LP TANKS

[Tracy]

I feel like this is an image people need to see when they're making the decision on cylinder material, pressure rating, and when and how much to exceed a rating. If you feel like you know enough about metallurgy, quality control, and tolerancing to make that decision I say right on. But I would argue this image illustrates pretty well that material strength and pressure vessel design is not strictly intuitive. Aluminum 3k psi cylinder on the left, not sure what the rating on the steelie on the right is (but it's a modern cylinder).

Cave folks take the risk to overfill their cylinders for a reason, it comes down to logistical feasibility of carrying a given gas volume for a given penetration vs. likelihood of material failure at said pressure from all the reasons a material can fail under load. There are volumes of required gas that are not feasible to carry at rated pressure that become feasible when you exceed the rating by say 40%. If the logistics of your diving are such that you can observe the rating of the cylinder, please do.

View attachment 884522

Why do you think this image is important to see? What are you trying to show?
I have lots of tank pictures as well, just not sure what you are trying to convey with your image?

 

[BRT]

Many/Some dive shops don't have the capacity to fill pressures beyond 3000-3200 PSI. So if you have a HP tank rated at 3442 psi you cannot get the tank filled to rated capacity.

LP tanks, rated at 2640 can get filled to, and beyond capacity at these shops.

I have a couple of LP tanks in Mexico. I also have a couple of AL100's with a service pressure of 3300. for several years we have rented a pair of HP120's while we are on Cozumel. They all get filled to 3000.

 

[missionmtb]

Why do you think this image is important to see? What are you trying to show?
I have lots of tank pictures as well, just not sure what you are trying to convey with your image?

View attachment 884537

I was talking about relative modulus and its influence on wall thickness. This sort of goes to my point.

 

[happy-diver]

I feel like this is an image people need to see

which is heavier a pound of bricks or a pound of sticks

 

[Garth]

I like LP tanks for rebreather sidemounted bailout for cave diving and aluminum for ocean/open water CCR rebreather sidemounted bailout. I don't prefer HP tanks for anything other than back mounted doubles or back mount single tank diving due to the trim and feeling they give when side mounting.

Another item to consider is galvanized versus faber/painted. I don't really care much about finish or the look, but do notice a significant difference in trim, preferring galvanized. Just a personal opinion of mine.

 

[boulderjohn]

Many/Some dive shops don't have the capacity to fill pressures beyond 3000-3200 PSI. So if you have a HP tank rated at 3442 psi you cannot get the tank filled to rated capacity.

LP tanks, rated at 2640 can get filled to, and beyond capacity at these shops.

Yes.

I own both HP and LP tanks. At my home shop, I pretty much cannot get the HP tanks filled to their rated capacity, but I have no trouble getting the LPs well beyond their rated capacity. It is not a matter of policy--it's the equipment.

 

[Tracy]

Many/Some dive shops don't have the capacity to fill pressures beyond 3000-3200 PSI. So if you have a HP tank rated at 3442 psi you cannot get the tank filled to rated capacity.

LP tanks, rated at 2640 can get filled to, and beyond capacity at these shops.

This is the argument that I haven't ever been able to understand, and I think it stems from how the tanks are measured in the US.

If you buy an lp85/hp100 and the shop can only fill it to 3200. You have the same amount of gas. It doesn't matter that one isn't full and the other is overfull. You still have the same amount of gas.
Same with LP95/HP117, LP104/HP130. They are all the same volume and will have the same amount of gas.

The nice thing about the HP tanks is when a shop can fill them properly, you get more gas than in the LP.
There can't be that many shops still running the old 3200psi pumps. They haven't been made since the late 70s / early 80s. They did tend to last a long time, but we are talking 40+ years old at this point.

Cave overfills are easy to get in north Florida and some specific tech diving shops, but 99% of shops aren't filling beyond rated pressure. So you wind up with less gas in an LP tank than in an HP.

 

[runsongas]

@Christopher McCaskie 3442/3500 psi aka hp tanks use a high carbon steel alloy that is stronger but more brittle compared to low carbon steel LP tanks. you can look up the alloy composition on the CFR if you are curious. but the TLDR version is that the LP tanks even when overfilled are within their elastic deformation region so it has little effect on longevity. doing the same relative overfill to a HP tank is worse for wear and tear and also why the hydro testing requirement is lower for HP tanks.

Filling above 3800 psi is also less common in the US and combined with compression non-ideality losses as you increase the pressure, the juice isn't worth the squeeze to overfill HP tanks much.

 

[rjack321]

Filling above 3800 psi is also less common in the US and combined with compression non-ideality losses as you increase the pressure, the juice isn't worth the squeeze to overfill HP tanks much.

And that non-linearity gets really bad with trimix fills above about 3400

I have some 9L 300 bar tanks. They hold 2700L of ideal gas or 95cf. They only hold about 80cf of 15/55

 

[flymolo]

And that non-linearity gets really bad with trimix fills above about 3400

I have some 9L 300 bar tanks. They hold 2700L of ideal gas or 95cf. They only hold about 80cf of 15/55

So if i'm filling lp steels to 3500 with 18/45 versus something like 12/65 it's not the same gas volume? How would you calculate the gas volume?