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View Full Version : why do I have to wait for banked nitrox fills



diver-wife
05-29-2008, 15:28
I am a new diver, so I might be completely off base about this, but why do I have to allow 24-48 hours for nitrox fills at my LDS? I can understand custom blends which take longer, only certain people can fill, but if they bank nitrox, why do I have to drop off my tanks and come back later? This same shop can fill air while I wait?

Rainer
05-29-2008, 15:30
Are you asking for EXACTLY the mix that they bank?

diver-wife
05-29-2008, 15:34
exactly the mix they bank = 30%, is there not any reason I can't wait around 10-15 minutes and pick up my tank then

RikRaeder
05-29-2008, 15:34
Maybe they "bank" it at another shop?

Rainer
05-29-2008, 15:36
That's an odd mix to bank, but I digress.

If you're asking for what's banked, and they're not having to blend down, I can't see why you'd need to wait either.

Have you bothered to ask them directly?

diver-wife
05-29-2008, 15:37
not yet, I will give them a call, why is 30% an odd mix? I have seen it at least three different shops

Rainer
05-29-2008, 15:41
not yet, I will give them a call, why is 30% an odd mix? I have seen it at least three different shops

Well, they're screwed if anyone wants anything richer. Many recreational divers use 32% or 36%, and you can't get those mixes if you're just banking 30%. GUE divers all use 32%, so again, they'd be screwed. Makes WAY more sense to bank something around 40% and then just blend down from there (allowing you to service many, many more divers).

comet24
05-29-2008, 15:41
Most shops bank 32%. What are the depths of most dive in your area. That may effect what is banked. Was this a one time thing are do they alway need to wait. Many places that bank Nitrox have only a few tanks dedicated to the Nitrox bank. If the bank was not full it could effect the wait.

diver-wife
05-29-2008, 15:44
most dives are in the 100 ft range, up to 130ft. I only called and asked if I have to wait, I will call in the AM to see if this is normal

awap
05-29-2008, 18:26
I'll bet it is a training issue and the wait time is to make sure the trained person will be available.

mike_s
05-29-2008, 20:17
I am a new diver, so I might be completely off base about this, but why do I have to allow 24-48 hours for nitrox fills at my LDS? I can understand custom blends which take longer, only certain people can fill, but if they bank nitrox, why do I have to drop off my tanks and come back later? This same shop can fill air while I wait?


why do you have to wait? well there is no technical reason.

only they are doing something different than the rest of the dive world. Either they don't have an employee there who can do it, don't have the nitrox mixed up already, or they have a dive shop owner who is a micromanager and won't let anyone else do it because "he's the expert.

My money is on the later guess.



not yet, I will give them a call, why is 30% an odd mix? I have seen it at least three different shops

30% is pretty common. most places that have do mixing with other that partial pressure often come up just a little short and you get a 30% or 31% fill. It's not that big of a deal with just one 1% or 2% off 32%.

PvtStash
05-30-2008, 00:55
Well, they're screwed if anyone wants anything richer. Many recreational divers use 32% or 36%, and you can't get those mixes if you're just banking 30%. GUE divers all use 32%, so again, they'd be screwed. Makes WAY more sense to bank something around 40% and then just blend down from there (allowing you to service many, many more divers).[/quote]


I dont understand... , apparently you can blend down a 40% mixture (by adding air I'm guessing?).... why could you not simply "blend up" by adding O2 ?

comet24
05-30-2008, 03:44
Well, they're screwed if anyone wants anything richer. Many recreational divers use 32% or 36%, and you can't get those mixes if you're just banking 30%. GUE divers all use 32%, so again, they'd be screwed. Makes WAY more sense to bank something around 40% and then just blend down from there (allowing you to service many, many more divers).


I dont understand... , apparently you can blend down a 40% mixture (by adding air I'm guessing?).... why could you not simply "blend up" by adding O2 ?[/QUOTE]

100% O2 is very reactive and can be very dangerous. When partial blending which is what you are suggesting you first add 100% O2 then air to get the mix you want. Your tank has to be O2 cleaned for this and the person doing the mixing need to know what he is doing. More training and much more dangerous. 40% nitrox doesn't require all the safety concerns you have with 100% o2.

navyhmc
05-30-2008, 04:14
From my class, I recall that 32% and 36% are by far and wide the most favorite mixes with 32% being the more favored of these two. This is why most rec nitrox classes have tables for 32% and 36%.

Disclaimer: I am not Nitrox blender. Yes, you can add additional O2 to a tank and top off with the mix on hand to acheive a higher o2% than is in the bank. So, if I am doing my math correct: If you had a AL 80, the Nitrox bank was 30%, you would need an additional 60 psi of 100% O2 to make it 32%. Not too hard to do, but you are now dealing with the realm of partial pressure blending and that has increased risks that comet 24 alluded to.

Here's how I figured: On a 3000 psi tank, the partial pressure of 30% O2 is 900 psi and the PP for 32% is 960 so an additional 60 psi of 100% O2 is needed. If I am incorrect, please let me know.

I didn't even try to figure how one would go about blending 32% out of 40%-that one made my head hurt and I didn't go online to see if a formula was available.

Scotttyd
05-30-2008, 05:18
for NC coastal dives - many times we do not know the dive site until we leave the inlet. Many dives sites are close to the 130 foot rec limit. A PO2 at 130 ft for 30% = 1.5. That is why the dive shops here bank 30%. You are not taking 36% of the coast of NC or you just get to look down from the anchor line!

Dive-aholic
05-30-2008, 09:17
100% O2 is very reactive and can be very dangerous. When partial blending which is what you are suggesting you first add 100% O2 then air to get the mix you want. Your tank has to be O2 cleaned for this and the person doing the mixing need to know what he is doing. More training and much more dangerous. 40% nitrox doesn't require all the safety concerns you have with 100% o2.

O2 is not reactive. Simply mixing O2 with something else does not set it off. O2 simply feeds fire. The hydrocarbons that could be in the tank valve are the reactive elements. Add heat to these and they begin to combust. Add oxygen, and they will really go boom because the oxygen causes the combustion to increase in amplitude.

Aside from needing an O2 clean tank (I'm guessing this shop my require it even though the nitrox is banked), the main reason shops don't blend up is because they would either need a booster to get the O2 pressure high enough to fill a tank that already has air in it or they need to empty the tank being filled so they can fill it with the right amount of O2 for the desired mix. This in turn means they need to use more of their banked air, which costs money.

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
05-30-2008, 11:11
That's an odd mix to bank, but I digress.



Odd, or not, 30% is the go-to mix at the NC coast. Lots of shops bank it instead of the standard NOAA mixes. Even so, we are still required by PADI shops to buythe EAN32 and EAN36 tables to get the Nitrox cert. These tables are the most useless pieces of dive gear I own. Even less useful than the snorkel I never use. But I digress :smiley36:

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
05-30-2008, 11:21
for NC coastal dives - many times we do not know the dive site until we leave the inlet. Many dives sites are close to the 130 foot rec limit. A PO2 at 130 ft for 30% = 1.5. That is why the dive shops here bank 30%. You are not taking 36% of the coast of NC or you just get to look down from the anchor line!

I have heard that at least one shop in Morehead City will not fill higher than 30 unless you have an instructor card. I don't know if that rumor is true. Personally, I think if I want to kill myself, I ought to be allowed to do it.

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
05-30-2008, 11:27
I'll bet it is a training issue and the wait time is to make sure the trained person will be available.


This is usually the case. It is very hard to keep a trained person on staff. One shop I know of is getting out of the Nitrox business because they can't keep a trained employee around.

Another reason I have had delays is because the banks are empty and they need to mix a new batch.

And yet another is that the booster is down and they are having to rebuild it.

CompuDude
05-30-2008, 13:50
30% is a strange mix to bank, but if it's the area standard, there might be some justification for it, in spite of the industry standard 32% mix.

As to the OP's question, I agree, there is no technical reason why he should have to wait overnight for a banked mix, as long as there is enough gas in the bank to do the fill. It's simply the shop's apparent policy, for entirely internal, rather than technical, reasons.

The reason you don't "enrich" a 30% mix to 32% by adding o2 is very few shops want to pump o2 at a high pressure. You're talking about pumping o2 at pretty close to 3000 psi at that point (in a standard aluminum tank), which gets a lot riskier than pumping it lower pressures. When mixing "up", you put in the pure o2 first, at a very low pressure, and then top off with air to hit the desired mix. (see above posts for discussions about the need for o2 clean tanks, etc, for this) Thus, only your air compressors need to work at the higher pressures needed to complete the fills.

jj1987
05-30-2008, 13:52
30% is a strange mix to bank, but if it's the area standard, there might be some justification for it, in spite of the industry standard 32% mix.

As to the OP's question, I agree, there is no technical reason why he should have to wait overnight for a banked mix, as long as there is enough gas in the bank to do the fill. It's simply the shop's apparent policy, for entirely internal, rather than technical, reasons.

The reason you don't "enrich" a 30% mix to 32% by adding o2 is very few shops want to pump o2 at a high pressure. You're talking about pumping o2 at pretty close to 3000 psi at that point (in a standard aluminum tank), which gets a lot riskier than pumping it lower pressures. When mixing "up", you put in the pure o2 first, at a very low pressure, and then top off with air to hit the desired mix. (see above posts for discussions about the need for o2 clean tanks, etc, for this) Thus, only your air compressors need to work at the higher pressures needed to complete the fills.
Couldn't you put 100% o2 into the tank, and then top it off with 30% after that?

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
05-30-2008, 14:17
30% is a strange mix to bank, but if it's the area standard, there might be some justification for it, in spite of the industry standard 32% mix.

The justification for banking 30% is that 32% is a little too hot for the historical wreck sites that people visit from Morehead City (typically 110 -140 fsw). Divers coming from out of town often don't research the depths and might ask for 32% out of habit. By banking 30% you keep the tourists within their MOD and you speed up the process since the locals are going to be asking for 30% (and sometimes less) anyway.

As for the GUI divers, the minimum deco table states that it's good for mixes in the 30-32% range, so I disagree with the poster who said the GUE divers are screwed by having only 30% banked.

Scotttyd
05-30-2008, 14:19
I'll bet it is a training issue and the wait time is to make sure the trained person will be available.


This is usually the case. It is very hard to keep a trained person on staff. One shop I know of is getting out of the Nitrox business because they can't keep a trained employee around.

Another reason I have had delays is because the banks are empty and they need to mix a new batch.

And yet another is that the booster is down and they are having to rebuild it.
Booster down, I heard that one today:smiley2:
Which shop is getting out of the nitrox business?

mike_s
05-30-2008, 14:20
Couldn't you put 100% o2 into the tank, and then top it off with 30% after that?


sure you could.... if your 30% is o2 clean.


but a lot of banked 30% isn't o2 clean if it's made by removing nitrogen.

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
05-30-2008, 14:33
a lot of banked 30% isn't o2 clean if it's made by removing nitrogen.

Really? Do you have a source for that statement? I'd like to know whether my tanks are still O2 clean since I regularly fill with banked 30%.

Scotttyd
05-30-2008, 15:11
Couldn't you put 100% o2 into the tank, and then top it off with 30% after that?


sure you could.... if your 30% is o2 clean.


but a lot of banked 30% isn't o2 clean if it's made by removing nitrogen.
How do you remove nitrogen from the air??

CompuDude
05-30-2008, 15:15
a lot of banked 30% isn't o2 clean if it's made by removing nitrogen.

Really? Do you have a source for that statement? I'd like to know whether my tanks are still O2 clean since I regularly fill with banked 30%.

You have to ask the shop how their banked are filled. Membrane systems (which are the ones that make Nitrox by using a special membrane to filter Nitrogen from air, leaving a gas with a higher-than-usual o2 content: Nitrox) rarely use hyper-filtered oxygen-compatible air, because it's simply not needed. It's only needed with partial-pressure blending.

That said, a lot of shops with a membrane system don't bother banking, or don't bother banking much, since they can custom mix anything up to 40% right out of the system. So it's likely that they're PPBing their banks, in which case you're fine... IF their compressors are kept o2 clean. And if they're using different compressors to PPB into their banks from the HP compressors needed to pump regular air tanks up, they probably aren't.

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
05-30-2008, 15:15
Couldn't you put 100% o2 into the tank, and then top it off with 30% after that?


sure you could.... if your 30% is o2 clean.


but a lot of banked 30% isn't o2 clean if it's made by removing nitrogen.
How do you remove nitrogen from the air??

It's possible to filter nitrogen. It's just hard to believe that a shop would do that without also filtering out enough hydrocarbons to make the gas O2 clean. If the standards allow them to do this, something is very wrong with the standards. If this is the case I'd need 3 sets of tanks: air, premix nitrox, and clean nitrox.

I'm pretty sure everyone who banks around here uses partial pressure blending, but its good to know that not all nitrox is O2 clean. The state of the industry when it comes to tank labeling and filling is almost absurd.

jj1987
05-30-2008, 15:28
a lot of banked 30% isn't o2 clean if it's made by removing nitrogen.

Really? Do you have a source for that statement? I'd like to know whether my tanks are still O2 clean since I regularly fill with banked 30%.
If the lines it goes through aren't clean (after it's mixed) then the nitrox isn't o2 clean or that's how I understand it....

For instance, if I pump banked nitrox into my unclean tank, and then use my tank to put it in another, the air would not be o2 clean. That's just my understanding.

CompuDude
05-30-2008, 15:29
Couldn't you put 100% o2 into the tank, and then top it off with 30% after that?


sure you could.... if your 30% is o2 clean.


but a lot of banked 30% isn't o2 clean if it's made by removing nitrogen.
How do you remove nitrogen from the air??

It's possible to filter nitrogen. It's just hard to believe that a shop would do that without also filtering out enough hydrocarbons to make the gas O2 clean. If the standards allow them to do this, something is very wrong with the standards. If this is the case I'd need 3 sets of tanks: air, premix nitrox, and clean nitrox.

I'm pretty sure everyone who banks around here uses partial pressure blending, but its good to know that not all nitrox is O2 clean. The state of the industry when it comes to tank labeling and filling is almost absurd.

Membrane systems don't work that way. They just filter out Nitrogen.

If you feed them OCA, you get o2 clean Nitrox. If you feed them regular air (i.e., just sucking ambient air through the intake), which is how most shops do it, you don't get o2 clean Nitrox.

There's no need for o2 clean Nitrox under 40%. That lets you switch between plain air and Nitrox at will. And it's the only way I'll do it. I have no interest in dedicating a set of tanks to exclusive Nitrox use, which can only be filled by places with o2 clean air (expensive) or PPB Nitrox. I'd rather use regular non-o2 clean tanks, which can be filled ANYWHERE with air, and which can be filled with Nitrox and any one of several local shops that have Membrane systems, and re-filled on any one of several local boats that have Membrane systems or banks on board.

As long as you avoid shops that only do PPB Nitrox, you only need one set of tanks that can be used anywhere other than those shops. (And even at those shops, you can always over-pay for an air fill, if you really want, you only have to pass on their Nitrox.)

Obviously, different needs and rules apply when you're talking about deco tanks than need 50-100% o2. Those have to be maintained o2 clean, naturally. Membrane systems don't put out more than 40% anyway, though, so that's outside the scope of this discussion.

Geoff_T
05-30-2008, 15:30
now here is a bit of a stupid question but in the case of a pp blend does it take a certan amount of time for the o2 and air to mix evenly? Or for example if you have someone do a custom blend on your tanks do you have to wait say 2 or 3 hrs for the mix to become uniform?

jj1987
05-30-2008, 15:38
now here is a bit of a stupid question but in the case of a pp blend does it take a certan amount of time for the o2 and air to mix evenly? Or for example if you have someone do a custom blend on your tanks do you have to wait say 2 or 3 hrs for the mix to become uniform?
It takes time, but not long enough to make you wait.

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
05-30-2008, 15:40
No one around here uses a membrane system. It's all partial pressure blending and banking. If you are not O2 clean you get pre-mix. I often get fills at a shop that does not bank nitrox, so my tanks need to be O2 clean. Many shops won't fill nitrox unless you have a big ugly wrap on the tank. I guess I could throw those stupid nitrox wraps on my air tanks.

CompuDude
05-30-2008, 15:40
now here is a bit of a stupid question but in the case of a pp blend does it take a certan amount of time for the o2 and air to mix evenly? Or for example if you have someone do a custom blend on your tanks do you have to wait say 2 or 3 hrs for the mix to become uniform?

No, gas diffuses through other gases pretty quickly. Don't forget that it's essentially being "stirred" through the filling process, as the fast-moving stream of air flows into the existing tank.

Think about a bucket of water. Now pour in a glass of dye. It doesn't happen instantaneously, but it won't take long for that dye to spread out and evenly color the whole bucket. (I'm ignoring heavier dye powder and settling issues, since we're talking about gas on gas with scuba tanks.)

Gas is a lot more mobile than liquids are, so it happens even faster. Brownian movement evens things out pretty quickly... there is no stratification between oxygen and nitrogen to worry about.

CompuDude
05-30-2008, 15:43
No one around here uses a membrane system. It's all partial pressure blending and banking. If you are not O2 clean you get pre-mix.

Then there is the answer to your question. If you want to keep o2 clean tanks, you can get them filled at the PPB shops (and ONLY those shops, unless the pre-mix shops are using OCA in their mix), and your other tanks can get filled anywhere (air or premix). If you don't want to bother maintaining a separate set of o2 clean tanks, you can only get air at the PPB shops, and premix or air at the other shops.

There's no need for a third category of tank that is premix-only.

jj1987
05-30-2008, 15:54
Many shops won't fill nitrox unless you have a big ugly wrap on the tank. I guess I could throw those stupid nitrox wraps on my air tanks.
My tanks have a VIP sticker that says o2 clean. If a dive shop requires more than that I don't do business with them. I'm not buying $20 in stickers so that some shop doesn't have to go through the effort of reading my VIP sticker.

mike_s
05-30-2008, 16:04
Many shops won't fill nitrox unless you have a big ugly wrap on the tank. I guess I could throw those stupid nitrox wraps on my air tanks.


My tanks have a VIP sticker that says o2 clean. If a dive shop requires more than that I don't do business with them. I'm not buying $20 in stickers so that some shop doesn't have to go through the effort of reading my VIP sticker.


I agree with you. But the number of idiots that work at (or own) dive shops and have "no clue" never seems to stop amazing us...

CompuDude
05-30-2008, 16:05
Many shops won't fill nitrox unless you have a big ugly wrap on the tank. I guess I could throw those stupid nitrox wraps on my air tanks.
My tanks have a VIP sticker that says o2 clean. If a dive shop requires more than that I don't do business with them. I'm not buying $20 in stickers so that some shop doesn't have to go through the effort of reading my VIP sticker.

I agree. There are a LOT of retarded policies in place in NC shops, from what I've read. I feel bad for people stuck with shops that have such asinine policies.

Better than staying dry, though!

fireflock
05-30-2008, 16:09
I agree. There are a LOT of retarded policies in place in NC shops, from what I've read. I feel bad for people stuck with shops that have such asinine policies.


For what it's worth, the problems you hear about are from inland shops. The coastal shops that traveling divers will visit have seen everything and know how to deal with it all.

In other words, divers traveling to the NC coast for some wreck diving don't have to be scared about running into silly shop policies about gas fills. We save the silly policies for the locals :smiley29:

jj1987
05-30-2008, 16:10
Better than staying dry, though!
Diving AND staying dry at the same time is where it's at :smiley2:

And before anyone asks, yes, I dive dry in 85 degree water lol

CompuDude
05-30-2008, 16:21
I agree. There are a LOT of retarded policies in place in NC shops, from what I've read. I feel bad for people stuck with shops that have such asinine policies.


For what it's worth, the problems you hear about are from inland shops. The coastal shops that traveling divers will visit have seen everything and know how to deal with it all.

In other words, divers traveling to the NC coast for some wreck diving don't have to be scared about running into silly shop policies about gas fills. We save the silly policies for the locals :smiley29:

That's good to know, since most likely the only reason I'd be diving in NC would be to hit some of the wrecks. :)

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
05-30-2008, 16:32
In other words, divers traveling to the NC coast for some wreck diving don't have to be scared about running into silly shop policies about gas fills. We save the silly policies for the locals :smiley29:

So true. The Raleigh-Durham shops leave something to be desired when it comes to fills. Discovery and Aquatic Safaris seem to be top notch. Probably same for Olympus.

texdiveguy
05-30-2008, 16:36
I did not read all the posts here on the thread.....DID the OP ever reply back as to her shops issue with having to wait on the 'banked' gas..?

Scotttyd
05-30-2008, 19:31
Couldn't you put 100% o2 into the tank, and then top it off with 30% after that?
sure you could.... if your 30% is o2 clean.


but a lot of banked 30% isn't o2 clean if it's made by removing nitrogen.
How do you remove nitrogen from the air??

It's possible to filter nitrogen. It's just hard to believe that a shop would do that without also filtering out enough hydrocarbons to make the gas O2 clean. If the standards allow them to do this, something is very wrong with the standards. If this is the case I'd need 3 sets of tanks: air, premix nitrox, and clean nitrox.

I'm pretty sure everyone who banks around here uses partial pressure blending, but its good to know that not all nitrox is O2 clean. The state of the industry when it comes to tank labeling and filling is almost absurd.
learn something new everyday, and I was the chemistry minor

mitsuguy
05-30-2008, 21:40
I'm kinda curious why I hear all about PP blending and banked (which in my understanding, is still PP blending just in a large reservoir), and not more shops do the stick fills... I watched as they filled a bunch of tanks the other day with 32%, and it didn't take long at all... a couple minutes (maybe) to dial in the o2%, then just let the compressor run, while monitoring the output... Once its setup, it takes no longer than a standard air fill... Not only is it easy, but it's relatively easy to change the mix on the fly...

mike_s
05-30-2008, 21:49
Couldn't you put 100% o2 into the tank, and then top it off with 30% after that?


sure you could.... if your 30% is o2 clean.


but a lot of banked 30% isn't o2 clean if it's made by removing nitrogen.
How do you remove nitrogen from the air??

It's possible to filter nitrogen. It's just hard to believe that a shop would do that without also filtering out enough hydrocarbons to make the gas O2 clean. If the standards allow them to do this, something is very wrong with the standards. If this is the case I'd need 3 sets of tanks: air, premix nitrox, and clean nitrox.

I'm pretty sure everyone who banks around here uses partial pressure blending, but its good to know that not all nitrox is O2 clean. The state of the industry when it comes to tank labeling and filling is almost absurd.


why would a shop go to the expense of filters to make the air o2 clean if it never exceeds 40% at any time?

as for having 3 sets of tanks, you don't need that either. If you have o2 clean tanks, you can still put air in them. You just do air fills from the o2 clean side of the fill whip, just like you would with a partial-pressure-fill, but just without adding the o2 first.

as for banking nitrox, some shops partial pressure fill their banks, some filter off the nitrogen. It all depends on what setup they have.


If you partial pressure fill your banks, you have to have a multi bank system to get full use of all the o2 you just "spent" filling the banks or have a booster pump to get it out.

Neither way, banking nitrox isn't cheap.

The reason that a lot of shops "partial pressure fill" is that it's the cheapest way to do Nitrox tanks in low volume.

The reason that shops in high volume areas bank nitrox is that they sell enough of it to justify a bank system and also it's much quicker (and cheaper on labor) to fill tanks from a pre-banked source.


If it was cheap, we'd all have our own nitrox systems at home :D

mike_s
05-30-2008, 21:52
I'm kinda curious why I hear all about PP blending and banked (which in my understanding, is still PP blending just in a large reservoir), and not more shops do the stick fills... I watched as they filled a bunch of tanks the other day with 32%, and it didn't take long at all... a couple minutes (maybe) to dial in the o2%, then just let the compressor run, while monitoring the output... Once its setup, it takes no longer than a standard air fill... Not only is it easy, but it's relatively easy to change the mix on the fly...


it's simple why not more shops do the stick fills...

it cost more money in overhead equipment purchases.


If they don't sell a lot of nitrox fills, then it's not really worth it to the shop. (they never get the return on their investment).

partial pressure fills is the cheapest way to do nitrox in lower number of fills.