Question regarding LP Tank

hartl0602 · Dec 29, 2009

Hey all I hope everyone had a good holiday. Scuba Claus was good to me

I received a full 7mm (no more rental for me) and an xs steal lp 108 tank. Which brings me to my question; is there anything special I should know about gas management as it relates to low pressure tanks? Is 500 psi still as low as you should go? Sorry for what Im sure are silly questions but Im still pretty new.

Thank you,

Sean

rstofer · Dec 29, 2009

Using PSI as an indicator of safe remaining gas is problematic. I know, I was taught the same thing: always surface with 500 psi. That just isn't a very safe approach.

You should search the board for things like "turn point" or "rock bottom". Perhaps someone will post a link.

The idea is that you should turn your dive while you still have enough gas for both you and your buddy to make a safe ascent including any required deco stops. You should probably also have enough gas to make the recommended safety stops.

Since the implication is that you will have enough gas to provide for your buddy, even if it is never required, you might find yourself surfacing with quite a bit of gas. Or at least ending the dive by spending time in the shallows.

In any event, think of 500 psi as 500/2640 of 100 cf or about 22 cf (assuming rated fill pressure is 2640 psi). For an HP 100 it is 500/3442 or less than 19 cf. It's cf and SAC rate (combined for both divers) that matters.

Richard

adm3745 · Dec 29, 2009

A nice thing about that tank is the baseline is right about 4 cubic feet per 100 psi. This maeans for simple on the fly gas management. If you breath .6 cf per min on the surface, and say at 100 ft, you would breath (4 atm * .6 = 2.4 cf per min). Round that to 3 cubic feet per minute. You know you ascend at 30 ft per min so it will take you about 3 minutes to surface. The ascent portion will take you 9 cf. a Safety stop by the same math would take around 5 cf. So thats 14 for you. Double it for a buddy.

Your rock botom tank presure with alot of consveratism is 28cf. Divided by 4 = you get 7, which is 700 psi.

Its a great idea to start using cubic feet instead of pressures.

timle · Dec 29, 2009

You should definitely learn the conversion factor for your tank so you can calculate Cubic Feet from PSI. A LP108 has a tank factor of 4.09 which means you get 4.09 cubic feet per 100 PSI.

108 cf/2640 psi * 100 = 4.09 cf/100psi

However SPGs are not very accurate at the far ends of the gauge. Add on that the average regulator needing about 130 PSI more than ambient pressure to work properly. However where 500PSI fails is it doesn't tell you HOW to get "back on the boat with 500". Rock Bottom is good for this - I believe Rock Bottom says the minimum pressure should never be less than 500PSI - but that is to start an ascent - it will be frequently more than that.

Rock Bottom and Gas Management for Recreational Divers

LeadTurn_SD · Dec 29, 2009

Here is a link to "Rock Bottom" calculations by scubaboard's very own Lamont. It is a very worthwhile read if you have not been exposed to the concept of calculating gas reserves.

Rock Bottom and Gas Management for Recreational Divers

In general, the "back on the boat with 500 psi" and "Rule of Thirds" will keep most recreational divers out of trouble, but it is far better to understand how depth, your personal consumption rate, tanks size, etc., all contribute to actually "planning" how much gas is required for a given dive.

Best wishes.

Edit: Oppps. Timle and I were posting at the same time

Wookie · Dec 29, 2009

LeadTurn_SD:
In general, the "back on the boat with 500 psi" and "Rule of Thirds" will keep most recreational divers out of trouble, but it is far better to understand how depth, your personal consumption rate, tanks size, etc., all contribute to actually "planning" how much gas is required for a given dive.

On my boat, the back on the boat with 500 PSI is used as a guideline rather than a hard and fast. The hard and fast is to be back on the boat with a breath left in the tank. We recommend 500 PSI so that if someone gets screwed up on the ladder, you still have a reserve to hang out with while they un-screw themselves. Also, SPG's are notoriously inaccurate, and that goes for electronic gauges, too. 500 PSI gives you some leeway to for inaccurate measurement.

If we see you climb the ladder with less than 500 PSI, we will ask you to take a breath from your regulator. Take a breath, all is dandy. Fail to take a breath, you will get extra assistance with your gas management skills.

Leadturn is exactly right, 500 PSI back on the boat gives most recreational divers a hour long dive and a target goal.

Nemrod · Dec 29, 2009

The only significance to "back on the boat with 500" is to keep the DM/boat captains happy otherwise I would and occasionally do suck the tank flat.

If you are depending on 500 psi to keep you out of trouble, probably not enough if you really need it.

N

hartl0602 · Dec 29, 2009

But does the 500 psi still apply to LP tanks? i would think that 500psi on a LP tank would be different than 500 psi on a HP tank and different again on an al80...maybe this will all make more sence once i read link. heads spinning a bit

hehe

SkimFisher · Dec 29, 2009

Why push the limit? I always turn at 1000. Nothing down there to see that's worth my life.

It's good info to have. But...

JMO.

boulderjohn · Dec 29, 2009

A couple of posters have given you the math to get a tank factor of 4 for doing quick calculations. In my experience, a lot of people's eyes glaze over when they see even simple math presented and they tend to skip over such advice. If by chance you are one of those, I strongly urge you to go back and take the time to understand the process. Knowing how you can use the number 4 to jump back and forth between PSI and cubic feet is a very valuable skill, and if you practice it a little it will become second nature.

BTW, I think the XS steel 108 is an excellent tank. They are what I use for my doubles.

Question regarding LP Tank

Please register or login

hartl0602

Registered

rstofer

Contributor

adm3745

Contributor

timle

Contributor

LeadTurn_SD

Contributor

Wookie

Proud to be a Chaos Muppet

Nemrod

ScubaBoard Sponsor

hartl0602

Registered

SkimFisher

Contributor

boulderjohn

Technical Instructor

Similar threads