LP vs HP tanks on doubles...

[DC53]

Don't think anyone is going to overfill an LP tank by 50% in this neck of the woods. I hadn't realized they were talking about THAT much overfill. My main shop will go a few hundred over. Another shop nearby won't put it out the door above rated pressure; he calls it a "bomb". Don't burst valves tend to go off if you go too high?.... LDS said he has seen burst valves go just by getting too hot like in a car. (Adding 50 degrees F in a hot car on a 90 degree day would add slightly less than 10 percent to the pressure, by the way.) I have heard of plugging burst valves but that seems over the top to me. Anyway if that is the main reason for LP's, that you have to exceed their rated pressure by 50% to beat HP's in terms of gas/pound of tank, then my inclination is to stick with HP's, either 100 or 120. So many other more controllable risks in scuba, why add in a wild card like REALLY stressing out your tanks.

 

[UCFKnightDiver]

Don't think anyone is going to overfill an LP tank by 50% in this neck of the woods. I hadn't realized they were talking about THAT much overfill. My main shop will go a few hundred over. Another shop nearby won't put it out the door above rated pressure; he calls it a "bomb". Don't burst valves tend to go off if you go too high?.... LDS said he has seen burst valves go just by getting too hot like in a car. (Adding 50 degrees F in a hot car on a 90 degree day would add slightly less than 10 percent to the pressure, by the way.) I have heard of plugging burst valves but that seems over the top to me. Anyway if that is the main reason for LP's, that you have to exceed their rated pressure by 50% to beat HP's in terms of gas/pound of tank, then my inclination is to stick with HP's, either 100 or 120. So many other more controllable risks in scuba, why add in a wild card like REALLY stressing out your tanks.

Track record's proven it to be pretty good, no bombs lol and tanks continue to pass hydro with no problems, and 3600 is in relation to 1/3rds. There's a common misconception, the number stamped into the neck of the tank isn't the fill pressure, it's the turn pressure . In regards to burst discs, some double disk, some use high pressure disks (in the 5000-6000 range usually), and some plug the disk. If in a fire, the neck o-ring should extrude before the tank fails.

 

[Captain12Pk]

I think I've missed something. Didn't you say you already have the 120's? and that you're comfortable with them? If that's the case, I don't see why you'd want to build another set just yet. I have several sets of tanks, but I built different sets for different uses. If you're looking for one set to use everywhere, and you have one set you're already comfortable with, the rest seems intuitive.

 

[DC53]

I think I've missed something. Didn't you say you already have the 120's? and that you're comfortable with them? If that's the case, I don't see why you'd want to build another set just yet. I have several sets of tanks, but I built different sets for different uses. If you're looking for one set to use everywhere, and you have one set you're already comfortable with, the rest seems intuitive.

Need more tanks for single tank dive trips, between the wife and myself. Wanted to get tanks that fill both shoes as opposed to remaining dedicated doubles tanks. At least that's the idea for now.

 

[RJP]

Anyway, (for possible future tanks) everyone seems to favor LP steel tanks.

Not sure if that's the case. Certainly not everywhere. I've got two sets of double HP 119's - a not uncommon configuration here in NJ. As Lynn pointed out, it's about how much gas you need and can carry easily. For my size and stature (6'2" and 175lbs) the 119's are a nice package. For my diving, it's enough gas to have full "lost deco gas" capability to do pretty much any deco necessary on back gas if I had to.

 

[Colliam7]

Currently my setup uses two steel HP 120’s as they are the only tanks that I have in an identical pair. However for the future I am wondering about input on tanks.

Double 120s are fine. A bit long for the divers shorter than 6’, perhaps, but not a management problem for the majority of divers that use them.

Anyway, (for possible future tanks) everyone seems to favor LP steel tanks.

Whoa! As RJP mentioned, I don’t think that is the case (at least not on SB) at all.

With this info, is it just tradition that speaks so much against the HP tanks or is there some reason that I am missing?

I suspect what you are missing is that the ‘tradition that speaks so much against the HP tank’ doesn’t really exist. LP tanks are fine. HP tanks are fine. I have NEVER had a problem getting HP fills, in FL or NC or VA or AL. That doesn’t mean you won’t ever have a problem. Rather, I just haven’t experienced it. And, if I did, I would change my gas fill venue. Any competent shop can fill their banks to well beyond 3500 psi, or they have a compressor, or both. I HAVE had a problem getting 100% oxygen bottle filled to 3000 psi, but have never had even a short fill on my 3500psi 120s, or my 3442 psi 100s, or 130s.

what are thoughts about choosing 120’s as opposed to 100’s?

I have both. My (slight) preference is my double 100s, simply because of trim characteristics. But, when I want more gas, I go to the 120s. Actually, I go to my double HP130s. BTW, if I want the best trim, I go with my double 80s.

 

[Superlyte27]

OMS used to claim 10,000 Fills at 4000psi on their LP tanks. Of course DOT made them remove that sales pitch from their advertisement. I used to hydro about a thousand tanks every year. It was very rare for an LP95 to fail. But, I have been told that fills above 3600 start to stretch the metal to fragmentation. I'm not sure how true it is. I try to keep my LP's under 3600 cooled. Oh, and I have 4500 burst disks in my doubles.

 

[iztok]

Whoa! As RJP mentioned, I don’t think that is the case (at least not on SB) at all.

Haha, I know the area and I know OP and people telling to go LP there is certainly some cave country tradition in this.

DC53, you need to do your own calculations. I did and ended up with HP119s (as singles and will get another set for doubles). Think this way, LP tank with about same dimensions filled at HP pressure holds about same amount of gas. LP is overfilled dramatically while HP is at rated working pressure. It goes the other way around. Same LP and HP pair filled at LP pressure have about the same amount of gas as well.

Here are calculations for HP 119 and LP 95 (from XS Scuba Worthington Steel Cylinder Specifications)

HP119:

123 3442
94.3 2640 (under-filled for 23%)

LP95

93.3 2640
121.6 3442 (over-filled for 30%)

So as you can see LP95 and HP119 are about the same in terms of capacity at the same pressure.

For your dive trips you have a couple of options. If two single HP120s are good for your two tank dives then one set of doubles HP120s should be the same. So instead of splitting them apart when you need singles and put them back together when you want doubles just dive doubles in both cases.

What you might consider is AL80 doubles for wetsuit diving.

 

[mark99]

I was in the same situation as you, first set was double hp120's, and I find the best second set to be double lp85's. They make a good second set for a second deco dive when you've used the 120's for the first, longer dive. They also make a good set for general recreational use if you get tired of lugging around the 120's- they're a lot lighter! (and cheaper than equivalent HP tanks).

 

[Scared Silly]

My main shop will go a few hundred over. Another shop nearby won't put it out the door above rated pressure; he calls it a "bomb". Don't burst valves tend to go off if you go too high?.... LDS said he has seen burst valves go just by getting too hot like in a car. (Adding 50 degrees F in a hot car on a 90 degree day would add slightly less than 10 percent to the pressure, by the way.) I have heard of plugging burst valves but that seems over the top to me.

Wow, I think you need to find a better set of shops.

A disc for 2400psi working should blow at 4,000ps.
A disc for 3000psi working should blow at 5,000ps.

If they were blowing before that then the disk was bad or the wrong disk was installed. Disks should be replaced at the time of hydro which is when a valve should be rebuilt.

As for increase in pressure:

A cylinder at 70F (20C) with a 3000 psi working pressure

at 90F (30C) would be ~3100 psi
at 120F (50C) would be ~3300 psi
at 140F (60C) would be ~3400 psi

All are well below the burst disk blow rating.

BTW it is within CGA standards to overfill a cylinder if when cooled to 70 degrees it is at it's rated working pressure. Further, the recommended filling rate is 600 psi/m. IF your shop does not know that then you might want a new shop.