What size of steel tank for beginner doubles?

[TSandM]

The Fabers are very negative, but my husband's Worthington HP 100s are only 1.5 pounds more negative than my LP 95s. They are, however, five pounds lighter when empty. I didn't realize that until just now.

 

[SparticleBrane]

Just remember to be specific when discussing "HP" tanks.
If you're talking about the true HP tanks, 3500psi, then say HP.
If you're talking about the 3442 tanks, say "exemption" or "special permit" tanks.

There ARE buoyancy differences, especially in the Faber line.

 

[bigcat]

Been diving dual HP 100's for 5yrs........ Love them!

 

[WD8CDH]

One thing not mentioned in the HP vs LP cylinder differences are the buoyancy differences. LP cylinders tend to be more buoyant than HP. Take the Faber LP85 versus HP100 and put the exact same amount of gas in them. I do not care if it is 100cuft or 10cuft. The HP cylinder will be around 3 pounds more negative. that becomes 6 pounds when doubled. Coincidentally, the HP100 is also 3 pounds heavier dry (6 pounds when doubled). This may not be much but for me it was. I do not need the extra 6 pounds even when diving dry. And I certainly do not want to be schlepping more weight than I need to. So I went with the LPs. So outside of the HP is really a LP but just put what you need in it argument I think there is a need for both.

But don't forget that if your tank is 3 pounds more buoyany, you need to carry 3 more pounds of lead to make up for it.

Also, adding the same amount of air to both an HP and an LP tank will not add any more weight to the HP tank than it will to the LP tank. Unless you violate Laws of Physics, 1 cubic foot of air weighs .0807 pounds no matter what pressure it is at.

Most modern HP tanks (usually 3442psi) don't get their higher pressure handling ability from more steel and thicker walls, most get it from stronger steel af the same amount and same wall thickness. Due to slight differances in hot forming stretch, there will be very very slight differences between HP and LP tanks that come from the same die.

That is why at the same pressure, an HP tank will have the same weight, buoyancy, trim and stored air as a LP tank of the same dimensions. The only real disadvantage is inital cost. The advantage is you can put more air in anHP tank if you fill it to higher pressures than a HLP tank allows. At 2640psi, they both (e.g. HP119, LP85) dive and carry the same.

If you include all the weight, including weight belt, you almost always shlep around more weight per cubic foot with LP tanks. LP tanks average usually over .360 pounds per cubic foot of rated capacity and HP tanks are usually less than .325 pounds per cubic foot of capacity.

 

[rjack321]

Just remember to be specific when discussing "HP" tanks.
If you're talking about the true HP tanks, 3500psi, then say HP.
If you're talking about the 3442 tanks, say "exemption" or "special permit" tanks.

There ARE buoyancy differences, especially in the Faber line.

There are far too many tank models to generalize nowadays.

By and large avoid all the HP Faber tanks. The 3180+ rated ones or above. They are built to a vastly different (heavier!) standard than the 2400+ LP tanks. Kinda amazing they would do that for just a wee bit more pressure eh?? I mean shoot what's -20% on tensile strength among steels - it works just fine on most bridges, except for one in Minneapolis.

When I started diving 15yrs ago it was damned hard to even get a 3000psi fill. Compressors frequently weren't setup to deliver 4000+psi. So I really question those who believe there's 15-30 yrs of good history behind filling a 2400+ tanks to 3900psi.

 

[Scared Silly]

But don't forget that if your tank is 3 pounds more buoyant, you need to carry 3 more pounds of lead to make up for it.

Not in my case I do not need that much weight which is what I was pointing out. Some might. Which in this case the HP 100 might make more sense.

Also, adding the same amount of air to both an HP and an LP tank will not add any more weight to the HP tank than it will to the LP tank. Unless you violate Laws of Physics, 1 cubic foot of air weighs .0807 pounds no matter what pressure it is at.

I do not think anyone has said that. If you note I said "put the exact same amount of gas in them. I do not care if it is 100cuft or 10cuft."

Most modern HP tanks (usually 3442psi) don't get their higher pressure handling ability from more steel and thicker walls, most get it from stronger steel at the same amount and same wall thickness. Due to slight differences in hot forming stretch, there will be very very slight differences between HP and LP tanks that come from the same die.

True - though I do know first hand if this what the mfg. are truly doing (using the same mold). I have heard this repeated several times. Perhaps someone from Worthington or Faber can chime in. However, it should also be pointed out that stronger does not mean better. There are many factors that go into choosing the right metal for a particular application. Irregardless of LP or HP if properly maintained and filled they will probably outlast most divers.

That is why at the same pressure, an HP tank will have the same weight, buoyancy, trim and stored air as a LP tank of the same dimensions. The only real disadvantage is inital cost. The advantage is you can put more air in an HP tank if you fill it to higher pressures than a HLP tank allows. At 2640psi, they both (e.g. HP119, LP85) dive and carry the same.

I think you meant an HP119 and an LP95 (that is for Worthington) - which is basically true - modulo 5%.

If you include all the weight, including weight belt, you almost always schlep around more weight per cubic foot with LP tanks. LP tanks average usually over .360 pounds per cubic foot of rated capacity and HP tanks are usually less than .325 pounds per cubic foor of capacity.

This certainly true in the case of my wife MP72s - she would have been better off with some HP72s as they are 8 pounds lighter. But considering we got them $200 well sometimes you gotta go with that.

The bottom line is that these are issues that folks should look at when buying cylinders. For me it made more sense to go with LP. This is certainly not the case for everyone.

 

[DA Aquamaster]

Just remember to be specific when discussing "HP" tanks.
If you're talking about the true HP tanks, 3500psi, then say HP.
If you're talking about the 3442 tanks, say "exemption" or "special permit" tanks.

Everything other than 3AL and 3AA tanks made under the established standards for aluminum and steel tanks are made under an exemption or special permit - including 3500 psi tanks.

But I agree we need to specify between the current 3442 psi PST E series and Worthington X series tanks and the older 3500 psi 7/8" valved Genesis tanks.

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The difference between 125,000 psi and 145,000 psi is potentially significant and treating a low pressure tank like a high pressure tank made from a stronger steel is not wise. We can compare tensile strengths for the steel used in low pressure and high pressure tanks, but that largely misses the point. Regardless of the material used or its tensile strenght, both LP and HP tanks are designed with a safety factor based on that material. The wall thickness of two tanks formed on the same tools may have different wall thicknesses based determined by the thickness of the disc of steel used to form the tank. The difference in weight may not be all that great (maybe .5 to 1.0 lb) but the difference in wall thickness can still be significant. The lenght fo the tank can also be significant as a tank that is only an inch or so shorter may will have much less wall stress than a tank only slightly longer. Those little details can make a big difference.

In any event, when you overfill a tank with a given service pressure to levels 150% over that service pressure, you are seriously reducing the safety margin for the tank and are running the risk of reducing that safety margin to critical limits.

 

[rjack321]

In any event, when you overfill a tank with a given service pressure to levels 150% over that service pressure, you are seriously reducing the safety margin for the tank and are running the risk of reducing that safety margin to critical limits.

I have no problem filling my lp85s and lp95s to 3000psi. There is ample experience that those kinds of overpressures are tolerated well. 3600-4000 psi, now that's just nuts. And neither the engineering nor decades of "field experience" can demonstrate that's prudent. And I agree with your earlier post that's just changing an OOA risk for another type of risk.

Rather than massively overfilling LP tanks I would prefer to bring a stage bottle.

 

[Mr. Sunday]

I started with LP 80's and they worked well for me. I have a set of lp95's that I'm currently considering banding up too although I may sell them and go with a different size altogether. Love my twins!

 

[Jimmer]

The Fabers are very negative, but my husband's Worthington HP 100s are only 1.5 pounds more negative than my LP 95s. They are, however, five pounds lighter when empty. I didn't realize that until just now.

Not all the Fabers are very negative. The 3180+ tanks are extremely heavy and negative. According to Blue Steel's website, my lp108's actually go positive when empty.