Plz explain the math - HP single transfill to LP doubles

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mwilding

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I am about to take my intro to tech class. I have a set of small steel 72's I am going to be learning on. I have just been reading about using an HP120 (or similar) to top off the doubles after a dive and am wondering about the math of it all.

Assuming a straight fill at 2250PSI, I would have 2X 65cu ft or 130cuft of gas for my first dive.

I have a nice dive and return to the surface with 1125PSI. This is a reasonable expectation for me as I have come up with similar PSIs when diving 120 singles. It is also similar to the amount of gas I would use diving a single AL80 if I surface with >= 500PSI left in the tank.

So, at this point I would hook up the transfill whip and move gas from the HP120 to the 72's. Now how much gas do I have in my doubles?

If I had an HP130, the math is easy (i think): (3300PSI + 1150PSI) / 2 = 2225 almost a full tank. Is this right?

OK, so now for the 120. intuitively, the pressure will be lower, but I can't quite get the conversion without over complicating it.


The bottom line for me is, once I am ready, I can start making boat dives in my doubles and taking along an HP single and a whip would be easier that buying or renting another set of doubles. Am I missing anything here?
 
Using the metric way of specifying tanks it would be pretty easy.

1. calculate total gas volume by figuring out free gas (gas volume at 1 bar) in each tank and adding.

2. Get your total tank volume by adding the volume (in liters) of the two tanks.

3. Divide total free gas volume by total tank volume to get pressure in bars

--------------------

With imperial method of specifying tanks you can do the same thing, but it involves a bit more math. The reason is that in metric system tanks are described by their internal volume (11 liter, 15 liter, etc.) at 1 bar while in the imperial system tanks are specified by their total free gas volume at a specified service pressure. To get internal volume one has to convert psi to ata, then divide that into rated volume to get the internal volume.

Assume that your tanks are a set of doubles with 130 cu feet at 2250 psi and an HP tank of 130 cu feet at 3300 psi (I don't think those are the actual specs of tanks, but they are the numbers from your post).

1. If you have 1125 psi left in your doubles, then you have 130/2250 * 1125 = 65 cu feet there, and you have a full 130 cu feet in your HP tank for a total of 195 cu ft.

2. Your doubles have 130cu ft/2250 psi = 0.0578 cu feet per psi (you will also see people refer to a "tank factor" of 5.78 cu feet per 100psi). You 130 cu ft at 3300psi tank has 130/3300 = 0.0394 cu feet per psi. This means that when joined together with the transfill, that the combined set of tanks is 0.0578 + 0.0394 = 0.0972 cu feet per psi.

3. Having 195 cu feet total volume of air in the combined tanks which are 0.0972 cu ft per psi means that the pressure will be 195/0.0972 = 2006 psi.

There may be simpler a simpler formula, but rather than memorizing specific formula I find it is easier to just go back and do the step by step calculation.

Charlie Allen
 
I guess the bottom line is, the HP transfill approach will work, especially if the second dive is less gas intensive as the first.
 
https://www.shearwater.com/products/teric/

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