Filling double LP 50's from a HP 120

[formernuke]

Ok a little more background.

I don't like the way the 120's feel in or out of the water. Right now I dive with HP 100's.

I'm starting tech training soon and will be doubling my 100's. In addition my current regs have a limited life span (not sure how long, they are no longer making replacement parts so once the current inventory is used up that's it). So my options are.

1. buy 2 more 100's and either buy a another reg set or convert my tech regs back and forth for single tank
2. buy a set or 2 of LP 50's and dive doubles all the time.

I will be using the single 100 or double 50's for low key shallow shore diving 20-40 feet max depth. If I stay with doubles then everything config wise stays the same just lower air volume. For any boat dives I'll use the double 100's. As any deep dives (nitrox) will be boat they will also be in the 100's.

So for consistency I'm thinking very strongly about just always using doubles, people I've talked to say that the double 50's trim very well, better than the single 100. This got me thinking maybe instead of buying 2 sets of 50's just get one and a transfer whip and refill the 50's at the site with a 120.

 

[NAM001]

Ok a little more background.

I don't like the way the 120's feel in or out of the water. Right now I dive with HP 100's.

I'm starting tech training soon and will be doubling my 100's. In addition my current regs have a limited life span (not sure how long, they are no longer making replacement parts so once the current inventory is used up that's it). So my options are.

1. buy 2 more 100's and either buy a another reg set or convert my tech regs back and forth for single tank
2. buy a set or 2 of LP 50's and dive doubles all the time.

I will be using the single 100 or double 50's for low key shallow shore diving 20-40 feet max depth. If I stay with doubles then everything config wise stays the same just lower air volume. For any boat dives I'll use the double 100's. As any deep dives (nitrox) will be boat they will also be in the 100's.

So for consistency I'm thinking very strongly about just always using doubles, people I've talked to say that the double 50's trim very well, better than the single 100. This got me thinking maybe instead of buying 2 sets of 50's just get one and a transfer whip and refill the 50's at the site with a 120.

I carry one or 2 lp120s over filled for this stuff. you dont get a full tank but it saves you the trip to the pump station for that last half of tank dive you want to do. Especially when you pay by the tank and not the amount of filled gas. One place I go to is 10 dollars a tank whether it is a 6 cuft or a 150 cuft tank and 15 for nitrox. yep fill that 120 cuft tank and transfer a usable amount of gas for several other tanks. I think a lp120 is a 20 liter tank and the al80 is a 11 liter tank. so to equalize them from empty al80s will draw teh psi down to half. even better if they have 5-600 in them already you can thaen have 2k or so for a couple more dives. soing that with lp 50's would be i would guess nearly a full tank of 2500. just a guess without doing the math. Im going to guess a lp50 is about a 6-7 liter tank. Then second round of sdoint that first frmoe the used tank and then getting another lp120 at 3500 to top off with.

 

[JahJahwarrior]

Double 100's should be good for 2 boat dives on same fill in rec limits. But for shore diving, the doubled 50's will be better than the double 100s, so I like the way you are thinking. Get the 50's for shore diving, keep the 120 for topping them off, and keep the 100's for boat diving.

Bonus plan: Get all tanks filled. Go for a boat dive with the 100's, don't refill.
Go for a shore dive with the 50's, use the 100's leftover gas first to fill, then do final top off with the HP120, then get them all filled or maybe even do a third shore dive that is shorter.

What do you see on shallow shore dives in new England?
What regs are you using?

 

[formernuke]

What do you see on shallow shore dives in new England?
What regs are you using?

We see a lot of starfish (some of them pretty colorful), lobsters, craps (several species I'm partial to the horseshoe ones). we have some corals though not as colorful as the warm water ones. I've seen tautog and baby striped bass along with other fish I'm not sure about. In late summer we get some cool tropical fish in RI.

Right now I'm using 12 year old zeagle flatheads. They breath great but at the time they were made by Apeks. Huish bought zeagle a few years ago, when that happened Apeks stopped supplying parts. Zeagle told me that they have parts on the shelf but once those were gone that was it.

 

[phoque]

We see a lot of starfish (some of them pretty colorful), lobsters, craps (several species I'm partial to the horseshoe ones). we have some corals though not as colorful as the warm water ones.

Yes, I confirm, that's what we see around here, although for craps I don't need to go diving, I see those around the block quite a bit when I walk my dog, especially lately for some reason I guess without the usual scrutiny around during these pandemic days, people are getting a little distracted and "forget" picking up.
More seriously though, a lot of nudibranchs take over during the winter months too.

 

[phoque]

Thank you for the better explanation Phoque. Thankfully even if our calculation was a little off, it's not crazy wrong. I also appreciated that you reran the calculations showing the pressures assuming he's not starting from empty.

Yes, in the end the results aren't that far apart, and this already gave a good idea of the kind of fill you'd get this way.
An easy way to estimate how much gas you'll have after transfilling in this case, is to do the following. Here, the volumes of the doubles and HP120 are almost equal (52/48 split), so you'll get very close to the actual number averaging the 2 pressures (doubles at end of dive 1, and full HP120), and subtracting 50psi.
Ex: empty doubles and full 120 @ 3440: (0+3440)/2 - 50 = 1670 (vs actual of ~1650).
Doubles @ 500 psi and HP120 @ 3100: (500+3100)/2 - 50 = 1750 (vs 1748 actual).
Doubles @ 1000 psi and HP120 @ 3300: (1000 + 3300)/2 - 50 = 2100 (vs 2104 actual).

If you decide to transfill sequentially each tiny tank, and then equilibrate them by opening the isolator valve, a good estimate will be mean pressure + 100 psi instead of mean - 50.
This way, you know easily how low you can go on your first dive to preserve enough residual gas to obtain a usable fill for dive 2 by transfilling during your SI. This allows you to know when to turnaround on dive #1. Whether that is viable or not for you depends on your RMV, the dive site chosen, and the bottom time you want to be able to have on both dives.

 

[MichaelMc]

I’d start with one 50s set and trans fill, just because you have the 120. If you tire of the second partial fill get the second set. I like my baby doubles for back mount.

 

[saxman242]

while in your calculation you have used the rated capacity of the tanks in cf

D'oh, yes, you're correct.

 

[formernuke]

Ok, I brought out the baby doubles yesterday.

First they dive great, I love the way they trim out.

I transfilled from the 120 with the doubles at 700 psi. I once I reopened the isolator 1900 pounds. So not bad, I thinking of maybe selling the 120 and a 149 instead.

 

[hroark2112]

Transfill calculations?