Transfill pressure between dissimilar tanks

[fnerg]

So never having done this, or even having a whip, if you're filling a smaller tank from a bigger tank, I'm guessing the resulting pressure would be something like

(tank pressure 1 - tank pressure 2)/((1- (tank size 1/tank size 2))/2)

So transfilling a 19 cf tank at 500psi from a 120 cf tank at 3500psi would give you a pressure closer to 3500psi in both tanks?

I just think about things like this sometimes.

[Jeff Pack]

I'm no tank math wiz, but 19 cf is like 15% of the 120. So I can't see how you could expect to loose 15% of a tank, and have the supply pressure stay at 3500psi... If the drop is linear, at that 15% loss, your supply pressure would be roughly 3k.

[fnerg]

Well right. When I said closer to, I meant closer to 3500 than 500, so 3000 is closer to 3500.

[fnerg]

I guess part of the reason I'm thinking about this is because I'm thinking about how best to keep a pony bottle filled without a compressor. I'm liking my new doubles a lot, to the point that I could see myself using my hp119s only a few times a year.

So would it make sense to get a whip, and transfill from the 119 to top the pony off every once in awhile?

[Norris]

I guess part of the reason I'm thinking about this is because I'm thinking about how best to keep a pony bottle filled without a compressor. I'm liking my new doubles a lot, to the point that I could see myself using my hp119s only a few times a year.

So would it make sense to get a whip, and transfill from the 119 to top the pony off every once in awhile?

How often are you sucking out of your pony? Is it a stage or just a redundant air supply?

[LCF]

Why would you need to top off your pony bottle? You aren't going to use it with doubles, are you?

[Mateo1147]

This is how I understand the relationship of pressure to cubic feet. A 120cf cylinder that operates at 3442 psi has a ratio of 28.68psi per cf. The math for that number is 3442psi / 120 cf. This is a good rough number because the pressure to cubic foot relationship isn't absolutely linear. So if you siphoned off 19cf of gas to completely refill an AL19 that would leave you with only 101cf with a resulting pressure of 2896psi. I am sure these numbers aren't absolutely correct but close enough for most calculations.
On another topic that comes up some times. Say your HP130 only has 3000psi in it. Is it enough gas for the dive you have planned or do you need to plan around the gas you have? Same math applies. 3442psi / 130cf = 26.48psi per cf. 3000psi / 26.48 = 113cf.
Now for the big kicker. Most tanks don't really hold what the name implies. A HP119 actually holds more gas than a HP120! A HP130 holds 131cf. An AL80 is really a 78. These two links should help anyone find the true capacity, full buoyancy, empty buoyancy and on the dock weights of most cylinders floating around out there.
http://www.xsscuba.com/tank_steel_specs.html
http://www.huronscuba.com/equipment/scu ... ation.html


I hope this helps you figure out what you would have left in your tank after a transfill and I hope this info is mostly correct. If it's not, I would gladly hear where I have gone astray.

[fnerg]

It's not even that I'm necessarily breathing off the pony, but opening the valve to pressurize the first and second stage uses air, and over the course of X dives, you won't have the 3000 psi you started with.

I'm getting two recreational dives per fill out of the doubles, so I'm thinking it's nice to have the pony along on the second dive.

[Mateo1147]

I have the same issue with my AL19 pony. After setting it up and tearing it down X times plus a little wave induced purge on entry and exit next thing you know it's at 2300psi. Since a AL19 is pretty minimal redundant gas I want it to be good and tight most of the time. My answer is to switch up to a stage rigged AL40 for redundant gas. I can still top it up from a HP cylinder and just get the viz done when needed.

[Mortuus]

Doubles along with proper gas planning should give you more than enough air for recreational dives though.... You already have redundancy with the doubles, as well as plenty of gas, so I am not seeing why you would need a pony here.

[Linedog]

Let's all go dive!

[LCF]

Well, if you are using the pony bottle the way you describe, what you are doing is basically carrying a very small stage bottle. In that case, that gas should be used FIRST (because it has no redundancy and cannot be shared) and the tank should be emptied to whatever degree you are comfortable emptying the tank. In that case, you won't get very many refills off transfilling. And for my money, if you are cutting your gas planning so close that you need an additional 19 cf to do the second dive, you are cutting it too close. JMO.

[Norris]

Seems like we should be careful and remember no where in the Ops original post is there a question as to what type of stage bottle they should use and when they should use it. Seems to me they had questions about filling a smaller tank with a larger tank.

fnerg
If it makes you feel more secure to dive with a 19cu foot tank while doing the second dive on some dubs, while not figuring that gas into your dive time, then there is nothing wrong with that. It is not unsafe and if you have a catastrophic air failure even a 19cu has more benefits than trying to breath water.


Just my opinion,
Rick

[fishb0y]

Back to your original question, I started trans filling my rebreather bottles (HP23) with some HP149s. Right now I am getting about 2-400psi drop in my 149's after a fill for my rebreather. I'm finding its more for convenience than anything else (I'm a little over an hour away from Tacoma Scuba... really the closest good dive shop for Kitsap County).

[fnerg]

Thanks for the replies and advice. So it might actually make sense to transfill a few times from a big tank, especially if it's just air lost from charging the first stage.

As far as the efficacy of using the pony/stage with doubles, I've only done about 10 dives with them so far, 3 of those being tech classes, meaning task loading, which sent my SAC through the roof, and a good part of the rest of those dives were spent adjusting weight, moving the tanks and wing so I didn't feel head-heavy all the time, getting used to the wing inflator being in a different spot, etc. I'm used to being able to get an hour out of a 100cf, so sucking down insane amounts of air for quite a few dives made me want extra insurance.

Happily, I managed a 0.58 SAC on my last dive, so I think I've got everything dialed in. I may or may not need the pony, but I still like to think about strategies for keeping the thing full without worrying about hook-up fees. (UWS seems not to worry so much about topping off small bottles, but I don't know if that's true for all shops)

[Mateo1147]

Seems like we should be careful and remember no where in the Ops original post is there a question as to what type of stage bottle they should use and when they should use it. Seems to me they had questions about filling a smaller tank with a larger tank.

fnerg
If it makes you feel more secure to dive with a 19cu foot tank while doing the second dive on some dubs, while not figuring that gas into your dive time, then there is nothing wrong with that. It is not unsafe and if you have a catastrophic air failure even a 19cu has more benefits than trying to breath water.


Just my opinion,
Rick

+1 and thanks for bringing this up. I have been thinking it for 24 hours but didn't want to

[CaptnJack]

Doubles and a pony seems like alot of crap for a recreational dive. But knock yourself out.

The ratio of pressure drop to pressure rise is about 9. So for every 1000psi you add to the 19, the supply will drop by about 125psi. The exact amount will depend on the interior volume of your whip which is "wasted" gas. If the doubles start off full, your bigger issue will be avoiding overfilling the pony.

[Tom Nic]

Yeah, perhaps I slept on this a bit as a mod.

The key is this - A ton of very unsolicited advice given here that was way off topic the question being about the math of transfilling, not stages and pony bottles. Mr. Merq did not get his nose bent - a credit to him - but this did go a bit farther afield than was bargained for.

[coulterboy]

Seems like we should be careful and remember no where in the Ops original post is there a question as to what type of stage bottle they should use and when they should use it. Seems to me they had questions about filling a smaller tank with a larger tank.

fnerg
If it makes you feel more secure to dive with a 19cu foot tank while doing the second dive on some dubs, while not figuring that gas into your dive time, then there is nothing wrong with that. It is not unsafe and if you have a catastrophic air failure even a 19cu has more benefits than trying to breath water.


Just my opinion,
Rick

+1 and thanks for bringing this up. I have been thinking it for 24 hours but didn't want to

Often times, the OP's question is answered with a "sermon" not totally relevant to what he was asking for.

Again, just my observation. Stick to answering the main question. Let the OP person probe for more questions, rather than giving him a sermon he doesn't need.

Josh (fnerg), there is nothing wrong with what you're doing, just like Rick said.

[Tom Nic]

Yep.

Not to mention that the other route violates nwdc TOS.

Just sayin'.

[kdupreez]

not trying to stir the pot here, but if someone asks a question that's in contrast with how a much more experienced group if divers would recommend an approach to such a topic, I see nothing wrong with offering some advice.

That's ultimately partly what this board is for; welcoming new divers and offer mentoring and advice based on our collective experience.

And when someone asks a question that's clearly born from something underlying that he/she might benefit from being elaborated on / discussed in a non demeaning and helpful way and we do so, then I'd say we've done our job as mentors.

Gas planning is a very near and dear topic to lots of folks here and that was the underlying concern.

that said, to the OP: in your tec class you should have learnt doubles is built for redundancy and if you plan/manage gas as taught in your class (I.e. enough reserve gas for 2 people back to next gas source), you can manage / isolate any catastrophic problem and call the dive with enough gas in your 1 available tank to make a slow / safe ascent. that's the whole idea of doubles..

its puzzling to me that you feel you need the assurance of a pony when diving doubles after taking a tec class. Please feel free to ping me via PM and I would love to go for a dive with you and we can talk min-gas / rock bottom strategies after the dive over some coffee.

But, if you really do want to carry a pony for some extra gas, id say the best top-off strategy is to leach off your high pressure doubles after they got filled. You will have less of a pressure drop on them and they are at full pressure after every dive shop visit. just be careful about over pressurizing the pony.

[LCF]

I'm sorry. I didn't intend a sermon, and didn't think what I wrote was one.

I know that the first time a friend and mine dove, carrying a bottle of extra gas because of our gas planning, we used the backgas first. I was subsequently educated on why that's not the best approach, so I was passing that info along. I didn't know the OP, and didn't know he had technical training. I was just trying to help someone not make the same mistake I made.

I guess the rules here are that you must only answer the strictest interpretation of the original question, or go on wild, taco-based segues. Nothing else.

[Tom Nic]

• do not give advice to others unless it’s explicitly asked for by that person

This is the line in the TOS.

I see nothing wrong with offering some advice.

And for the most part, neither do I. However... either we eliminate that line in the TOS (beyond my paygrade) or we think about the reason it is there and the spirit behind it.

I guess the rules here are that you must only answer the strictest interpretation of the original question, or go on wild, taco-based segues. Nothing else.

I don't think so Lynne, nor is that what I am trying to convey.

Taco-based segues are a ton of fun (rule #2, btw), as is a good hi-jack - at least for me. But there are some of our members who HATE thread hi-jacks. Do we ban them? No.

But frankly, the above mentioned rule in the TOS is one of the most under enforced or not enforced at all "rule" on NWDC, so I would say that strictness is not going to be a problem here.

I certainly don't think there was anything mean spirited or out of line in any of the responses, and again, because the OP did not take offense it is really all good. I didn't use the word "sermon" (and I know a sermon when I see one!)... but for some people it feels that way even when it isn't. And frankly, sometimes it IS a sermon. On NWDC you don't need a license to preach, just a C-Card and an opinion.

We "all" have a ton of knowledge and experience, and being willing to share that is part of what makes NWDC a cool place. We also have opinions, and of course we think there are reasons for those opinions and that they are good ones. And that is where it can get dicey. WE immediately see the connection from X subject to Y subject, even if the OP does not. And so I think we (myself included!) need to be reminded to somewhat restrain ourselves to the advice asked for as much as possible. Throwing out questions and seeing if the OP is thinking that way or wants to go there is often a good way to do that.

I think this is a simple reminder that can be good for us - and it is not intended to stifle or shut anyone up.

Carry on.

[Joshua Smith]

Sorry, I wasn't paying attention to this thread until just now.


It's a fine line, folks. That line in the TOS is the best we've been able to do. Our intent was to prevent the all-too-familiar scenario wherein a newbie comes on excited about.....whatever. New split fins, weight integrated BC, 19" dive knife that they're gonna strap to their shin....fill in the blank. And then 10 local hardcore cyberdivers pile on them crying "backplate and wing! Jetfins! Lil steak knife with the tip busted off!" and so on.

I didn't find any really awful TOS violations in this thread, but it was heading that way, and I thank Norris for intervening.

I just want to ask everyone to try and refrain from being too forward when topics like "transfilling my 19 cf pony bottle" come up. Not everyone dives the same way, and there are legitimate arguments for having a 19cf pony. (Anyone who wants to rip on spare airs will probably get a free pass, though )

I do realize that these comments are coming from a good place, and that safety is a paramount concern. But, really, there was nothing.....seriously, nothing....in the OPs post that caused me to think he was doing anything dangerous.

[pensacoladiver]

Josh. The dives I did today with Chris put me over 225 for the year. I think this qualifies me as NOT a cyber diver. As such, I am going to comment.

I routinely dive with a 19 cuft pony bottle strapped to my leg. My dive knife/sword is held securely between my teeth. I simply alternate the knife with the reg. I also dive one split fin and one Hollis.

In all seriousness though, when it comes to transfilling, it has already been said... Top it off from one of your full HP tanks. I used to do that with my 19 before I got my compressor and hardly noticed a difference.

If you drain it all the way down, transfill from a used hp tank then top it with a full one. If I go into any more detail, I will be discussing cascade systems.