Question Why are most HP tanks 3442 psi?

[sunny_diver]

3442 psi seems like a random number for the service pressure on HP steel tanks. It's not even a nice pressure in bar.

Anyone know why this pressure was chosen?

I know some older PST HP tanks are 3500 psi. Seems like that would have been a better number to standardize on.

 

[Manatee Diver]

3,500psi requires 300bar valves that are purposely designed to not work with yoke regs.

 

[tursiops]

3442 psi seems like a random number for the service pressure on HP steel tanks. It's not even a nice pressure in bar.

Anyone know why this pressure was chosen?

I know some older PST HP tanks are 3500 psi. Seems like that would have been a better number to standardize on.

Yoke attachmenrs were only allowable to 230bars, ie 3442psi. DIN required for higher tgan that.

 

[runsongas]

its due to a very loose interpretation of the maximum rating for yoke valves with metric to imperial conversion so that they can take a regular 3/4 NPS valve instead of the mandated 3500 psi 7/8 UNF 300bar valve that previously was needed for the 3500 psi tanks

 

[runsongas]

@tursiops there was also a temperature conversion to get to 3442 psi because the european spec for yoke was at a different temperature than the 75F room temp that US DOT uses

 

[tursiops]

@tursiops there was also a temperature conversion to get to 3442 psi because the european spec for yoke was at a different temperature than the 75F room temp that US DOT uses

Go ahead and fill in the details.

 

[greeniguana]

@tursiops there was also a temperature conversion to get to 3442 psi because the european spec for yoke was at a different temperature than the 75F room temp that US DOT uses

Like the Europeans, I keep my HP cylinders in my keg fridge.

 

[lowwall]

From Calculating SCUBA Cylinder Capacity | Dive Gear Express®

"Did you ever wonder how the peculiar service pressure value of 3442 came to be established? The first US manufactured high pressure steel SCUBA cylinder (designed by Pressed Steel Tank Company circa 1987) had a 3500 psi service pressure that could only be used with 300 bar DIN valves and regulators; a considerable annoyance to sport divers in North America who overwhelmingly use yoke fitting regulators. In 2004, PST introduced a new design that could be used with yoke (aka A-clamp) regulators. The international standard at the time for a yoke connection maximum cylinder working pressure was 230 bar at 15°C. However, DOT specifications are in pounds per square inch at a different temperature, so some conversions had to be done...

(Nerd alert!) The Gay-Lussac's law of pressure–temperature says that the pressure of the ideal gas will vary in direct proportion to it's temperature (for a constant volume.) Every diver understands the effect of this gas law as what happens when they get a "hot fill" that cools with a resulting pressure drop, but the inverse is also true. Instead of using ideal calculations, PST used software to model the properties of dry Air as a mixture of pure nitrogen, argon and oxygen. The engineers determined for a cylinder filled to a gas density of ≈9.24 moles per liter and pressure of 230 bar at 15°C then warmed up to 70°F, the pressure would have to be {3442.7 psi | 237.4 bar} to maintain the exact same density. Thus the 230 bar @15°C metric standard became the 3442 psi @70°F imperial standard in the US. The original 230 bar has since been superseded by the ISO 12209:2013 standard of 232 bar, so the DOT maximum cylinder service pressure could hypothetically be 3472 psi if it were established today using the same method. "