why the "LP" versus "HP" service pressure rating in the US?

[oldschoolto]

Adding a very small amount of a Element can have major changes in the property of the steel... Most alloys are very closely held secrets... Also the forming and heat treatment...

Think about two wrenches that look the same , But... One is super strong and the other is junk...

Jim...

 

[MichaelMc]

Blue Steel specs for Faber tanks: Blue Steel Scuba - Cylinder Specs

..... L... cu.in. L.... D... wt..
LP95. 15.. 915... 23.82 8.02 37.2 3AA
FX117 15.0 915... 24.21 8.02 38.9 13488
Diameters the same. FX is heavier, i.e., has more steel.

 

[Scared Silly]

I agree. That's the first thing that occurred to me as likely accounting for the difference. Can anyone confirm these are different between a Worthington LP85 and a Faber FX100, or between other "LP" and "HP" tanks that are nearly identical in dimensions and weight?

Yes, they are different. How do I know because I talked to the mfg. (plus being practicing engineer at one time, I learned stuff like this issue) And when the whole discussion about the Faber LP/HP baloney was going on I got Lee from Sea Pearls (who used to be the rep. for Faber before Bud took over) to make a statement about this nonsense. Somewhere in the bowels of SB there is a thread on it.

Here is it: Difference between LP and HP tanks

 

[Lorenzoid]

Adding a very small amount of a Element can have major changes in the property of the steel... Most alloys are very closely held secrets... Also the forming and heat treatment...

Think about two wrenches that look the same , But... One is super strong and the other is junk...

Jim...

So the reason no one has ever definitively answered this question may be that there are few people in the world who would know for sure, but the alloys and manufacturing process are the most likely suspects. I'll buy that.

 

[tbone1004]

@Lorenzoid did I mention they had thicker walls? that is sometimes the case, but not always.

@boulderjohn your 108's vs his 130's hold 5% more regardless of pressure. Important to remember that scuba tanks in this country are based on nominal volumes not actual volumes so if your tank can hold 5% water, it will always hold 5% more at the same pressure, regardless of what that pressure is.

to the original question. Comparing Faber to Worthington is not fair because the Worthingtons are using a hot dip galvanizing and zinc is pretty heavy. That doesn't technically go to wall thickness, but it is there. The alloys are almost guaranteed to be different as well because they are made to different design standards. You can see that in the tensile strength requirements that @Charles2 quoted.
A better comparison is the Worthington LP95 to the X8-119. They hold the same water volume, and have the same dimensions, yet the X8-119 is heavier on land and more negative in the water. This is likely a function of the alloy used in the X8 series being more dense than the alloy in the LP series.

Regarding the Worthington lp85 vs the Faber FX100. We know that the external treatments are different *hot dip zinc vs spray*, and we know that the engineering standards are different *3aa vs. the exemption*, and we know that the internal water volume is slightly larger on the Faber at 12.9l vs 12.8l, so there are a lot of different things going on.


Comparing the Worthington to the Faber HP alloys. Faber has a pretty good amount of nickel, there is no nickel in the Worthington alloy, they also have a good bit of copper in there which is not present in the Worthington alloy. No idea what they chose for the 3aa alloy as it's a pretty loose spec, but it's highly unlikely it's the same alloy and process as the exemption tanks

 

[Diving Dubai]

A
Most alloys are very closely held secrets... Also the forming and heat treatment...
Jim...

Not so, I'm afraid.

All commercial alloys are published, You get a design handbook and each alloy is listed with it's composition and properties (Corrosion strength weight etc) So design engineers can pick the most suitable one for their design

All alloys are "standardised" i.e. they have to meet a national or international standard (EN, BS, ANSI etc)


Similarly the heat treatments are standardised the basic one's are listed from t1 through to T10 condition. Again the process must meet the requirements of an international standard, thus a Billet of Aluminum 7075 in the T8 condition from maunfacturer A, will exibit the same properties as the same material from Manufacturer B

New alloys are developed at the request of say Defence or space industries but they're not magical, they may just offer a slight improvement in a couple of areas over the standard materials. Because their composition is different it falls outside the standards. Because it's made in small quantities it isn't worth getting it approved by putting it through the standardisation process.

 

[Diving Dubai]

@Lorenzoid, @boulderjohn

Often the same product is certified against different standards to meet a specific market/purchasing requirement. Also standards are generally not withdrawn nor superceeded hence you end up with a mine field of differing standards to do the same job (just look at the number of differing international standards for Scuba cylinders)

What the manufacturer will do is take a product and certify it to a number of differeing standards (lets say LP and HP for simplicity) It will be the same product put forward for both but given different part numbers, thus the LP cylinder will be an HP cylinder down rated, because that part number is certified only to that standard.

Often (and this is generally driven by departments who bulk purchase) don't want a higher spec cylinder because they think it costs more. The manufacturer isn't goin gto go through the expense of a seperate line so in realitly they are the same item

However you can't know that, because they may run a batch with a lower grade material to meet teh requirements of the lesser standard. All you can ever know is that product only meets the requirement of the standard it was offered against

As to why something as simple as a scuba cylinder is sold world wide under so many standards... Just go read all about the historical battles over different bolt threads or screw head types.. UNS and BSW threads used to drive me insane touring shows around the world Or imagine the sh1t storm if each country had to agree on a standard type of electrcal plug for all their donestic appliances - Not going ot happen unfortunately

 

[oldschoolto]

Not so, I'm afraid.

All commercial alloys are published, You get a design handbook and each alloy is listed with it's composition and properties (Corrosion strength weight etc) So design engineers can pick the most suitable one for their design

All alloys are "standardised" i.e. they have to meet a national or international standard (EN, BS, ANSI etc)


Similarly the heat treatments are standardised the basic one's are listed from t1 through to T10 condition. Again the process must meet the requirements of an international standard, thus a Billet of Aluminum 7075 in the T8 condition from maunfacturer A, will exibit the same properties as the same material from Manufacturer B

New alloys are developed at the request of say Defence or space industries but they're not magical, they may just offer a slight improvement in a couple of areas over the standard materials. Because their composition is different it falls outside the standards. Because it's made in small quantities it isn't worth getting it approved by putting it through the standardisation process.

Yes... The base metals and standard alloys like 6061 or 6063 or 4130 steel are all known.. But, Company's have products made to their specific needs and wants..

And then you have the working and treatment of the metals that are in house secrets.. simply shot beading a metal changes it working properties.. Or heat treatment at 500° vs 600°...

Jim...

As a side note, When I drove Flatbeds and hauled 100's of loads of raw material to factories the first thing they wanted to see was the spec sheets for the load.. And heat treatment.... It was specifically made for that product...

 

[Charles2]

At the risk of getting a little bit out there and just for fun, let’s calculate the new service pressure of a 3AA-2400 when built to SP13488 specifications.

• First, the wall thicknesses are calculated based on test pressures, not service pressures – so we will first have to determine the test pressure of the 3AA-2400.
  • Test pressure specification for 3AA is equal to 5/3 times the service pressure.
    
  • 2400 psi times 5 divided by 3 equals 4000 psi – Test pressure
• Next, the test pressure can be raised by the ratio of the different allowable alloy strengths in 3AA vs SP13488.
  • 3AA max strength allowable = 70,000 psi
    
  • SP13488 max strength allowable = 90,500 psi
    
  • New test pressure = 4000 psi times 90,500 divided by 70,000 = 5171 psi
• We can now calculate the new service pressure
  • Service pressure specification for SP13488 is 2/3 times the test pressure
  • 5171 psi times 2 divided by 3 equals 3448 psi

3448 psi – Hmmm! Sound like any other number that we use in SCUBA? So is it any wonder that there could be two different specification tanks that could possibly have similar dimensions and weights? Could a manufacturer use the same tooling but with different alloys to make two very different tanks?

A big caution remains – and that is that the two specifications are very, very different – not only in strength of the materials, but also in the factor of safety. You cannot overfill the low pressure cylinder to the high pressure ratings without assuming an unknown risk that could be very hazardous to your health. Do not overfill cylinders.

 

[Lorenzoid]

@Lorenzoid, @boulderjohn

Often the same product is certified against different standards to meet a specific market/purchasing requirement. Also standards are generally not withdrawn nor superceeded hence you end up with a mine field of differing standards to do the same job (just look at the number of differing international standards for Scuba cylinders)

What the manufacturer will do is take a product and certify it to a number of differeing standards (lets say LP and HP for simplicity) It will be the same product put forward for both but given different part numbers, thus the LP cylinder will be an HP cylinder down rated, because that part number is certified only to that standard.

Often (and this is generally driven by departments who bulk purchase) don't want a higher spec cylinder because they think it costs more. The manufacturer isn't goin gto go through the expense of a seperate line so in realitly they are the same item

However you can't know that, because they may run a batch with a lower grade material to meet teh requirements of the lesser standard. All you can ever know is that product only meets the requirement of the standard it was offered against

As to why something as simple as a scuba cylinder is sold world wide under so many standards... Just go read all about the historical battles over different bolt threads or screw head types.. UNS and BSW threads used to drive me insane touring shows around the world Or imagine the sh1t storm if each country had to agree on a standard type of electrcal plug for all their donestic appliances - Not going ot happen unfortunately

So, different alloys or manufacturing treatments was the first possibility that occurred to me. This is the other possibility that occurred to me. That is, two tanks could be physically identical, and one could be rated "LP" and the other "HP" for business decision reasons that have nothing to do with the physical characteristics. I can see how this theory is appealing, since we hear of LP tanks routinely being filled to high pressures in cave country and are said to pass hydro again and again.