Tank Sizing Questions

[Vlane]

Originally Posted by in_cavediver

Navy, your numbers are off. The OMS LP121 is 29" and the OMS LP 131 is 31". The PST LP 121 is about 29.4" .This is compared to the PST E7-120 which is 28.5"

IN general - they are pretty close in length with a slight nudge longer to the LP tanks.

I've been looking, out of curiosity, and LP tend to only be an inch or two taller than HP tanks. I can see how that might slightly affect trim but not by much.

Can someone elaborate on the + that some LP tanks have? The LP tanks that ST carry have the +, but is it uncommon to not have the +? Also, can a tank lose the + rating after years of having the 10% overfill or cave fill?

[in_cavediver]

Originally Posted by Vlane

I've been looking, out of curiosity, and LP tend to only be an inch or two taller than HP tanks. I can see how that might slightly affect trim but not by much.

Can someone elaborate on the + that some LP tanks have? The LP tanks that ST carry have the +, but is it uncommon to not have the +? Also, can a tank lose the + rating after years of having the 10% overfill or cave fill?

The '+' rating allows for a 10% overfill of the tank. IE, most LP tanks are rated at 2400psi fill pressure but with the '+', they get filled to 2640. It should also be noted that the rated capacity of most LP tanks is at 2640, not 2400 psi. This is also limited to the 3AA steel tanks - commonly referred to as LP steels in the scuba arena.

As for which is elgible - any 3AA steel tank is eligible to qualify for the '+' rating. All it has to do is pass hydro at the '+' standard. This standard is valid only for the hydro period in question. Its possible for a tank to have the '+' rating for 5 years, not have it for 5 years and then again have it for 5 years. It all depends on the hydro test performed.

The last bit is cave fills. These simply are gross overfills in LP steel tanks and are not legal, allowed by the DOT or recommended by any tank manufacturer. They are also fairly common in North Florida. For an example - take my PST LP104's they are 106cft@2640. A cave fill puts them between 3600-4000psi. At 4000psi, they have around 160cft of air in them. (145cft at 3600psi). Cave fills are all about getting more air. A set of LP121's pumped to 4000 is almost 400cft of air. More cft means more time underwater on thirds. These do cause damage to the tanks and will shorten the life of the tank. That said - I have seen quite a few 15-20 year old steel LP tanks that have known a life of cave fills which keep passing hydro.

[navyhmc]

Originally Posted by in_cavediver

Navy, your numbers are off. The OMS LP121 is 29" and the OMS LP 131 is 31". The PST LP 121 is about 29.4" .This is compared to the PST E7-120 which is 28.5"

IN general - they are pretty close in length with a slight nudge longer to the LP tanks.

Oops, you're right...sorry. Note to self, don't post when you're THAT tired.

[CompuDude]

Originally Posted by Vlane

I've been looking, out of curiosity, and LP tend to only be an inch or two taller than HP tanks. I can see how that might slightly affect trim but not by much.

Can someone elaborate on the + that some LP tanks have? The LP tanks that ST carry have the +, but is it uncommon to not have the +? Also, can a tank lose the + rating after years of having the 10% overfill or cave fill?

In_Cave gave the detailed response. I'll summarize... sort of.

Most new LP tanks (if not all) are sold with the + rating stamp, which allows a 10% overfill. The tanks are all rated to 2400, so adding 10% gets you 2640.

LP tanks are sold assuming the plus rating. So an LP 95 only carries 95 cf of gas if you fill it to 2640 psi (ie, 10% over the stamped rating of 2400 psi)

After 5 years pass, and it's time for hydro, you have to take the LP tank to a hydro facility that is willing and able to give it the extra test needed to extend the + rating. A tank can pass the hydro but not the + test, at which point you can only (legally) fill it to 2400 psi, and it no longer holds 95cf of gas with a legal fill, it now holds 86cf. (Many simply continue to overfill and ignore this, but ...)

As I alluded earlier, there are a number of hydro stations that cannot (or at least that's what they claim) or will not perform the extra steps needed to restore the + rating to the tank... sometimes they simply don't know, other times they just can't be bothered, who knows. It's not supposed to be that way, but such is life. Most places around Los Angeles fall into this category, unfortunately. So unless you're an overfiller in cave country, if you choose LP tanks you'll want to be sure you have access to a hydro facility that can and will do the + rating steps.

That's pretty much the basics.

[Vlane]

Thanks for clearing the + up. It seems odd that a tank is advertised as 95 cf tank, but if it doesn't pass a certain aspect of hydro it is no longer a 95 cf tank.

[WD8CDH]

I have only seen two 3AA tanks manufactured after 1960 that did not have the + from the factory. I was able to get them (yes, they were mine) tested and stamped for the + on subsequent hydros.

Some, but unfortunately not all 3AA tanks have their REE value stamped on the tanks. That value is what the hydro shop needs to determine if the tanks can be stamped with the +.

[in_cavediver]

Originally Posted by WD8CDH

I have only seen two 3AA tanks manufactured after 1960 that did not have the + from the factory. I was able to get them (yes, they were mine) tested and stamped for the + on subsequent hydros.

Some, but unfortunately not all 3AA tanks have their REE value stamped on the tanks. That value is what the hydro shop needs to determine if the tanks can be stamped with the +.

The sad part is the REE values are published in the books/documentation the Hydro shops are supposed to have. As Compudude has stated, the quality of hydro shops seems to vary geographically.

Good for me - I have a top notch hydro facility who are willing to answer all of my questions and know that some tanks have recommended procedures (PST HPs for instance)

[redsoxphan]

Thanks everyone!! This is all great info and something that I've not given much thought until reading these posts.

So, an air hog like me needs more CF (bigger tank), not necessarily more working pressure.

[Flatliner]

Originally Posted by navyhmc

Not to mention LP's are a lot bigger LP 121 is 38", a HP 120-7 is 28" and HP1119-8 is 24 a 130 is 25. the LP121 is huge!

I LOVE my LP121!!!!

[navyhmc]

Originally Posted by Flatliner

I LOVE my LP121!!!!

I like mine as well, but you're juuust a few inches taller than me!