Air Fill Protocol

[DA Aquamaster]

I get the impression that shops consider that higher pressure air needed to overshoot as compensation for cooling to be more expensive and don't want to deplete their banks.

As much as possible I leave them so I can get a final top-off before I take them.

That may well be true, but whether they charge a diver a flat rate per fill, or by the cubic foot, the odds are they are ripping that diver off by stopping at 3000 psi and then leaving the diver with only 2800 psi when it cools to room temp. Assuming you started with 500 psi and left with 2,800 psi (after it cooled) that's only 71.8 cu ft not 77 for a final volume, and it's only 59 cu ft actually pumped, not 64 cu ft. So the diver got shorted 5 cu ft, no matter how he was charged for the fill.

It's true that the last 200-300 psi to get an honest fill comes off the top and will lower the bank pressure, but if you think about it, taking another 5 cu ft off the bottom of the bank would reduce the bank pressure by the same amount and require exactly the same amount of compressor time to refill the bank to the original pressure, so it's not about saving PSI, it's about releasing less gas from the bank in total - i.e. screwing the customer who has a 3000 psi tank and should get a full 3000 psi fill.

Kudos to you for expecting the shop to do that. The shop may have come to expect it, but if they understood the bigger picture, they'd just hot fill your tank to 3300 psi to start with and save the staff time and gas lost in connecting, disconnecting, reconnecting and disconnecting your tank for a fill and a top off.

 

[spectrum]

DA I'm with you 100%

One of my points is that we can rant all day about what is right or better but unless each customer speaks up we will often be at the mercy of lesser trained service providers. I think it's safe to say that the average poster in the thread has a greater understanding of the physics behind this issue that the average fill person and probably more than the average proprietor. With the dive industry as loosely structured as it is we need to focus our message at the shops.

At some point this begins to be like "why don't we train everyone in a BP&W" or "to do the frog kick from day 1". All are good, all have advantages but for 1 thing the the body of shops and trainers aren't on that page and nothing is going to get them there in a hurry. Maybe in time but there are lots of other forces in the cosmos.

There is an educational value in these threads. I have been involved with pneumatics for over 30 years but when I came to scuba and ScubaBoard 7 years ago there were considerations than never mattered to me. Reading through threads like this of the day helped awaken me to what mattered to me as a diver. Without a doubt there are many more readers of this thread than contributors and I feel it's important that they get some useful tools to work with, not just contempt for the local dive shop.

Pete

 

[GrumpyOldGuy]

I am in the habit of checking my fills with a tank checker at the shop. I almost always drop off a load of 3-4 tanks and pick them up later when cool. A few times some have been low and I ask for a top off even it is just 100lbs. They guys are not stupid, pretty soon they learn to anticipate my checking and I find 3100-3200 in the tanks by magic.

 

[Diver Tim]

5) misquoting the CFRs and referencing sections that refer to actual hazardous materials rather than gases such as air and nitrox does not help. I realize it is confusing but you have to pay close attention to the sections and sub sections you are in to properly interpret the meaning and interpretation of a specific regulation.

Just have to clear something up. When you say "...referencing sections that refer to actual hazardous materials rather than gases such as air and nitrox...", I assume you're referring to my post where I said:

...regular old air becomes a "hazardous material" when compressed.

You may disagree, but Title 49 is pretty clear. A non-flammable, non-poisonous gas (like air) at a pressure greater than 29 psig is defined as a hazardous material. You do "...have to pay close attention to the sections and sub sections you are in to properly interpret the meaning and interpretation of a specific regulation."

My point is that a full scuba tank may seem pretty benign, but we need to keep in mind that there's a lot of potential energy cooped up in there... Enough that the DOT felt it necessary to set regulations about transporting them.

The Defense Logistics Agency has some guidelines about filling compressed gas cylinders as well.

DoD 4145.19-R-2

Tim

 

[Paulmal]

Just to clarify. It is turn pressure that is stamped on LP steel tanks not fill pressure, anything under 3200 is a short fill.

 

[NAM001]

I MUST BE IGNORANT HERE OR JUST DO NOT RECALL THE TERM sorry for the caps

what is turn presure??? perhaps you have some text to help me understand the term? thanks

Just to clarify. It is turn pressure that is stamped on LP steel tanks not fill pressure, anything under 3200 is a short fill.

 

[shoredivr]

Shops, for the most part, fill their tanks, with the tanks submerged in water.

Not around here.

 

[Paulmal]

I MUST BE IGNORANT HERE OR JUST DO NOT RECALL THE TERM sorry for the caps

what is turn presure??? perhaps you have some text to help me understand the term? thanks

Turn pressure is part of a gas management plan for advanced diving. Typically rule of thirds, one third of your gas for the decent, one third for the assent, and the last third for emergencies. ie you buddy looses all his gas due to a valve failure. So on LP steels you start your dive with 3600 psi you turn at 2400 psi and get out of the water with 1200 psi.

 

[boulderjohn]

Just to clarify. It is turn pressure that is stamped on LP steel tanks not fill pressure, anything under 3200 is a short fill.

Turn pressure is part of a gas management plan for advanced diving. Typically rule of thirds, one third of your gas for the decent, one third for the assent, and the last third for emergencies. ie you buddy looses all his gas due to a valve failure. So on LP steels you start your dive with 3600 psi you turn at 2400 psi and get out of the water with 1200 psi.

Sorry, but the markings on a tank signifying fill pressure have nothing whatsoever to do with how the tank is to be used during a dive. Nothing at all.

 

[eponym]

I thought that was FL cave humor . . .