Transfilling LP50 doubles from HP100 doubles

[flymolo]

Hi all,

New to transfilling here, so bear with me-

I have a set of LP50 doubles (with flexible manifold). They are at approximately 2300 psi. I also have a set of HP100 doubles, which are full at around 3500 psi. I'm looking to "top-up" the LP50 doubles from the HP100 doubles. Some have suggested isolating the HP100 doubles and using them as a sort of mini cascade.

What's the most efficient way to top up the LP50s off the HP100s? Isolate both, and transfill each tank individually? (i.e. transfill left side lp50 from left side hp100, and right side lp50 from the right side hp100)? Or isolate the HP100s and fill into the LP50s, with the soft manifold left open? Or is there another method I'm not thinking of?

 

[JahJahwarrior]

@phoque gave a good explanation of this recently, and his solution is really easy to mock up in Excel.

I took the liberty of adding it to the bottom of my gas planning tool, because in reality it can be helpful to throw together a few scenarios quickly to see how to best scavenge gas from tanks.

Gas Planning

Option 1: Hooking both doubles up together, all tanks open, should equalize at 3100 psi in the all tanks.
If your goal is to get the LP50's back to 2640 pressure, the lowest you can take them is 1,100 psi, at which point the fills will equalize to 2700 psi.

Option 2: fill the doubled LP50 first from one side of the HP100s with both 50's open, then close and fill off the other side of the HP100s. Off the first tank, they will equalize at 2900 psi, and then from the second tank they will equalize at 3200 psi. If you then open the isolator on the 100's they will equalize to 3,050 psi. This method gets more gas out of the 100's than straight equalizing.

Option 3: Isolate all tanks. Fill one LP50 from first one HP100 tank, then the second HP100 tank, then fill the second LP50. This will result:
Round 1: both tanks equalize at 3100 psi
Round 2: LP50 1 is up to 3,366 psi
Round 3: LP50 2 gets up to 2833 psi
Round 4: Lp50 2 gets up to 3,188 psi
To compare to the other methods, we could equalize the two 50's to each other and get 3,277

 

[tursiops]

@phoque gave a good explanation of this recently, and his solution is really easy to mock up in Excel.

I took the liberty of adding it to the bottom of my gas planning tool, because in reality it can be helpful to throw together a few scenarios quickly to see how to best scavenge gas from tanks.

Gas Planning

Option 1: Hooking both doubles up together, all tanks open, should equalize at 3100 psi in the all tanks.
If your goal is to get the LP50's back to 2640 pressure, the lowest you can take them is 1,100 psi, at which point the fills will equalize to 2700 psi.

Option 2: fill the doubled LP50 first from one side of the HP100s with both 50's open, then close and fill off the other side of the HP100s. Off the first tank, they will equalize at 2900 psi, and then from the second tank they will equalize at 3200 psi. If you then open the isolator on the 100's they will equalize to 3,050 psi. This method gets more gas out of the 100's than straight equalizing.

Option 3: Isolate all tanks. Fill one LP50 from first one HP100 tank, then the second HP100 tank, then fill the second LP50. This will result:
Round 1: both tanks equalize at 3100 psi
Round 2: LP50 1 is up to 3,366 psi
Round 3: LP50 2 gets up to 2833 psi
Round 4: Lp50 2 gets up to 3,188 psi
To compare to the other methods, we could equalize the two 50's to each other and get 3,277

GREAT post!

 

[flymolo]

Excellent. Thank you for this!

I bookmarked that page of yours. Hope it comes in handy!

 

[SurfLung]

Are you thinking just to "top off" your 50s or refill them? Because the most efficient use of a cascade fill is to transfill into an empty or near empty tank.

- At the most recent "Sea Hunt Forever" event, I was diving vintage Twin 38s. These tanks are marked 1800 psi. I did 10 Dives of 30 minute duration and kept my tanks filled by transfilling from 3 AL80s and 2 steel 72s. At first I was filling my tanks 10% over to 2000 psi. And after a 30 minute dive, I'd have about 1000 psi left and transfill them back up to 2000 psi. But then I figured maybe I shouldn't use up my hi pressure transfill capacity just to top off at 2000 psi. So I began transfilling my tanks to just 1200 psi and breathing them to nearly empty on each 30 minute dive. I think this was probably the most efficient use of my transfill pressure capacity. Later, I seemed to have plenty of air left so I began to transfill all the way up to 2000 psi but I now made TWO dives on the one transfill... Still breathing the tanks down to nearly empty for best Transfill efficiency.

- I didn't have the option to separate the Twin 38s but I would definitely recommend separate tanks to transfill FROM.