are heading.

But I know of a gal that handles double 120s so I expect if you are determined enough, 104s would be doable.

Another issue to think about is length and how that will affect your trim UW. The longer 104 works great for most but might be a tad long for you... and trim you in a feet down attitude. You would be able to correct for this by moving the tanks up in the bands so it is not an insurmountable problem... going the other direction is more difficult... i.e. a tall fellow using short 95s (though Yooper makes it work.)

So... if your game for it... get the 104s

 

[roakey]

If weight's not an issue, get the cylinders that the other folks you're going to be cave diving with have. When caving you limit your penetration to 1/3 of the SMALLEST gas capacity in the team, so if your buddies have 104s, you want to go that route. If they has 95s, no sense in getting 104s.

Roak

 

[CapeFearDiveGal]

Would the 104's offer the same trim as the 95's? I'm thinking that the 95 on a 5'4" person would offer better horizontal trim than a 104 sticking out down my back. But, since the height difference is so small, perhaps I'm splitting hairs.

Also, Roakey, you bring up another question: If I'm diving with big ole' beefy guys with 120's (which I don't, just hypothetical), should I dive with 120's as well, so we all have the same tank(s)? Or should I find different dive partners? I'm thinking this is so we all have the same amount of gas at the start of the dive. My SAC rate is better than Big Beefy Guy, but I do admit that with a set of 120's I would have enough for him to get out of a cave as well in an OOA situation. But again, 120's are awfully big for me. That makes me think of another situation. If I have 95's and BBG has 120's, and I'm sipping air and turn on 1/3's, if BBG has an OOA situation I wonder if I will have enough gas to get us both out? I haven't had cave training yet, so this is probably all been figured out, but what's the answer?

-christen

 

[SeaGypsy]

I'm about the same height. I've used both. In the water there's not much difference but for getting in and out of some caves or up a boat ladder I don't like the extra weight of 104s.

You'll do turn pressures for different tanks in a cave class. The short answer is that both divers use 1/3 of the smallest amount of gas.

 

[Rick Murchison]

Originally posted by CapeFearDiveGal
If I have 95's and BBG has 120's, and I'm sipping air and turn on 1/3's, if BBG has an OOA situation I wonder if I will have enough gas to get us both out? I haven't had cave training yet, so this is probably all been figured out, but what's the answer?

-christen

Strictly from an air management standpoint, the heaviest breather should have the smallest tanks. That way he'll always hit thirds first and turn the dive with plenty of gas for the team to exit, and no fancy figuring needs to be done.
If you put the BBG on the big tank, *his* turn pressure must be figured at a third of *your* tank size... for ease of math, let's assume that with both of you at 3000 psi, you have a 150 CF supply on you and a 200 CF supply on the BBG. Your turn pressure is 2000 psi, but what's his? Since at 2000 psi you have only 100 CF left, you only have 50CF to give in an OOA situation (technically you'd have a bit more 'cause you don't breathe as much, but you can't count on that in figuring reserves). Therefore, you can only allow your BBG buddy to use 50 CF of his 200, or 750 psi, before you must turn the dive... so *his* turn pressure is 2250, while yours remains 2000.
Clear? I hope...
Rick

 

[Cave Diver]

Originally posted by CapeFearDiveGal
Also, Roakey, you bring up another question: If I'm diving with big ole' beefy guys with 120's (which I don't, just hypothetical), should I dive with 120's as well, so we all have the same tank(s)? Or should I find different dive partners? I'm thinking this is so we all have the same amount of gas at the start of the dive. My SAC rate is better than Big Beefy Guy, but I do admit that with a set of 120's I would have enough for him to get out of a cave as well in an OOA situation. But again, 120's are awfully big for me. That makes me think of another situation. If I have 95's and BBG has 120's, and I'm sipping air and turn on 1/3's, if BBG has an OOA situation I wonder if I will have enough gas to get us both out? I haven't had cave training yet, so this is probably all been figured out, but what's the answer?

-christen

Christen...

The simple answer revolves around your statement 'the same amount of gas.' When you take your full cave training you will learn how to figure out 1/3 air rules and just as importantly, dissimilar tank turn pressures. The briefest summation I can give you on that is that you must figure out the total tank volume for each member of your dive team based on tank size and fill pressure, and then calculate your turn point based on one third of the SMALLEST volume of air. That way you can ensure that you have adequate air to get even the heaviest breather out from the point of furthest penetration. If you wish a more detailed answer on exactly how this is figured, send me a Private Message and I will explain the procedure based on your 95 vs 120 cft scenario.

Dive often, but dive safe

John

 

[roakey]

Hey folks, she was asking ME!

Cave Diver, Rick and SeaGypsy covered it well.

Though let me show you what happens when you do it wrong. I'm going to make up cylinder sizes to make the math easy.

Let's say you have 60cf of air and big guy has 120cf and consumes air at twice the rate you do. Cylinder sizes look about right, yes?

So you head into the cave and both hit thirds at the same time. You've consumed 20cf and big guy 40cf. Just then big guy has a catastrophic loss of gas. Now you're both on your cylinder. You need your 20cf to get out plus 40cf for him. But you only have 2/3 of your cylinder left, which is 40cf.

You both gonna die.

Moral of the story, match cylinders and match divers.

Roak

 

[Cave Diver]

Hey folks, she was asking ME!

Awww Roakey... I'm from Texas, and yanno us Southern Gentlemen can't leave a lady in distress... And since you weren't around <shrug> well, someone had to take care of her.

You both gonna die.

We would never have let that happen to her. [:]-)

Dive often but dive safe

John

 

[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? I read trip reports all the time where Tom met up with Bill for a dive, then later on Bill went in another cave with Mary, and so on and so on. 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.

-christen