Air Fill Protocol

[redacted]

Put a gauge on it before you leave. 2200 should be unacceptable. I have no problem with 2700+ as long as it will not limit my planned dive. I tell the shop when I need a "full" fill. If the shop tries to tell you 2700 is OK. pay them 90% of the charge and see how that works out.

 

[NAM001]

BRAVO
last i remember the standard was when filling tanks do not exceed the fill presure/temp of the tank. that was ie. 3000 at sea level at 60 degrees ( actual temp may not be correct.) fill presure may exceed so that it does not exceed the presure at the standard environment. Of course all that means nothing when your insurance carrier otherwise limits you.

Just about any shop near me thinks that a 3000 PSI rated tank cannot be filled past 3000 PSI even during a fill. For the same reason one cannot every actually receive a fully rated fill -- even after a top-off.

An 80cf rated volume of a tank rated at 3000 at room temperature (72*) would acutally be about 77 CF but lets just say 80 for this argument. If that volume warms up by some degrees you may have 3200-3300 PSI but it is still the rated volume and will cool off to about 3000 PSI. Even during Hydro they fill the tank to 5/3's of the volume (meaning 166%of its rating which means a very high PSI) and if it passes hydro then the tank can handle a few hundred extra PSI during a hot fill even when that extra PSI is the same volume rating. If you have a full 3000 PSI fill and leave it in the summer heat it may expand to about 3200 PSI but it is fine -- it will cool off when you hit the water and contract back to 3000 PSI.

When I do my own fills I always overfill a tad so it cools off to its rated pressure but no shops around me will do that. Sometimes I attest it to not fully undersanding the laws/rules. Other times I attest it to insurance paranoia. Either way the only times I ever truly get a 3442 fill on my HP doubles sets is when they blend with Helium because then they compute exact PSI's for He and O2 and watch it during the top-off. But then I also have to leave my tanks with them over night. For any other fill I usually get about 3300 PSI but have yet to have that get in my way though it would if I were on 2640/3000 PSI tanks.

 

[shmuggy]

any shop that gives you a tank with 2200 PSI should be challenged. Hopefully there are other shops you can do business with in your area. I have usually found the tanks are filled to 3200-3300 PSI and on rare occasion get one that is at 2800. You shouldn't have to check behind them on this- they should give you what you bargained for. I would find another dive shop to fill my tank.

---------- Post added ----------

any shop that gives you a tank with 2200 PSI should be challenged. Hopefully there are other shops you can do business with in your area. I have usually found the tanks are filled to 3200-3300 PSI and on rare occasion get one that is at 2800. You shouldn't have to check behind them on this- they should give you what you bargained for. I would find another dive shop to fill my tank.

 

[mictrik]

I never agreed with that article you linked to from "Fillexpress". It makes some false assumptions in my opinion. I take far longer to fill an empty 80 then 5 minutes (usually 20 minutes). I use the bath when I know someone is waiting and it makes a huge difference!! I can speed the fill up to lets say 15 minutes and i usually let the cylinder sit in the tank until it is cool to the touch. When filling another al80 outside of the tub at the same time, at the same speed the difference is great. Sure if you fill an empty al80 in 5 minutes (as per the example in the article) and you remove the tank from the bath right away you see only a small difference. Water conducts heat 24 times faster than air. Compare the speed of cooking a potato by boiling vs baking it. How fast will you become hypothermic in 50º water vs 50º air? That article is hogwash, what should be recommended is that a procedure be developed for using a bath that reduces the risk of water vapor being transmitted into the cylinder.

I also do not believe by any stretch that fill baths are the primary vector for water vapor intrusion into SCUBA cylinders. I believe the primary causes are: poor drying after hydrostatic testing, Poor compressor maintenance (ie. not replacing dryers at proper intervals), not ensuring the fill whips and tank valves are dry before filling; both should be quickly purged before commencing a fill. Even at the slower speeds of filling a cylinder the internal temperatures of the tank will exceed 70º F which is the temp that the service pressure is expected to yield it's rated volume. Slightly overfilling though not expressly sanctioned is also not expressly forbidden by DOT rules; especially since filling increases the cylinder temp above 70º and this is more quickly cooled in a bath so the pressure will soon settle down to it's rated service pressure. This way the fill is not short or over filled. Shops often do not fill to rated pressure in the interest of saving time and because they run their compressors less as higher pressure air costs more. Filling a al80 starting from 700psi to 3250 (3250 at a temp above 70º) when filled even slower than recommended will result in about 3000psi at 70º in my experience and I get their much faster when using a bath. In addition cylinder temps stay lower throughout the fill. All bets are off of course if you fill fast and remove the cylinder immediately from the bath after fill.

Bottom line is this; what should be delivered to customers is a fill that is at the rated service pressure at 70º! There are many methods to get there and slightly over filling when the cylinder is warmer than 70º is one of them. Using a bath to limit just how high you go and to facilitate cooling works and makes sense when used properly.


DOT Reg quoted from article:
"DOT 49CFR 173.302 (Charging of cylinders...),(b) Filling limits (Ref.173.301 (e)): “Container pressure.The pressure in the container at 70°F. must not exceed the service pressurefor which the container ismarked...”. That’s straightforwardenough and no one should have anytrouble understanding it."

That is straight forward "The pressure in the container at 70°F"

 

[NAM001]

I agree with you, but unfortunately you are using a common sence approach to filling tanks. Things are not like in the past. in the 60's you could fill a steel 72 to 2200 relativly quickly. putting in a water bath was no problem. now the good do'ers have got into things and have prescribed a process for "zero problems". the water bath as with the dot reg you cited is has become moot when relaced by slow fill rates. I have watched tanks in a water bath fall over, pulled out and have the fill whip connected with out purging the valve from water. Once again that is moot if you fill slow and remove the need to bath cool. I am with you physics are physics. I have no problem with bath fills. I have watched 60% overfills. I have watched bathed fills to +60. In those cases i have noticed that fills will take 30 min or beter to do. 600 psi a min. no way cut it back to 100 and you avoid a lot of issues. Normal fills do not exceed rated fill psi, you can fill faster. There is a big difference in a pro filling tanks and a monkey. Most of these rules are to protect us from the monkeys, not the pro's. From what i have seen we should be more concerned about air quality issues (filter systems) than fill issues. and i stand corrected on the 70 deg vice the 60 i posted earlier.

I never agreed with that article you linked to from "Fillexpress". It makes some false assumptions in my opinion. I take far longer to fill an empty 80 then 5 minutes (usually 20 minutes). I use the bath when I know someone is waiting and it makes a huge difference!! I can speed the fill up to lets say 15 minutes and i usually let the cylinder sit in the tank until it is cool to the touch. When filling another al80 outside of the tub at the same time, at the same speed the difference is great. Sure if you fill an empty al80 in 5 minutes (as per the example in the article) and you remove the tank from the bath right away you see only a small difference. Water conducts heat 24 times faster than air. Compare the speed of cooking a potato by boiling vs baking it. How fast will you become hypothermic in 50º water vs 50º air? That article is hogwash, what should be recommended is that a procedure be developed for using a bath that reduces the risk of water vapor being transmitted into the cylinder.

I also do not believe by any stretch that fill baths are the primary vector for water vapor intrusion into SCUBA cylinders. I believe the primary causes are: poor drying after hydrostatic testing, Poor compressor maintenance (ie. not replacing dryers at proper intervals), not ensuring the fill whips and tank valves are dry before filling; both should be quickly purged before commencing a fill. Even at the slower speeds of filling a cylinder the internal temperatures of the tank will exceed 70º F which is the temp that the service pressure is expected to yield it's rated volume. Slightly overfilling though not expressly sanctioned is also not expressly forbidden by DOT rules; especially since filling increases the cylinder temp above 70º and this is more quickly cooled in a bath so the pressure will soon settle down to it's rated service pressure. This way the fill is not short or over filled. Shops often do not fill to rated pressure in the interest of saving time and because they run their compressors less as higher pressure air costs more. Filling a al80 starting from 700psi to 3250 (3250 at a temp above 70º) when filled even slower than recommended will result in about 3000psi at 70º in my experience and I get their much faster when using a bath. In addition cylinder temps stay lower throughout the fill. All bets are off of course if you fill fast and remove the cylinder immediately from the bath after fill.



Bottom line is this; what should be delivered to customers is a fill that is at the rated service pressure at 70º! There are many methods to get there and slightly over filling when the cylinder is warmer than 70º is one of them. Using a bath to limit just how high you go and to facilitate cooling works and makes sense when used properly.


DOT Reg quoted from article:
"DOT 49CFR 173.302 (Charging of cylinders...),(b) Filling limits (Ref.173.301 (e)): “Container pressure.The pressure in the container at 70°F. must not exceed the service pressurefor which the container ismarked...”. That’s straightforwardenough and no one should have anytrouble understanding it."

That is straight forward "The pressure in the container at 70°F"

 

[mictrik]

I was going to touch on air quality as I too believe this is an issue that has been all too neglected in the industry. One of the owners of of my local shop takes what many people consider to be "too much" concern (I disagree) for the compressor & air quality as well as checking for cracks in 6351-t6 alloy cylinders. We send out air samples to a lab 4 times per year (comes back near medical grade everytime), religiously changes compressor oil at proper intervals, changes filters, dryers, and record in a log book every time the compressor is run. We also record every VIP, check all Aluminum cylinders with Eddy & Scope for cracks. When we do find cracks they are of course recorded and we show the customer on the scope so that they can confirm the issue. So far not one customer has argued with our decision to take a tank out of service. We have 3 banks with the high bank at around 4000-4100 and we really try to ensure that customers get fills to service pressure at 70º. I think it has worked because I see divers (especially commercial oriented) coming to our shop from areas that are serviced by other shops; short fills are often mentioned for the reason. We have seen some 300bar cylinders come in and at this time we will only provide around 275 BAR (4000psi) for those customers. though we have no plans to bank at 300 BAR we are considering putting an air booster online to meet those needs and a few others.

We have seen cylinders come back with rust that we have visually inspected the year before. I believe the reason for much of this is moisture in the fills. I can only say that the moisture is most definitely not coming from our fills and that the customers that fill exclusively with us have never shown such changes in the condition of their cylinders year to year. More attention needs to be focused on air fill quality for sure.

Regarding fast fills of old steel 72's in the 60's. Well, to be honest I do not think that was good practice! When the steel is in the tub it will certainly cool faster than when in air; unless of course the tube is very hot. but a slow filling is always better for the cylinder and for the safety of all involved. i try to fill as slow as time will allow and never faster than 200-250 PSI per minute. Cylinders still get quite warm at that rate and the bath definitely helps to cool them quicker. The bath also comes in very handy when checking for leaks. Some knowledge and some applied common sense certainly go's a long way.

 

[NAM001]

my comment was primarily directed twords heat build up when pumpimg to 2200 vs 3500. 2200 generated much less heat. and then most of the heat was from hot gas from the compressor and not from fill rate. fililng from a cascade was always cooler. as such they could handle faster fill rates. Slower is always better tough.

Regarding fast fills of old steel 72's in the 60's. Well, to be honest I do not think that was good practice! When the steel is in the tub it will certainly cool faster than when in air; unless of course the tube is very hot. but a slow filling is always better for the cylinder and for the safety of all involved. i try to fill as slow as time will allow and never faster than 200-250 PSI per minute. Cylinders still get quite warm at that rate and the bath definitely helps to cool them quicker. The bath also comes in very handy when checking for leaks. Some knowledge and some applied common sense certainly go's a long way.

 

[Griffo]

I do wonder how many "short fills" are the result of people accidentally cracking the valve a smidgen when putting the tank into or out of their car.

 

[Diver Tim]

Very interesting subject. For those readers in the United States, from the Code of Federal Regulations:

"Title 49: Transportation
PART 173-- SHIPPERS--GENERAL REQUIREMENTS FOR SHIPMENTS AND PACKAGINGS
Subpart G--Gases; Preparation and Packaging
§ 173.301 General requirements for shipment of compressed gases and other hazardous materials in cylinders, UN pressure receptacles and spherical pressure vessels.
(a) General qualifications for use of cylinders.
...
(8) The pressure of the hazardous material at 55 °C (131 °F) may not exceed 5/4 of the service pressure of the cylinder. Sufficient outage must be provided so the cylinder will not be liquid full at 55 °C (131 °F)."

Electronic Code of Federal Regulations:

Note that in this usage, regular old air becomes a "hazardous material" when compressed. It's all about potential energy... I didn't see anything in that part that mentioned any other pressure restrictions. I also could not find a definition of "liquid full."

It seems to me, the bottom line is that if you're filling an AL80 to 3000 PSI, you're cramming 77.4 cubic feet of air (at ~14.7 PSI) into a space less than half of a cubic foot (at ~3000 PSI). That's inherently dangerous. If you cram in more than that much air ("overfilling"), it's more dangerous; if you cram in less air ("under filling"), it's less dangerous. If you keep cramming air into the tank, eventually it WILL fail, likely catastrophically. And you don't know at exactly what pressure it will fail. Is it possible to fill a tank to significantly more than its service pressure without failure? Sure. We've all gotten AL80s that read more than 3000 PSI at 70F. Is it possible that a tank will fail at less than it's service pressure? Absolutely.

You can dissect the numbers all you want, but if YOU'RE the one filling the tank, it comes down to how much risk YOU are willing to accept.

Tim

 

[AfterDark]

Years ago New England Divers had a super cooled air fill system. The guy connected my tank and within a few minutes, a very few minutes, my tank was full and only warm to the touch. I was amazed and taken a back. I wasn't too keen on my tank being expanded that quick but damn if that was the fastest fill I've ever seen. I wonder sometimes what became of that system, it was very big.