NEWBIE Q: Service Pressure vs. Working Pressure

[rjack321]

I don't know if there is a standardized definition of service versus working pressure, but here is my definition:

• Service pressure is the pressure stamped on the tanks. It is the basic tank pressure use to do any tank calculations... it is used to figure out the tank hydro test pressure, etc.
• Working pressure would be the same as the service pressure unless it has an allowed overfilled "+" stamp on the cylinder. On some steel tanks (normally the 3AA type tanks) the tank can be filled to 10% over the service pressure as long as it has a current "+" stamp next to the most recent hydro test date.

Below is an example of a tank that has different service and working pressure:

72 cu ft Standard (vintage)
Service pressure: 2250 psi
Working pressure: 2475 psi (2250 psi +10%)
Actual air capacity: 71.2 ft3 (at a working pressure of 2475 psi)
Outer diameter: 6.9 in
Length without valve: 25.1 in
Empty weight: 26 lbs (w/o valve)
Buoyancy Empty: 0 lbs (w/valve)
Buoyancy Full: -5.4 lbs (w/valve)


When calculating SAC, what maters is that you know the actual air volume at whatever pressure it is specified.

For example in the steel 72 mentioned above:
At the working pressure of 2475 psi the actual volume is 71.2 cu ft.
At the service pressure of 2250 psi the actual volume is about 65 cu ft.
Either pair of numbers will give you the correct results as long as you use them together.

I actually looked this up last week. DOT does not define working pressure for scuba cylinders, they do for other types of gas storage stuff though. Hence its technically a 10% overfill of the service pressure.

Here were the definitions/citations I found:
49 CFR § 171.8 Definitions and abbreviations.
Working pressure for purposes of UN pressure receptacles, means the settled pressure of a compressed gas at a reference temperature of 15 °C (59 °F).

49 CFR § 173.301a Additional general requirements for shipment of specification cylinders.
(c) Cylinder pressure at 21 °C (70 °F). The pressure in a cylinder at 21 °C (70 °F) may not exceed the service pressure for which the cylinder is marked or designated, except as provided in §173.302a(b).

 

[whodunit68]

I actually looked this up last week. DOT does not define working pressure for scuba cylinders, they do for other types of gas storage stuff though. Hence its technically a 10% overfill of the service pressure.

Here were the definitions/citations I found:
49 CFR Ž§ 171.8 Definitions and abbreviations.
Working pressure for purposes of UN pressure receptacles, means the settled pressure of a compressed gas at a reference temperature of 15 Ž°C (59 Ž°F).

49 CFR Ž§ 173.301a Additional general requirements for shipment of specification cylinders.
(c) Cylinder pressure at 21 Ž°C (70 Ž°F). The pressure in a cylinder at 21 Ž°C (70 Ž°F) may not exceed the service pressure for which the cylinder is marked or designated, except as provided in Ž§173.302a(b).

now this is an interesting definition that contradicts each of the earlier. The settled pressure as the gas/tank cools will likely be less than the pressure that was initialliy read. For example, I dove yesterday with LP 95s. The tanks have 2400 stamped on them. I read the pressure initially at 2685 on one dive. By dive time, maybe 20 minutes later, the pressure read 2570. 2685 is less than the 2400 + 10%, of course, and it was hot - just a few minutes after being filled. By this definition, if it were 15 Ž°C, I'd have the settled (or working) pressure.
S

 

[DA Aquamaster]

I actually looked this up last week. DOT does not define working pressure for scuba cylinders, they do for other types of gas storage stuff though. Hence its technically a 10% overfill of the service pressure.

Here were the definitions/citations I found:
49 CFR Ž§ 171.8 Definitions and abbreviations.
Working pressure for purposes of UN pressure receptacles, means the settled pressure of a compressed gas at a reference temperature of 15 Ž°C (59 Ž°F).

49 CFR Ž§ 173.301a Additional general requirements for shipment of specification cylinders.
(c) Cylinder pressure at 21 Ž°C (70 Ž°F). The pressure in a cylinder at 21 Ž°C (70 Ž°F) may not exceed the service pressure for which the cylinder is marked or designated, except as provided in Ž§173.302a(b).

This is not complete unless you also look at and cite Ž§173.302a(b).

In general a fill is "legal" if, when the tank cools to room temp (70 degrees) the service pressure stamped on the tank (with an extra 10% allowed on plus rated tanks) is not exceeded. This means hot fills are fine and tank monkeys can exceed the stamped pressure if the tank is warm during the fill.

With experience, a good one can feel the tank shoulder and guestimate the "overfill" needed to give a full fill when the tank cools to room temp.

At the other extreme, those who do not understand the reg, will not exceed the stamped pressure and will underfill the tank by 200-300 psi.

 

[rjack321]

This is not complete unless you also look at and cite Ž§173.302a(b).

This section allows 10% overfills if a 3AA tank:
has a burst disk,
is in hydro,
the wall stresses meet a mathematical formula that is impractical and never actually calculated post manufacture.
has a + after the latest hydro date