Low pressure steel 72s

[dagrath]

I tried out a new LDS, because they listed HP ST100s for rent, and I wanted to try them out. We got there, and it turned out they really only carried ST72s (they didn't even rent AL80s). These tanks were tall(ish) and skinny. They plugged each one into the compressor to check fill before giving them to us, and off we went. (They hadn't just been filled afaik, so it should have been a cold reading, not a hot one)

When we got to the dive site, we hooked up our gear and saw 2100psi. I had thought most LP tanks were rated at 2400psi. Is anyone familiar with any 2100psi LP steel 72s? I couldn't find them listed anywhere online, and I am just curious if we were shorted. We rented 6 tanks, and all of them were filled to 2100... which seems more like spec than short.


On a side note, I realized a key difference between LP and HP tanks yesterday. It is probably common knowledge to most long time divers, but for a newbie like me it was interesting.

If you return with 500psi, the percent of the air in the tank you use is a lot lower in a LP tank. For example:

My LP Steel 72s from yesterday, at 2100psi service pressure
500psi is almost a quarter of the tank. So I return with 17 cubic feet of air in the tank, meaning I used 55 cubic feet.

Lets take a similar tank, rated as an HP tank with 3442psi. Now you are returning with 10.5 cubic feet of air, and you used around 61.5 cubic feet. Around a 12% increase in air time even though both tanks are 72 cubic feet.

If I had been able to rent an AL80 at the LDS (which I wanted, once I learned they didn't have ST100s), I would have returned with 13 cubic feet of air at 500psi. That means I would have used around 67 cubic feet of air, about a 22% increase in air time.

Needless to say, I was disappointed with the tanks I ended up getting. I have been jonesing for HP120s or 130s. I am 6'4" and athletic, I don't mind the weight and want more bottom time. LP72s weren't the answer.

[BSea]

Originally Posted by dagrath

I tried out a new LDS, because they listed HP ST100s for rent, and I wanted to try them out. We got there, and it turned out they really only carried ST72s (they didn't even rent AL80s). These tanks were tall(ish) and skinny. They plugged each one into the compressor to check fill before giving them to us, and off we went. (They hadn't just been filled afaik, so it should have been a cold reading, not a hot one)

When we got to the dive site, we hooked up our gear and saw 2100psi. I had thought most LP tanks were rated at 2400psi. Is anyone familiar with any 2100psi LP steel 72s? I couldn't find them listed anywhere online, and I am just curious if we were shorted. We rented 6 tanks, and all of them were filled to 2100... which seems more like spec than short.


On a side note, I realized a key difference between LP and HP tanks yesterday. It is probably common knowledge to most long time divers, but for a newbie like me it was interesting.

If you return with 500psi, the percent of the air in the tank you use is a lot lower in a LP tank. For example:

My LP Steel 72s from yesterday, at 2100psi service pressure
500psi is almost a quarter of the tank. So I return with 17 cubic feet of air in the tank, meaning I used 55 cubic feet.

Lets take a similar tank, rated as an HP tank with 3442psi. Now you are returning with 10.5 cubic feet of air, and you used around 61.5 cubic feet. Around a 12% increase in air time even though both tanks are 72 cubic feet.

If I had been able to rent an AL80 at the LDS (which I wanted, once I learned they didn't have ST100s), I would have returned with 13 cubic feet of air at 500psi. That means I would have used around 67 cubic feet of air, about a 22% increase in air time.

Needless to say, I was disappointed with the tanks I ended up getting. I have been jonesing for HP120s or 130s. I am 6'4" and athletic, I don't mind the weight and want more bottom time. LP72s weren't the answer.

Most 72's are rated at 2250, and are older tanks. But you only get 72 cubic feet if they are filled to their "+" rating, or 2450 psi. The "+" rating means they can be filled to 10% beyond their stamped pressure. You are right that most newer steel LP tanks are rated at 2400. With their "+" rating, they can be filled to 2640, and that is where they get the volume rating. However, many LP & HP steel tank's capacity also differ from their stated capacity. Some are a few cubic feet less, and a few actually hold more than their stated capacity.

Now I like lp 72's for what they are, but I also would have been disappointed if I expected 100's. However, if I had the choice between a lp72, or an al 80, I'd take the 72. Its lighter, has better buoyancy characteristics, and only 5 cubic feet less. Most people don't know that a standard AL 80 is really only 77 Cubic feet.

[MSilvia]

That's yet another reason the "500 psi rule" is a poor rule of thumb. It doesn't account at all for the size or service pressure of the tank.

[CompuDude]

Originally Posted by dagrath

Needless to say, I was disappointed with the tanks I ended up getting. I have been jonesing for HP120s or 130s. I am 6'4" and athletic, I don't mind the weight and want more bottom time. LP72s weren't the answer.

I'd be disappointed too, in that situation. LP72s (those old 2200psi models in particular) are not the answer to your needs. The one advantage they have is buoyancy compared to aluminum.

And yes, you're right (as is Matt), the "500 psi rule" is wildly inaccurate once you stop using Al.80s. 10-13cf is a reasonable cushion, so better is to know what you're diving and plan your gas needs in advance. Bear in mind, however, the other reason for the 500psi rule, being that gauges tend to be less accurate at the extremes, so that 500psi may not actually be 500psi. Thus the cushion.

Time to save up for HP100s.

[Rileybri]

Originally Posted by BSea

Most 72's are rated at 2250, and are older tanks. But you only get 72 cubic feet if they are filled to their "+" rating, or 2450 psi. The "+" rating means they can be filled to 10% beyond their stamped pressure. You are right that most newer steel LP tanks are rated at 2400. With their "+" rating, they can be filled to 2640, and that is where they get the volume rating. However, many LP & HP steel tank's capacity also differ from their stated capacity. Some are a few cubic feet less, and a few actually hold more than their stated capacity.

Now I like lp 72's for what they are, but I also would have been disappointed if I expected 100's. However, if I had the choice between a lp72, or an al 80, I'd take the 72. Its lighter, has better buoyancy characteristics, and only 5 cubic feet less. Most people don't know that a standard AL 80 is really only 77 Cubic feet.

So I dive ST72's (71.2 to be exact.) and I love them. As most of the people I am diving with are using al80's I get more than enough gas and more often than not I am getting out of the water with more gas left in my tank than the AL80 user. The reason to get then has nothing to do with the + rating. It is my understanding that the + is only good through the fi9rst hydro date?
The reason I use/got them is that it gives me a comparable amount of gas, in a tank that is negatively buoyant even when empty, as an AL80. In addition my 2 ST72's cost me a whopping total of $75, for both, with current hydro, viz and filled.

[CompuDude]

Originally Posted by Rileybri

So I dive ST72's (71.2 to be exact.) and I love them. As most of the people I am diving with are using al80's I get more than enough gas and more often than not I am getting out of the water with more gas left in my tank than the AL80 user. The reason to get then has nothing to do with the + rating. It is my understanding that the + is only good through the fi9rst hydro date?
The reason I use/got them is that it gives me a comparable amount of gas, in a tank that is negatively buoyant even when empty, as an AL80. In addition my 2 ST72's cost me a whopping total of $75, for both, with current hydro, viz and filled.

Plus rating is good until the next hydro date... unless you get another hydro that does the extra testing and puts a new "plus" next to the new hydro date. Then, again, it's good until the next hydro.

If you skip a plus on one hydro cycle, you can still have it put back on at the next hydro.

[BSea]

Originally Posted by CompuDude

Plus rating is good until the next hydro date... unless you get another hydro that does the extra testing and puts a new "plus" next to the new hydro date. Then, again, it's good until the next hydro.

If you skip a plus on one hydro cycle, you can still have it put back on at the next hydro.

This is exactly right. I've seen them where the + was 30 years after the last + with several regular hydros in between.

[Rileybri]

Originally Posted by CompuDude

Plus rating is good until the next hydro date... unless you get another hydro that does the extra testing and puts a new "plus" next to the new hydro date. Then, again, it's good until the next hydro.

If you skip a plus on one hydro cycle, you can still have it put back on at the next hydro.

Originally Posted by BSea

This is exactly right. I've seen them where the + was 30 years after the last + with several regular hydros in between.

Sure now you bums tell me! lol well regardless I love my ST72's!! So (and sorry for the thread hijack) if I were willing, even with current as of last moth for month of them and this month for the other, can I send them back for another hydro for the + rating??

[WD8CDH]

You should always ask for testing for the + rating testing with a 3AA tank. It is very rare for a tank to pass hydro and not also pass for the + rating.

By the way, like others have said, the LP72 is a very nice size tank. It's a shame that the same size tank is not made as an HP tank. It would still have the nice buoyancy but would hold 100 CF. I prefer the smaller diameter over the more common 7.25" and 8.0" diameter tanks. Especially for doubles.

[Luis H]

Originally Posted by Rileybri

Sure now you bums tell me! lol well regardless I love my ST72's!! So (and sorry for the thread hijack) if I were willing, even with current as of last moth for month of them and this month for the other, can I send them back for another hydro for the + rating??

A good hydro facility can just look back at their records for the recent hydro numbers and stamp the “+” based on the recent hydro.

There is no extra testing required. It is just a calculation based on the data already taken.

The only number must hydro facilities are missing is the REE number for a steel 72. I have estimated it to be around 55 to 60 cc (others have come up with the same estimate).

Therefore, if the elastic expansion is less that 55 cc it is a pass for the “+” stamp.


One of my local hydro facilities did “+” stamped 5 of my steel 72 a few months after they did the hydro when I went directly to the facility and asked them if they would (I bought the 5 tanks from a dive shop that had them hydro at that facility).

You can find who did the last hydro by the number in the last hydro date.