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maverick
04-14-2010, 23:35
I had 1000lbs left in my Nitrox tank recently dated and the LDS had me empty the tank so he could 32%, is emptying a Nitrox tank with a little Nitrox left standard procedure 4 a refill which was also 32%?

navyhmc
04-15-2010, 00:30
I had 1000lbs left in my Nitrox tank recently dated and the LDS had me empty the tank so he could 32%, is emptying a Nitrox tank with a little Nitrox left standard procedure 4 a refill which was also 32%?

It depends on how they do a fill. If they do a partial pressure fill, then they usually will empty it so that there's not excessive use of the higher pressures on the o2 supply tank. If they used a bank of 32% then there usually is no reason to empty.

CompuDude
04-15-2010, 01:16
Most places doing partial pressure will empty the tank just because they're feeling lazy and would rather waste the existing gas than do the math to figure out how to top off the stuff in there. For empty tanks, they most likely have the simple numbers memorized.

If it was banked, they just weren't thinking... or didn't trust the analysis of the original fill and didn't want to re-do it?

skdvr
04-15-2010, 05:05
More than likely they were doing PP fills. I personally would dump it too just to save the higher end of my O2 bottles. It probably cost the same to you no matter what. If you were paying by cu.ft. then I would not be happy with them wanting to dump residual gas.

Phil

maverick
04-15-2010, 14:18
Thanks 4 the information.

scubadiver888
04-18-2010, 13:06
I kind of wonder about mixing as well. If I have a cylinder with EAN32 but filled at a different shop then I ask your shop to fill my cylinder with EAN32 from a pre-mixed bank, doesn't the gas in my cylinder and the gas in the bank mix a little? Sort of like backwash when drinking from someone else's bottle. They might want to avoid the 'backwash' potential since they don't know how clean the gas you have is.

Is this a possibility?

navyhmc
04-18-2010, 19:17
I kind of wonder about mixing as well. If I have a cylinder with EAN32 but filled at a different shop then I ask your shop to fill my cylinder with EAN32 from a pre-mixed bank, doesn't the gas in my cylinder and the gas in the bank mix a little? Sort of like backwash when drinking from someone else's bottle. They might want to avoid the 'backwash' potential since they don't know how clean the gas you have is.

Is this a possibility?

I won't say it isn't a possiblity, but I will venture to say that the possibility is very, very limited. The would be the same a saying you must empty your standard air filled tank because you might have tainted air in it. Just isn't done.

Personally, I don't think backwash is possible.

fire diver
04-19-2010, 12:43
I kind of wonder about mixing as well. If I have a cylinder with EAN32 but filled at a different shop then I ask your shop to fill my cylinder with EAN32 from a pre-mixed bank, doesn't the gas in my cylinder and the gas in the bank mix a little? Sort of like backwash when drinking from someone else's bottle. They might want to avoid the 'backwash' potential since they don't know how clean the gas you have is.

Is this a possibility?


No, it's not possible. If the fill bank has a higher pressure than the tank you are filling (which is a MUST to fill a tank) then it's impossible for any gas to go backwards into the bank. On top of that, a good shop will have check valves that make it impossible flow backwards if a high pressure tank is hooked into the bank.

But.... Oxygen is cheap. There's no sense in wasting time checking your gas contents to make sure they do thier math right. Dump it, and PP fill from scratch.

UCFKnightDiver
04-19-2010, 15:02
I kind of wonder about mixing as well. If I have a cylinder with EAN32 but filled at a different shop then I ask your shop to fill my cylinder with EAN32 from a pre-mixed bank, doesn't the gas in my cylinder and the gas in the bank mix a little? Sort of like backwash when drinking from someone else's bottle. They might want to avoid the 'backwash' potential since they don't know how clean the gas you have is.

Is this a possibility?


No, it's not possible. If the fill bank has a higher pressure than the tank you are filling (which is a MUST to fill a tank) then it's impossible for any gas to go backwards into the bank. On top of that, a good shop will have check valves that make it impossible flow backwards if a high pressure tank is hooked into the bank.

But.... Oxygen is cheap. There's no sense in wasting time checking your gas contents to make sure they do thier math right. Dump it, and PP fill from scratch.

There's several programs that will do the math for you, and the equations arnt that difficult, why waste the money if you dont have to? VPlanner and GUE's Gas Management Program come to mind.

Rorschach
04-19-2010, 15:28
Backflow prevention should be standard with any filling operation, but that doesn't mean every filling operation is configured or maintained that way.

In the industrial world we usually specify two check valves in series if we need to count on backflow prevention.

Like the others, I can understand 'better' if they are using partial pressure method for gas fills, but as to whether that affects the consumer's end price, it really should - if the gas fill price is based on raw materials cost, labor, overhead, and a profit margin. But that's not to say every business sets up and operates their cost of gas fills based on that methodology.

mitsuguy
04-19-2010, 16:48
If doing partial pressure fills, it doesn't make a whole lot of sense to trans fill from an O2 cylinder, that, when full, only has 2200-2500 psi worth of gas in it, into a tank that already has 1000 psi of gas in it...

For gas mixing, we use DSAT's gas mix calculator... does everything I need it to... In the above example, I'd need to transfill O2 until tank pressure reached 1279 psi, then top off with air, which would work just fine, until I had an O2 cylinder (we use two at a time) that was less than 1279 psi... In that case, I could use a booster, but, that is more time consuming, and, as mentioned, O2 is relatively cheap, even here on the island, and my time is worth a decent amount, actually...

In the long run, its just easier to drain and refill to whatever percent I want...

UCFKnightDiver
04-19-2010, 18:11
If doing partial pressure fills, it doesn't make a whole lot of sense to trans fill from an O2 cylinder, that, when full, only has 2200-2500 psi worth of gas in it, into a tank that already has 1000 psi of gas in it...

For gas mixing, we use DSAT's gas mix calculator... does everything I need it to... In the above example, I'd need to transfill O2 until tank pressure reached 1279 psi, then top off with air, which would work just fine, until I had an O2 cylinder (we use two at a time) that was less than 1279 psi... In that case, I could use a booster, but, that is more time consuming, and, as mentioned, O2 is relatively cheap, even here on the island, and my time is worth a decent amount, actually...

In the long run, its just easier to drain and refill to whatever percent I want...

So instead you want to use 418 psi worth of 02 or 10.45 cuft by draining the tank vs 278 psi of o2 or 6.95 cuft if you leave it at 1000psi, this makes no sense to me, sorry maybe I'm missing something.... I guess if you had a couple of pretty low pressure o2 bottles this would kinda make sense but... if all your bottles were full or over 1k then this wouldnt make sense at all, so I would say it depends.

mitsuguy
04-19-2010, 18:46
So instead you want to use 418 psi worth of 02 by draining the tank vs 278 psi of o2 if you leave it at 1000psi, this makes no sense to me, sorry maybe I'm missing something.... I guess if you had a couple of pretty low pressure o2 bottles this would make sense but... if all your bottles were full then this wouldnt make sense at all, so I would say it depends.

By the time you put a gauge on the tank and analyze the current contents, plug the numbers into the software, make sure that the O2 cylinder has more than enough to be sure you can transfill, in the situation above, you need to have more than the 1279 final pressure, because the pressure in the O2 cylinder will drop as well, so, you need to have at least 1500 psi, which is almost 3/4 full... Of course, the lower the pressure in the scuba tank, the less you have to have in the O2 cylinder, but then again, the less you gain by filling on top of the gas already in there... Anyways, by the time you do the extra work, you have spent that much more time doing the extra work... In a busy shop like we have, labor costs are way high, and far outweigh a dollar a fill in O2 savings...

If we were to do enough Nitrox to matter, we would (and will most likely anyways) invest in a continuous flow blending system... Waiting on a new compressor before we do that though...

Either way, if I was doing it for myself, in my free time and there weren't a million things going on, then I would do it your way, even using a booster if I had it (which I do), but in the shop environment, time is money too...

UCFKnightDiver
04-19-2010, 18:48
So instead you want to use 418 psi worth of 02 by draining the tank vs 278 psi of o2 if you leave it at 1000psi, this makes no sense to me, sorry maybe I'm missing something.... I guess if you had a couple of pretty low pressure o2 bottles this would make sense but... if all your bottles were full then this wouldnt make sense at all, so I would say it depends.

By the time you put a gauge on the tank and analyze the current contents, plug the numbers into the software, make sure that the O2 cylinder has more than enough to be sure you can transfill, in the situation above, you need to have more than the 1279 final pressure, because the pressure in the O2 cylinder will drop as well, so, you need to have at least 1500 psi, which is almost 3/4 full... Of course, the lower the pressure in the scuba tank, the less you have to have in the O2 cylinder, but then again, the less you gain by filling on top of the gas already in there... Anyways, by the time you do the extra work, you have spent that much more time doing the extra work... In a busy shop like we have, labor costs are way high, and far outweigh a dollar a fill in O2 savings...

If we were to do enough Nitrox to matter, we would (and will most likely anyways) invest in a continuous flow blending system... Waiting on a new compressor before we do that though...

Either way, if I was doing it for myself, in my free time and there weren't a million things going on, then I would do it your way, even using a booster if I had it (which I do), but in the shop environment, time is money too...

I hear you, also continuous flow is awesome! Let's you use practically all the o2 in the bottle, though this is something you can easily build on the *cheap*... PVC, o2 sensor, some plastic hoses etc. Versus a system like this Tri-Hunter 6000 (http://www.amigosdivecenter.com/stick.html)