Filling your own tanks (normoxic & other)

[ScottCarullo]

Hey FredT, I had a question about your post (under bands( but wanted to ask it here thought others may be interested.

You mentioned filling your own tanks with various percentages (from 21-40%) of 02.

I am very interested in doing this. It was very interesting that you fill the 02 and let the shop finish it off with the high-pressure air.

What precautions do you take / are necessary when filling with 100% 02. I just assumed it was way too hazardous for the common joe to deal with 100% 02 but it sounds as though you have figured out how and managed to avoid incidents.

I also assume you use partial pressure mixing just putting the 02 first.

Do you worry about the 'tank jockey' at the store opening your tank up while conected to his bank and feeding your 100% 02 into their bank prior to him opening his pressure valve to his primary's? I guess maybe you wouldn't lose enough to make a difference mix wise but could this cause a problem if they have components not compatible with 100% 02 or am I reading too much into the dangers in the info I've seen?

It should be also very safe to assume you use an 02 analyzer after filling the tank. You would have to double check, I'm sure you do. Which one do you have and would you buy it again?

Thanks for the info, I hope you are willing to share it with us.

Scott

[BobK]

Go here.

http://www.airspeedpress.com/newoxyhacker.html

[Stone]

The "OXYGEN HACKER'S COMPANION" (BobK's link takes you to it) is an excellent resource. I think has good information even if you don't do your own mixing.

Vance Harlow (oxyhacker) is an occational poster on the Scubaboard.

[swimndive]

Like everybody has already said, get the Oxyhacker's Companion. In some areas the local shops frown on "homebrewers" and they'll actually request to see a blender's certificate before they do any top offs for you.
That's what happened over here when too many people started to do this. It finally got to the point where the only shop I could do this with was too far away. That's when I bought a compressor and just started banking the mixes. ed

[ScottCarullo]

Have you found it cheaper to do this yourself or did you decide to do it yourself just for the convenience/pleasure/piece of mind doing it yourself?

How many tanks per week would you have to use to make the better financial decision in your opinion?

Since I fish a lot, I'd say six tanks per month probably avg for me - at least while I catch more than I can shoot :)

I think I probably know the answer to this already but maybe I can get several of my friends to chip-in and we can do it together - might make it more worth while...

Thanks for the information, I'm getting ready to check out that link.. Scott

[ScottCarullo]

Cool...

Just went to that link and ordered the book... Thanks again for the info. It may be cheaper than I thought, but we'll see.

Scott

[swimndive]

ScottC
Whether it's cheaper depends on what your shop charges for a nitrox fill vs. an air fill. Chances are he's not getting rich off nitrox sales.

Your greatest costs are going to be the hardware and will vary depending on whether you rent the storage cylinders or buy them. At your usage rate you would only need to buy 5 "T" bottles/year assuming your using 32% and diving 80's. A whip can be had for anywhere from 50 to 300 dollars. Bottle rental can range from 3-5 dollars/month. So for your first year you might spend 150 for a whip 60 for tank rental and 100 for O2 which comes out to 260 bucks divided by 72 tanks /year = 3.61/fill plus the cost of the air top off. The following year you wont have the startup hardware costs so your cost per fill should come down.

For me it was all about the convenience. My local shop gave me good prices on nitrox. Only problem was the 60 min rt drive to the shop and the fact that I had to leave the bottles there for filling. It took two trips/week for one load of nitrox fills. Once I got set up to home brew it was back to one trip per week, or so I thought.

Make sure your that the shop you are going to use for top offs has a sufficient number of whips, as well as a sufficient quantity air banked to get you in an out in a reasonable amount of time. Otherwise you are going to feel really stupid standing around wasteing all that time you tried to save. After a while it will dawn on you that you really need a compressor. Tell your wife/gf that it will save you x-hours/week and that's quality time you can spend with her (yeah right).

ed

[FredT]

Originally posted by ScottCarullo


What precautions do you take / are necessary when filling with 100% 02. I just assumed it was way too hazardous for the common joe to deal with 100% 02 but it sounds as though you have figured out how and managed to avoid incidents.Scott

Dealing with O2 is not difficult, but it is necessary to remember the rapidly expanding big orange ball your garage could turn into if you neglect cleanliness rules. Pressure boosting O2 is the worst thing to try, and is NOT suggested for anybody. Even the gas manufacturer's are skittish about this and use O2 dedicated stuff to do it.

Originally posted by ScottCarullo


I also assume you use partial pressure mixing just putting the 02 first.

Right, It's just necesssary to give the fill station the target pressure for the full tank.

Originally posted by ScottCarullo

Do you worry about the 'tank jockey' at the store opening your tank up while conected to his bank and feeding your 100% 02 into their bank prior to him opening his pressure valve to his primary's?


That is part of the the negotiation to get top offs. Anybody used to doing this will pressurize the whip a bit before opening the tank isolation valve, but you have to judge just how on the ball the shop is. It may require a secondary device you put on the tank to prevent backfill. Circle Seal check valves are cheap, reliable, and small. Combined with a fill block and yoke this prevents gas form leaving the tank during the fill process and kills most of the shops "real" safety concerns. The insane and delusional concerns of their gas handling ignorant lawyers are another thing entirely.

Originally posted by ScottCarullo

I guess maybe you wouldn't lose enough to make a difference mix wise but could this cause a problem if they have components not compatible with 100% 02 or am I reading too much into the dangers in the info I've seen?


Nope, you're reading the rules correctly there, but then if they do not have clean air to start with you shouldn't be getting a top off, or any other air from them. Perhaps it's wise to let others in the area know the shop doesn't trust their air quality.. (evil grin)The sword does have two edges. Perhaps it's just my bad attitude showing through again but either air is clean enough to put into ANY of my tanks, or it's not clean enough to use at ALL.


Originally posted by ScottCarullo

It should be also very safe to assume you use an 02 analyzer after filling the tank. You would have to double check, I'm sure you do. Which one do you have and would you buy it again?
Scott

Post check should ALWAYS be done, but this is even more of a mandatory thing if using custom mixes on deeper dives where a few% error can end up putting you at risk for an oxtox hit. I made my own analyser, and I'd do it again if I needeed to replace it.

FT

[fishhunta]

i work at a shop and to let you know- i don't even use o2 clean tanks myself. they are statistically as safe as a "cleaned" tank. and that "special" whip with the extra filtration, it does nothing. for o2 to be explosive, it either has to have tremendous friction(fill slowly), or be at pressures greater than 10,000psi(i don't think so), so there is no danger at all in filling your own tanks, just fill them slowly.

[ScottCarullo]

Hey fishhunta...

Sure your name isn't Cale?
I read about you in a book I just bought!

haha Anyone who has the book in question should be smiling :)

Great reference and thanks to the guys here that recommended it.

Scott

[ScottCarullo]

Ok, I've read and talked, now for the million dollar question.

I've verified equipment, prices, and locations. I have everything I need to fill my own nitrox tanks except one very important piece of the puzzle which tells me how much I'll really save dollar wise.

From a 337cf oxygen tank from the local gas company how many 80cf tanks of ~500psi O2 can you fill....

Thanks.
Scott

[ScottCarullo]

Oops, forgot one important bit of info.

337cf tank is 2300 psi I think he said.

Anyone know the equation to figure this out mathmatically?

337cf 2300psi ---> how many 80cf 500psi knowing you have to leave the last 500psi in the large tank unusable.

Thanks,
Scott

[richhermes]

14.7 plays into the calculation. I can't remember what part of air that relates to (weight or psi), but I remember that number for some odd reason.

[swimndive]

ScottC, I thought I went over most of this in my previous post. If you decant 500 psi of O2 from a T bottle into an al 80 it will drop the supply tank pressure by roughly 95psi. If your supplier gives you a bottle with 2200 psi, you'll be able to fill 17.7 80's. You will also be returning that T bottle to the supplier when the time comes with 525 psi. Your actual annual fill cost to homebrew nitrox will be the sum total of the following expenses divided by 72(your # of tanks per year): One whip, one analyzer, 5 T bottles of O2, 12 months rental fee for the single supply bottle, plus delivery charges or gasoline costs. Don’t forget to add in whatever it costs you for air top offs. Ex:(100+100+100+60+10)+(72 * $3.00)) / 72 = $8.13/tank for the first year and $5.36/tank for year two. (Your mileage and costs may vary) . In terms of volume you will use roughly 13 cf of O2 per fill, you will end up throwing away about 67 cf, your supplier will charge you for 337cf and you will probably receive around 300cf and this is ONLY if you hold your mouth right.

[FredT]

You can use this with the appropriate numbers filled in for your tank mix.

----------------------------Bank useage-----------
To keep this simple let's assume all tanks hold "nominal volume at rated pressure" and that all tanks to be filled are dead empty.
A 300 cu ft bottle @ 4500 psi will loose 15 psi for every cubic foot of air you take out of it. To totally fill a empty HP100 you would drop the bank tank pressure by 1500 pounds. This means the first tank you fill from a single bank bottle will NOT get a full fill!
The static equation at equalized pressure is:
Scuba tank Pressure initial [P1] +( Rate of fill [R] psi/ft^3 * (Volume added to SCUBA tank)) = (Bank bottle beginning pressure in psi)- 15psi(Volume subtracted from bank bottle)
By definition {Volume added to SCUBA} = {Volume subtracted from bank bottle}
The static equation then becomes:
P1 + RV = Pbs - 15V
Solving for V you get:
V = (Pbs-P1} / (R+15)
To get the ending pressure in BOTH tanks at equilibrium you simply multiply R times V to get the pressure added and add the initial pressure in the scuba tank. Pf= P1 + RV
To run through this for the first tank:
A HP100 fill rate is 3500/100 or 35psi/ft^3
V = (4500 - 0) / (35 +15) = 4500 / 50 =70 ft^3
Pressure at equalization is P1 + RV = 35*70 = 2450
To top off this tank you would tap bank bottle 2 [B2] for 30 ft^3 dropping it's pressure 450 psi to 4050.
B1=2450; B2=4050; B3...=4500
To fill the second HP100 you'd first equalize with B1
V=(2400 - 0) / (35+15) = 2400/50 = 48
RV = B1 = 35 * 48 = 1680 psi
now check for final fill from B2
4050 - {52 * 15) = 4050 - 780 = 3270
This is less than the 3500 full pressure so we have to equalize with B2
V= (4050-1680)/50 = 47.4
P= 1680 + 47.4*35 = 3339
So far we've added 95.4 ft^3 to the second scuba tank.
We then pull 4.6 cubic feet out of the third bank bottle dropping it to 4431 PSI
Now go ahead and crunch the numbers to fill 2 HP 80s followed by a half dozen Al 80s. A HP 80 will build up pressure at 3500/80 psi/ft^3 or 43.75psi/ft^3. An Al 80 builds pressure 3000/80 or 37.5psi/ft^3.
Real quick you'll see that a bank of less than 5 bottles won't go far if you insist on full tanks. I prefer a bank of 10 stage tanks to do a bunch of bottles. I had a bank of 5 300s and 5 400s in the Bahamas I filled SCUBA tanks off of, but the highest pressure we could get from the compressor 3500psi. I could still get almost 30 AL 80s to over 2800 psi before I had to turn the compressor back on.
All this number crunching is not necessary during an actual fill, but it's the only way to get a feel for the process without having to actually haul tanks and do plumbing.

---------------------------------

This can easily be adapted to a spreadsheet with the real values for the tanks you use.

FT

[ScottCarullo]

I appreciate your information.... Thanks a lot.

I'll put my actual numbers out here because maybe it will help someone or save them a trip to figure it out (for locals).

Dive shop was willing to top off the tanks with ~500psi O2 and understood how to deal with it. Top-off $2.50

Local gas company willing to provide 337cf O2 tank delivered to my house for free (no deposit-based on credit & no delivery fee due to proximity to a regular stop of theirs). Only cost is $0.28 per day tank rental (about $100/year) with no min/max time limits on rentals and 43.03 per fill of O2 (they just come and swap tanks). They proved willing to assist me getting the parts necessary to build the required whip as well with flow meter / precision guages.

Hope all this information has helped others besides me, thanks again for your input.

Scott

[swimndive]

Fred,

I'm not trying to bust anybody's nuts on this, but I ran through the numbers in that exapmle you posted and noticed a some major math errors. You might want to double check that other source, from which you pulled that information and scrutinize it for other inaccuracies.

In your example a 300cf @ 4500 psi supply tank, when equalized with an empty 100cf @3500psi tank, supposedly equalizes at 2450 psi. This would mean that 2050 psi, (4500-2450) which is roughly equal to 136 cf of gas, somehow transfers into an empty 100 cf tank and somehow manages to only fill it to only 70% of it's capacity. This is simply not possible.

The easiest way I've found to calculate equalization pressures is to first convert everything to standardized volume. Using your example of a 300 cf supply bottle at 4500 psi do the following. A single hp 100cf cylinder holds 100 cf at 3500psi. In order to standardize the volume calculations, we need to first figure out what volume of gas this bottle will hold at 4500 psi. To do that we take 100cf and divide it by 3500psi which equals .02857cf/psi. We then multiply this number by 4500psi. This gives us this cylinder's volume at 4500 psi. Ex: (100cf/3500psi)*4500psi =128.57 cf.

Armed with this information, we now can calculate the pressure at which both cylinders will equalize when connected together with a transfill whip, assuming that the supply bottle is full and the other bottle is entirely empty.

Begin by taking the volume of the supply bottle, and then divide that number by the sum of the volume of the supply bottle plus the standardized volume of the desired fill bottle. This number is a constant, and it can be used to calculate the equalization pressure no matter what the supply pressure is. In this case that would be 300/(300+128.57) which yields a constant for these two bottles of .70. Multiplying this constant (.70) by the starting pressure in the supply bottle (4500 psi) gives us the final equalization pressure between our two bottles (3150 psi.).

In terms of volume, that equates to .02857 cf/psi*3150 psi and equals roughly 90 cf. of gas in the empty tank. This gas volume must be equal to the volume of gas that has been decanted from the 4500 psi supply bottle. To check this, we multiply 1350psi (4500-3150) by .06666 cf/psi (300 f/4500psi) which also yileds 90 cf of gas. Now that we know that the volume of gas missing form the supply tank is equal to the volume of gas in the fill tank we can be sure that our calculated equalization pressure is also correct.

We are not quite finished because we still need to add another 10 cf or 350 psi to the hp 100 for a complete fill and we'll need to tap another supply bottle to accomplish this. To figure out how many psi this will draw down from the new supply tank, we need to calculate one more volume constant between the two tanks. We'll use the standardized volume of both tanks at 4500psi to do this by taking 300cf and dividing it by 128.57cf to yield a constant of 2.333. We now take the remaining 350psi that we need to add to the hp100 and divide it by this constant. And come up with the actual pressure drop from the new supply bottle (350psi/2.333=150psi.) Checking the volume of gas that 150 psi represents, we find that 150psi multiplied by .06666 equals 9.99cf and is also equivalent to the remaining volume of gas we needed to add, so we are finally done.

I didn't really want to open a can of math worms over here, but when I saw that it was too late, and that the example was flawed, I thought I should try to clear some of this up. Hopefully this will simplify things for those people who deal with cascade calculations and help them to better plan their gas needs. When in doubt, order another bottle, be it Helium, Oxygen, Bourbon, Scotch or Beer and be done with it.

[Speargun]

:confused: :confused: :confused:
Anyone..... Anyone.... Buller?

Let's take those math worms, put them on a hook & feed them to the fish... :D
Maybe I'll just keep giving the guy at the dive shop my money to fill my tanks.:cool:

[dothanfrogman]

I agree with you spear gun. I have thought about doing this as I am a blender. But to worry about cost over peace of mind that is the question.