too big? 120, 130 larger What is a good size.

[imjustdave]

Sorry if these seems like a duh Q but.
Deciding on size of tank? How do you pick a size? Im 5"10 and have a PST 130 tank. I have borrowed some blue steel LP 95 2400PSI and it just seems like a much smaller tank in terms of dive time. They say its a 95 but It just doesn't seem like a 95. My last dive on the 95 was a meer 22 min but my last dive on the 130 was 45 min, yes some of that time was at 20 ish feet but still came back with the same amount. I know I'm a gas hog but any reason to not have 130 tanks? As another reference in mexico on a AL 80 I got about 30+ min dive time, yes warmer, and no dry suit but 30 VS 22 min with a bigger tank just doesn't seem right. All dives were about the similar depth.

I know as I get more experience my bottom times will get better as 1 I will get in better shape and 2 my breathing will improve. but when we have options of 110 120 130 I even heard of 145... why not have more gas?

Also LP VS HP Is there a difference beyond being LP and HP ?

 

[Agility]

Heating your body consumes a lot of energy, which produces a lot of CO2, which in turn increases your ventilation resulting in running through your gas faster.
Take the largest tank you are comfortable with. You can always switch to smaller tanks when you come out of the water with half full tank.

 

[jcaplins]

You probably got a short fill. HP tanks are usually a little harder to get topped off properly. Even LP tanks get shorted, depending on the patience of you and the filler.

My air consumption can be all over the place with what I perceive as very little difference in exertion. I'm getting better, starting to average below 0.7 ft3/min now.


You need to figure out your surface are consumption (SAC) rate and use that as basis for determining your gas requirements at various depths. Then you can determine what size tank you need for whatever dive you are planning. (I assume you are staying withing the NDL)

If you have an Air integrated computer that you can download the data then this is done for you automatically.

If no AI computer, then do this:
1. Your sac rate can be determined in a pool, at the surface or 1 --> 5 feet (close enough to be considered the surface).
2. note your starting pressure and time
3. swim in place up against the side of the pool trying to simulate the amount of exertion you would normally do during a typical dive. (can also do this without swimming, and swimming hard, for more data and comparison)
4. stop after a certain time interval (1 min, 2, min, 5 min, whatever you want)
5. note end time and end pressure. Be as accurate as possible.
6. calculate SAC rate. this will be cubic feet per minute.

To calculate the SAC you need to know you cylinder specs and the notes you made from above.
If using a standard aluminum 80, it will actally be 77.4 cubic feet. (Catalina Aluminum Scuba Cylinder Specifications - Imperial)
calculate the "tank factor" (I'm sure this is a real term, I doubt I just made it up) = (tank capacity)/(working pressure of tank)

assuming the standard AL80.... TF = 77.4 ft3 / 3000 psi = .0257 ft3/psi

How much air did you use during your test? lets say start PSI = 2800, end PSI = 2500. = 300psi
time = 5 minutes.
We also need to know what pressure we are at (atmospheres, ATM). since we are at the surface(or pretty close) this will end up being = 1
ATM = (depth +33)/33
= (0+33)/33 = 1

for our example:
SAC = (([(PSI used)*TF] / min) / ATM)
= (([300 psi * .0257 ft3/psi]/ 5min) / 1)
= 1.542 ft3/min

With your SAC in hand (hehe) you can now determine your expected air consumption at any depth.
- if you decent to 33 feet, then ATM = 2. SAC *2 = 3.084 ft3/min
- descend to 100 feet, then ATM = (100+33)/33 = 4.030; SAC*4.030 = 6.206 ft3/min

With this example of a high SAC rate, if you wanted to do a 100 foot dive to the limits of the NDL of 22min (Naui chart). You would need: 22min *6.206 ft3/min = 137 ft3 of gas just for the bottom time alone.


(gawd, I hope this is legible)

 

[claymore]

For the most part gas consumption is independent of tank size. A larger tank will give you more time on a specific dive than a smaller tank but has very little to do with the volume/size of the tank it's self and more to do with your SAC rate for that dive.
Deep, cold, exertion and stress can increase you consumption greatly.
Your physical profile in the water will effect your SAC rate. Every thing that increases resistance as you move though the water will increase your SAC rate. Larger tank/tanks, trim position, exposure protection.
The frog kick will help your SAC rate because it uses less energy.

 

[drbill]

I'd find ways to reduce your gas consumption first. Amazing how much of a difference that can make. I've had dives as long as 2 hr 25 min on an Al 80 in the Philippines (although generally 70-90 min on one here in the colder waters off SoCal). When I was diving very deep (to 200 fsw) I'd use an HP120.

 

[fisheater]

But, Dr. Bill, you have gills!

 

[imjustdave]

You probably got a short fill. HP tanks are usually a little harder to get topped off properly. Even LP tanks get shorted, depending on the patience of you and the filler.

My air consumption can be all over the place with what I perceive as very little difference in exertion. I'm getting better, starting to average below 0.7 ft3/min now.


You need to figure out your surface are consumption (SAC) rate and use that as basis for determining your gas requirements at various depths. Then you can determine what size tank you need for whatever dive you are planning. (I assume you are staying withing the NDL)

If you have an Air integrated computer that you can download the data then this is done for you automatically.

If no AI computer, then do this:
1. Your sac rate can be determined in a pool, at the surface or 1 --> 5 feet (close enough to be considered the surface).
2. note your starting pressure and time
3. swim in place up against the side of the pool trying to simulate the amount of exertion you would normally do during a typical dive. (can also do this without swimming, and swimming hard, for more data and comparison)
4. stop after a certain time interval (1 min, 2, min, 5 min, whatever you want)
5. note end time and end pressure. Be as accurate as possible.
6. calculate SAC rate. this will be cubic feet per minute.

To calculate the SAC you need to know you cylinder specs and the notes you made from above.
If using a standard aluminum 80, it will actally be 77.4 cubic feet. (Catalina Aluminum Scuba Cylinder Specifications - Imperial)
calculate the "tank factor" (I'm sure this is a real term, I doubt I just made it up) = (tank capacity)/(working pressure of tank)

assuming the standard AL80.... TF = 77.4 ft3 / 3000 psi = .0257 ft3/psi

How much air did you use during your test? lets say start PSI = 2800, end PSI = 2500. = 300psi
time = 5 minutes.
We also need to know what pressure we are at (atmospheres, ATM). since we are at the surface(or pretty close) this will end up being = 1
ATM = (depth +33)/33
= (0+33)/33 = 1

for our example:
SAC = (([(PSI used)*TF] / min) / ATM)
= (([300 psi * .0257 ft3/psi]/ 5min) / 1)
= 1.542 ft3/min

With your SAC in hand (hehe) you can now determine your expected air consumption at any depth.
- if you decent to 33 feet, then ATM = 2. SAC *2 = 3.084 ft3/min
- descend to 100 feet, then ATM = (100+33)/33 = 4.030; SAC*4.030 = 6.206 ft3/min

With this example of a high SAC rate, if you wanted to do a 100 foot dive to the limits of the NDL of 22min (Naui chart). You would need: 22min *6.206 ft3/min = 137 ft3 of gas just for the bottom time alone.


(gawd, I hope this is legible)

Don't think I had a short fill, we had 4 tanks between the 2 of us and all were at 2400 PSI, and pretty sure that is what they are stamped for. To be honest the 2 of us had pretty much identical psi starting and ending"500" on both dives. Im just thinking I have a bad SAC rate and to have any decent amount of time underwater will need more experience diving, all of my own gear, better trim, and larger tanks.

I will try and compute my SAC with your info, only disadvantage to having the petrel.

---------- Post added April 22nd, 2014 at 09:08 AM ----------

For the most part gas consumption is independent of tank size. A larger tank will give you more time on a specific dive than a smaller tank but has very little to do with the volume/size of the tank it's self and more to do with your SAC rate for that dive.
Deep, cold, exertion and stress can increase you consumption greatly.
Your physical profile in the water will effect your SAC rate. Every thing that increases resistance as you move though the water will increase your SAC rate. Larger tank/tanks, trim position, exposure protection.
The frog kick will help your SAC rate because it uses less energy.

Correct SAC is the biggest issue. Frog kick I will have to look into that,

Thankyou.

 

[TSandM]

Dave, a 130 is half again as much gas . . . so it will definitely give you a longer bottom time. And if you are big and strong enough to carry them, 130s are nice tanks. I swear at them when I have to move them around on land, though!

It IS a very strong possibility that your gas consumption can be improved. If you're in the Seattle area, let's get together at Alki some time and see what we can identify to improve.

 

[serpentara]

Usually LP tanks give their c.f. rating ar their + pressure. So your 95 is only 95 c.f. at 2400+10% overfill for ~2650 PSI. That's why so many people get mad if their l.p. tank looses its + rating during hydro. At 2400 PSI it's smaller, only 86 c.f.

Sent from my HTC One X using Tapatalk

 

[Randy g]

2400 is a short fill in a LP tank here! We regularly fill them to 3600+.