Question regarding LP Tank

[timle]

500 psi on a LP tank is different than 500 psi on an HP tank. Any 2 tanks that are different will have a different amount of gas at 500 psi - that is why you need to learn tank factors and calculate the cubic feet.

FWIW - It is best to avoid HP/LP/etc and use service pressure when talking specifics

3500 - HP steel tank
3442 - DOT exempt "HP" steel tank
3300 - compact aluminum tank
3000 - average aluminum tank
2640 (2400 + 10%) - average steel LP tank

I recommend you read and understand about converting back and forth from PSI to CF. If you don't feel bothered enough to do this, I suggest you treat 500 PSI as the minimum even on your LP tank.

 

[boulderjohn]

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

That's why it is good to think in terms of cubic feet, especially if you will be diving different tanks in different conditions. Earlier posters showed how to get a tank factor by comparing the rated pressure with the volume. A standard AL 80 (believe it or not) has 77.4 cubic feet when filled to 3,000 PSI. It has a tank factor of about 2.5. At 500 PSI, it will have about 12.5 Cubic feet. An HP 120, with a tank factor of 3.5, will have about 17.5 cubic feet at 500 PSI. An LP 108 will have about 20 cubic feet.

It is also good to remember that the volume of a tank is only true at the rated fill pressure. At the shop at which I work there are 2 tanks that look just like standard AL 80s, but they are actually AL 90s--if they are filled to 3300 PSI. If you fill them to the standard 3,000 PSI, they will have about the same amount of gas a standard AL 80.

An HP 120 will only have 120 cubic feet if it is filled to about 3,500 PSI. I was on a boat recently where the operator bragged about the extra bottom time you got in his operation with his HP 120s. On almost every day of the week, we got fills at about 3,500 PSI, but one day all our tanks were at about 3,000 PSI. When people started mentioning the short fills, the DM insisted that 3,000 PSI was a standard fill, but he was wrong. I suspect the boat crew wanted to get done early and bled the tanks.

 

[adm3745]

The 500 psi is a general rule so people don't have to think. It really depends on the situation. I've done a Cavern dive and finish my safety stop with 500-600 psi left, and there is stuff to see at 15-20 ft, I'll burn my tank right down to 100 psi if I have the time. A good balanced reg will provide easy breathing at low tank presures.

For each dive, you should just conservatively estimate how much gas it will take you and your buddy to do a full ascent. Maybe plan 200 extra lbs for gauge error and such. If there is no emergency, Then you'll be left with around 600 psi on my earlier calculation. IF there is an emergency, you'll be left with that 200 lbs, but you'll both be alive. (Which reminds me, if you start planning in cubic feet, I like to alter the emergency SAC rate for more safety to maybe 1.2 incase your buddy is panicked).

I would just recomend getting completely out of the pressure mentality. It's what that number represents that is more important.

 

[rstofer]

One reason for the invention of the felt marker was to mark the cf per 100 psi on dive tanks. You'll see the number every time you handle the tank and pretty soon you will be thinking in terms of cf instead of psi.

This thing gets far more critical with deep dives. Tanks just don't last long at 100' or so. And the turn pressure needs to be high enough to serve both divers. If you think about a SAC rate of 1.0 scfm, you'll be using 4 scrm at 100' or 8 scfm for two divers (maybe more if there is panic). That's a lot of air and the SPG will be falling like a stone - about 200 psi/minute for the OP's LP tank.

Richard

 

[M_Bipartitus]

One thing to mention is that if your tank pressure gets too close the the regulator's intermediate pressure the possibility of getting water in you tank goes up a fair amount. I don't think you want water in that nice steel tank. It still shouldn't happen, but that doesn't stop one of the local shops near me from charging a "mini-vis" fee for tanks that come in below so many hundred PSI (300 or 500, I don't recall.)

However, if your tank is that low, then I suspect your not following suggestions like rock bottom for your gas planning.

 

[boulderjohn]

One thing to mention is that if your tank pressure gets too close the the regulator's intermediate pressure the possibility of getting water in you tank goes up a fair amount. I don't think you want water in that nice steel tank. It still shouldn't happen, but that doesn't stop one of the local shops near me from charging a "mini-vis" fee for tanks that come in below so many hundred PSI (300 or 500, I don't recall.)
.

I had someone borrow my tanks and brought them back just plain empty. I immediately took them apart to do an examination of my own. You don't want to take a chance.

 

[JahJahwarrior]

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.

500 psi and the rule of thirds are VASTLY different rules, assuming your tank holds more than 1500psi.

...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 I’m sure are silly questions but I’m still pretty new.

There is nothing special about gas management for LP tanks. However, 500 psi is a very liberal rule. The rule of thumb for basic conservative diving actually begins at your "turn pressure" not your "get back on the boat with" pressure. You should turn your dive when you have just enough air to get yourself and your buddy to the surface, leaving a little wiggle room for errors in gauges and to avoid bleeding the tank dry.

 

[gcbryan]

There's nothing special about LP tanks as there is nothing special about 500 psi in general. The idea with the 500 psi is to just make sure you have some air left in case you need it ( buddy needs help, you drop something, you fall back in from the boat, sudden current comes up while trying to get out, kelp, etc).

The bigger point is not how much to have as you are stepping back on the boat but rather how much do you have at depth in order to then be able to make it to the surface and back on the boat with 500 psi (or however much you choose).

Look up Rockbottom with the search feature on this board and you will get the more pertinent questions to be answered.

 

[sphillips3]

The important thing to key on is the volume of the tank. In the US, the "volume" of the tank (66 cft, 80 cft, 108 cft etc) is based on the amount of "standard" air contained in the tank when filled to the rated pressure (including any "plus" rating). So, 500 psi in a 108 cft tank is more air than 500 psi in a 80 cft tank. So assuming you trained like most folks on an aluminum 80, your 108 will have a bunch more air than you are used to at the same pressure. The fact that you start at a lower pressure (2640 vs 3000) is irrelevant. Full is full. 500 psi in a big tank is more air than 500 psi in a small tank.

Secondly and as others have mentioned, the "500 psi rule" is a rough guideline to help you keep out of trouble. 500 psi in an aluminum 80 should be enough to get your and your buddy (air share) to the surface from the depth of any recreational dive, but don't dawdle! <grin>. However, I have some thoughts on the subject.

As others have alluded to, 500 psi is the "back on the boat" pressure, not the "lets start looking for the boat" pressure. That 500 psi belongs to your buddy - not to you. You and your buddy need to agree on a turn pressure based on what ever rules you are comfortable with (rule of thirds, 1/2 of the pressure above 500 psi, whatever). You and your buddy must agree on this and then plan the dive/dive to plan to get back on the boat with that 500 psi in the tank (assuming no emergency).

For normal dives you should plan to be back on the boat (shore) with 500 psi on the tank. Now, I'm not super rigid on this, and if you and your buddy are both on the anchor line doing your safety stops while breathing off your own tanks and you dip into that 500 psi while the gaggle on the ladder/current line clears that's ok by me. But if my students are farting around at 100 feet with 600 psi in the tank they are going to get a friendly reminder about dive planning.

Another thing to remember is that regardless of the volume of the tank, some regulators start to act funky if tank pressure gets too low. The typical IP for a regulator system is 140 psi above ambient. If you attempt to stay deep with low tank pressure breathing effort will start to increase rapidly. Again, this behaviour is independent of tank volume. A bigger tank will get to lower pressure slower, but once you get to low pressure, the regulator behaviour will be the same. As others have mentioned, SPGs are none to accurate at the low end of the range, so getting way into the red is not a good idea in any event.

Bottom line - do your dive planning the way you normally do. You and your buddy must agree on a maximum dive depth, maximum dive time, and minimum tank pressure (turn pressure). If you have a bigger tank, your buddy may reach turn pressure before you. That does not matter, someone has to be first.

 

[JahJahwarrior]

Several people have said that "the 500 psi is for emergencies/your buddy."

I was always taught that the 500psi rule was to make sure tanks aren't bled dry, to allow for wiggle room in guage error, etc. I do not believe it is wise to assume that the last 500 psi is an effective safety reserve.

Granted, 500 psi in your tank is more than 500 psi in an al80.

Much of the world is taught to dive with extremely liberal and simplified gas management rules. Just as much of the world is taught that the DM will take care of them. When the poop hits the fan, liberal gas management rules don't hold up nearly as well as careful planning by a thoughtful and considerate diver. That said, the most important thing is that you dive safely, no matter what gas management rules you use. Diving the 500psi rule and living to 90 is just as good as diving the rule of 99% and living to 90.