Transfer from one tank to another

[mainedvr]

So I have two tanks one has 100 psi and the other 500 psi hook up a transfere whip and they will equalize. Volumn should not matter as its all about the pressure, so for those of you that transfere remaining gas from one tank to another what is your end goal. I can see filling my 1800 psi tanks from one of my hp tanks I could actually just about fill one with a good 3600+ fill but but then I have two half full tanks and I still pay for a whole fill. Now if I am where they charge by the cuft for say nitrox then it may be something worth doing?

My question is just a curiousity as I have heard others talk about it and wondered if there is anything to gain.

Thanks

 

[Caveeagle]

So I have two tanks one has 100 psi and the other 500 psi hook up a transfere whip and they will equalize. Volumn should not matter as its all about the pressure,

Volume does matter. If you are transferring gas between tanks of dissimilar size, you need to understand how much gas you can actually transfer before the pressure equalizes.

...so for those of you that transfere remaining gas from one tank to another what is your end goal.

There can be several reasons to do this. Here are just a few that I can think of..

a) I keep a small tank around the shop to use for reg testing and other stuff. I can used a trans whip to easily keep that tank set up with around 500-600psi without dragging it down to the shop.

b) Most folks do not own multiple sets of double tanks, and transporting them takes up a lot of space. then you typically don't breath them down below 1/2 capacity. So, you can use a transfill whip to move some gas from a single HP tank to top them off for a second dive.

c) When I am down in South FL, most shops do partial pressure blending of nitrox. And I don't keep my tanks stickered and clean for PP blending because its not needed in the shops near me that back 32%. So, I can pump up a large set of doubles with 32% and it will serve as a mobile fill station for a couple smaller tanks.

d) I don't blend my own gas, but I know some that do. So if you want to do this, you will need some way to transfer some gas.

 

[mainedvr]

Thanks, that is what I was looking for. I do have multiple tanks but not that many probably not worth it for me currently to spend the money on a whip. I do have a small set of doubles they are 1800 psi Steel 67's, so I could probably fill them with left over air from my HP tanks (some shore dives I do not use more than half a tank when diving with my kids. Yes I can see where volume does matter how you put it I just ment that it would only equalize at to a certain pressure but definitly need to know how much is in there. That is what I like about my 1800 doubles if I get a 2200 fill which is actually normal for me it gives me similar volumn as double al 80s (if I did the math correctly) and the tanks are light and are great in the water, as well as they double as light single tanks for my kids.

 

[ScubaInChicago]

So I have two tanks one has 100 psi and the other 500 psi hook up a transfere whip and they will equalize. Volumn should not matter as its all about the pressure, so for those of you that transfere remaining gas from one tank to another what is your end goal. I can see filling my 1800 psi tanks from one of my hp tanks I could actually just about fill one with a good 3600+ fill but but then I have two half full tanks and I still pay for a whole fill. Now if I am where they charge by the cuft for say nitrox then it may be something worth doing?

My question is just a curiousity as I have heard others talk about it and wondered if there is anything to gain.

Thanks

I like to daisy chain a few sets of doubles with whips and ask for ONE fill with all manifolds and whips open

About the only thing I transfill anymore is a 6cf airgon bottle.

 

[lermontov]

I like to daisy chain a few sets of doubles with whips and ask for ONE fill with all manifolds and whips open

About the only thing I transfill anymore is a 6cf airgon bottle.

and they only charge for one fill?

 

[tbone1004]

that was tongue in cheek I assume. Most places I get fills from charge by the cubic foot because it isn't fair to charge someone the same amount of money to fill an AL80 vs an FX149.

That said, the importance of understanding the transfill math for dissimilar tanks is something that can't be stressed enough. You mention 1800psi 67's which have a tank factor of roughly *for easy math here* 7.5cf/100psi. If you try to transfill them over from something like an AL80 which is roughly 2.5cf/100psi, you aren't going to get very far. For every 300psi you take out of the 80, the double 67's will only go up by 100psi *again, the 67's aren't exactly 7.5 and the 80 isn't exactly 2.5, but it's close enough to make me not have to use a calculator*.

I.e. if your AL80 is full at 3000psi, and the 67's are empty at 0, they will equalize at around 700psi. If you had a second full AL80 and were able to cascade into it, you'd end up with 1x AL80 at 750psi, 1x AL80 at around 1300psi, and the 67's at around 1300psi.

Transfilling is not something you do to get out of paying for fills because the math just doesn't work out

 

[rhwestfall]

mostly "b" stated above. I bring along an HP120 to boost my LP85 or HP100 twinset between dives for a two-tank trip... Yes, you do waste gas.... I will scavenge it occasionally to keep a "shop" tank handy for tunes, or other uses, but as the shops only PP blend, and without a booster, they are dumping your tank every time anyways, and it's just how it works around here....

Diving and economy are not to be used in the same sentence.

 

[rx7diver]

...understanding the transfill math for dissimilar tanks...

Sounds mysterious. Or impossibly complicated. Really isn't. Suppose you will dive your old-school LP 67, but you have two HP 120's that you will bring along with you additionally. Then think this way:

Caveat: I know nothing about old-school LP 67's. For this example let's assume that a LP 67 will hold 67 cu ft of gas at a service pressure of 1,980 psig (= 1,800 psig + 10%).

1. Step #1. The internal volume of an HP 120 is calculated by solving 14.7 * 120 = 3,500 * x for x. (Boyles Law.) This yields x = 0.5040 cu ft.

2. Step #2. Similarly, the internal volume of this old-school LP 67 is calculated by solving 14.7 * 67 = 1,980 * x. (Boyles Law.) This yields x = 0.4974 cu ft.

3. Step #3. By "equalizing" an "empty" LP 67 and a full HP 120, you will be putting 120.4974 cu ft (= 0.4974 + 120) of gas into a 1.0014 (= 0.4974 +0.5040) cu ft fixed volume. To find the resulting pressure, solve
14.7 * 120.4974 = x * 1.0014 for x. (Boyles Law.) This yields x = 1,769 psig.

That is, after "equalizing", the pressure in both the old-school 67 and the first HP 120 will be 1,769 psig.

4. Step 4. So, use the second HP 120 to top up the old-school 67 to it's final 1,980 psig service pressure. Then go do your second dive!

When you finish your dive, can you again fill your old-school LP 67, again treating your two HP 120's as if they were mini-cascade tanks, for another--the third--dive? How many times can you do this? The answers to these questions are provided by computations as straightforward as those shown above.

(Never underestimate the power of simple middle school algebra for these types of scuba problems. You never have to resort to "tank factors" and other concepts of dubious "added value." Fundamentals.)

Safe Diving,

rx7diver

 

[mainedvr]

that was tongue in cheek I assume. Most places I get fills from charge by the cubic foot because it isn't fair to charge someone the same amount of money to fill an AL80 vs an FX149.

That said, the importance of understanding the transfill math for dissimilar tanks is something that can't be stressed enough. You mention 1800psi 67's which have a tank factor of roughly *for easy math here* 7.5cf/100psi. If you try to transfill them over from something like an AL80 which is roughly 2.5cf/100psi, you aren't going to get very far. For every 300psi you take out of the 80, the double 67's will only go up by 100psi *again, the 67's aren't exactly 7.5 and the 80 isn't exactly 2.5, but it's close enough to make me not have to use a calculator*.

I.e. if your AL80 is full at 3000psi, and the 67's are empty at 0, they will equalize at around 700psi. If you had a second full AL80 and were able to cascade into it, you'd end up with 1x AL80 at 750psi, 1x AL80 at around 1300psi, and the 67's at around 1300psi.

Transfilling is not something you do to get out of paying for fills because the math just doesn't work out

Good point, I was mainly thinking of when I dive with my kids I have more gas in my tank than I want to pay for a fresh fill but not enough sometimes for another dive with my wife, but if I have 2000 left in my HP 100 and their LP 67 (1800 working pressure) is at 500 I may be able to put the remaining gas in there tank and get close to a full fill. I see now there is more to it again more of a thought. And I would get much more precise if I decided to do that.I just really trying to see if it is worth it. Thanks

Here is CA I get a 32% fill for $10 but no fills by the cuft like Florida.

 

[oldschoolto]

I just use the whip to bump up a tank from a full one to make a dive... Or to bump up the pressure on my steel 72's Doubles. other then that... I just get my tanks filled like normal...

Jim...