NEWBIE Q: Service Pressure vs. Working Pressure

[whodunit68]

greetings scubalings,
I'm yearning for learning and spending time reading and absorbing as much as I can.
I view my computer logs and look at the graphs as part of my plan.
rhyme off.
I come across terms such as "working pressure" that's required to determine SAC rate, for example. I search goodsearch.com (better'n google) for these phrases yet I can't find something that says: "same thing" or gives an explanation. Can someone break it down? Are working and service pressure the same thing? The volume versus capacity issue - does that come into play here?
Thanks in advance. BTW, if this Q should be in another forum, do let me know and I'll take it there. thanks!!!
Steve

 

[Luis H]

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.

 

[DA Aquamaster]

If you divide the working pressure or service pressure by the capacity at that pressure, you get the cu ft per psi. (71.2/2475=.0287) If you take that times 100 you get the cu ft the tank holds per 100 psi - in this case 2.87 cu ft per 100 psi.

Once you know your SAC rate, that can give you a pretty good idea how mnay psi per minute you will burn at depth.

For example I have a real world cold water dry suited working SAC of about .6 cu ft/min. So at 100 ft I will be at 4 ATM, (100/33+1= 4.03) and will use 2.4 cu ft/min. In the steel 72 above that woudl then be just a bit less than 100 psi per minute.

 

[knotical]

On this page:
Luxfer: Serial Numbers & Markings
Luxfer cites European cylinder markings that differentiate between working and service pressures.

Scroll down to:
"Typical Markings for a SCUBA Cylinder According to PED" and look at items 9 and 11

9. PW 232 BAR = (Working Pressure) - Shows the pressure that the cylinder should be filled to - this must not be exceeded.
. . .
11. PS 287 BAR AT 60Ž°C = (Service Pressure) - Shows the maximum service pressure and operating temperature for which the cylinder design is approved. This must not be confused with the working pressure (9).

So it appears that, at least in PED parlance, service pressure covers the eventuality of a scuba cylinder getting exposed to a hot environment after being filled.

< Note: PED = Pressure Equipment Directive of the EU >

 

[Luis H]

Interesting, thanks for the information.

The 10% overfill is a US DOT thing, I am not aware of the Europeans ever using any type of overfill in there regulations.
I have no idea how the overfill would play with those definitions.

 

[whodunit68]

great, thanks guys. Here's where the question comes from...I'm new (just finished #s 47 & 48) and I bought the cable for my dive computer. in the software (which, btw, has no help to speak of - I checked), the SAC rate is not automagically computed. So, you know us newbies, we wanna know! I set out to calculate it and it seems to be asking for working pressure. I typically dive AL80s since they're always rentals (in fact, I don't think I have done anything else - will likely try steel 100s this weekend) so it should be pretty simple to find working pressure. I believe the service pressure of an AL80 is 3000 psi, right? So, what's the working pressure? Is it also 3000 psi? I thought it was service pressure, however, that was stamped on our tanks.
Thanks again.

 

[knotical]

For determining surface air consumption, just use the pressure in the cylinder. That is the working pressure for your calculation at that time with that cylinder. The cylinder's rated working or service pressures don't really enter in to the calculation.

There are lots of threads about SAC on ScubaBoard, including:
http://www.scubaboard.com/forums/basic-scuba-discussions/117504-sac-formula.html
http://www.scubaboard.com/forums/archive/index.php/t-54655.html
http://www.scubaboard.com/forums/archive/index.php/t-476.html

Depending on the degree of accuracy you want, it can be helpful to know the actual capacity of the cylinder. It's typically less than you'd think. For examples (if memory serves), aluminum 80's are closer to 77 at 3000 psi. Full classic steel 72's are closer to 65 if not +rated (and around 71.2 if +rated). I'm sure the actual numbers can be found somewhere here on ScubaBoard, but I didn't look.

And recall that simply placing your cylinder in the water can reduce your gauge pressure, so be sure to temperature-stabilize everything before taking actual data.

 

[whodunit68]

For determining surface air consumption, just use the pressure in the cylinder. That is the working pressure for your calculation at that time with that cylinder. The cylinder's rated working or service pressures don't really enter in to the calculation.

Isn't that the same as the starting pressure then? It has a separate entry for that as well.

There are lots of threads about SAC on ScubaBoard, including:
http://www.scubaboard.com/forums/basic-scuba-discussions/117504-sac-formula.html
http://www.scubaboard.com/forums/archive/index.php/t-54655.html
http://www.scubaboard.com/forums/archive/index.php/t-476.html

Depending on the degree of accuracy you want, it can be helpful to know the actual capacity of the cylinder. It's typically less than you'd think. For examples (if memory serves), aluminum 80's are closer to 77 at 3000 psi. Full classic steel 72's are closer to 65 if not +rated (and around 71.2 if +rated). I'm sure the actual numbers can be found somewhere here on ScubaBoard, but I didn't look.

That's ok - you are right for the AL80s. Until Saturday, that's all that matters. Once Saturday comes, it's a different story. I then have to figure out Steel 100s.

And recall that simply placing your cylinder in the water can reduce your gauge pressure, so be sure to temperature-stabilize everything before taking actual data.

Good tip. I'm looking to be conservative for now and as I get more comfortable, it may be interesting to know more exact to increase bottom time. thanks for taking the time.

 

[DA Aquamaster]

A vintage steel 72 holds 71.2 cu ft at 2475 psi and 65 cu ft at 2250 psi.

An AL 80 holds 77 cu ft at 3000 psi - not 80 cu ft.

As long as you know the tank's capacity at some specific psi you don't need to know anything else.

Assuming you have an AL 80:

1. Swim for a few minutes at a normal cruising pace.

2. Note your depth, time and SPG reading on a slate. Noting the time exactly helps with either a digital watch or watch with a second hand or waiting to start the timing and noting of pressure until the dive computer clicks over to the next minute on the bottom time.

3. Swim at the same constant depth for at least 5 minutes (10 minutes is better, 15 minutes is even better. When exactly 5, 10 or 15 minutes elapses, note the SPG reading.

4. Do the math part....

Assume you dove for 10 minutes a 40 feet and used 550 psi.

A. First you need to figure the atmospheres at 40 ft.

The formula is: (depth/33)+1= atm, or in this case: (40/33)+1=2.21 atm

B. Next you need to figure out how much gas you used in the timed trial:

You know you used 550 psi out of an AL 80 that holds 77 cu ft when full at 3000 psi.

Divide the full capacity by the full pressure and then multiply it by the psi you actually used in the 10 minutes: (full capacity/full pressure)*PSI used = cu feet used, or in this case: (77/3000)*550= 14.12 cu ft

C. Then figure out how many PSI you used per minute:

Since 14.12 is how much you used in 10 minutes, dividing by 10 gives you 1.412 psi per minute.

D. Finally convert this to your Surface Air Consumption.

Divide the gas used at depth by the atm at that depth you figured out in "A": Gas used at depth/atm at depth = SAC, or in this case. 1.412/2.21=.64 cu ft per minute.


This approach works well as it takes into account a realistic consumption at a fairly rapid swim rate, in real water temps and on average will be reasonbably close to your SAC on an entire dive once the harder working portions and gas used for inflation is averaged out with the low workload, resting at safety stop, etc portions of the dive (assuming of course your buoyancy is ok and you are not constantly chasing proper bouyancy with near constant adding and dumping - which will partially be accounted for in your timed trial as you struggle to maintain depth and use mroe gas doing so.) It works a lot better than calculating a resting SAC and then multiplying by 1.5, 2.0 or some other wild guess of a constant to estimate a normal working SAC.

I have found with a computer that computes average depth that my SAC over the entire dive is almost always within .05 cfm of what I used to get figuring my SAC using this method.

 

[knotical]

Isn't that the same as the starting pressure then?

Yes, it is.

I then have to figure out Steel 100s.

I think most of them actually hold 100 cubic feet at nominal fill pressure. more info at:http://www.huronscuba.com/equipment/scubaCylinderSpecification.html