How much air in an HP100 (3500psi capacity) when filled to 3000psi?

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Hi All, wondering if anyone would know the amount of air in a PST HP100 with a 3500psi rating when it only gets filled to 3000psi?

Mostly wondering for the purposes on knowing the fill amount when on a boat that can only get the tank to 3000-3100.

Thanks in advance.

I've usually gotten hot fills to 3000 or so on boats, so unless they have time to cool and top off before the dive, look for 2800 or so when you splash. This is one reason I use an LP95, in the rush I wind up with 2400 to 2800 for the splash, depending on the attention they pay to the cylinder and if the DM filling likes me.



Bob
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Even a blind pig gets an acorn once in a while.
 
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Thanks guys I'm slowly getting a handle on this :D
 
I assume it would be 6/7ths, or roughly .87 expressed as a decimal. A box that holds 100 ping pong balls with only 87 in it to start.
 
Here's the quick math on this:

100 Cubic Feet / 3500 PSI = .0285 Cubic Feet for everyone 1 PSI
3000 PSI *.0285 = 85.5 Cubic Feet
 
I assume it would be 6/7ths, or roughly .87 expressed as a decimal. A box that holds 100 ping pong balls with only 87 in it to start.

Or you could just play ping pong, assuming that that that you would not want to address fractions while diving.



Bob
 
Here's the quick math on this:

100 Cubic Feet / 3500 PSI = .0285 Cubic Feet for everyone 1 PSI
3000 PSI *.0285 = 85.5 Cubic Feet

Actualy math is
102cf/3500psi= 0.0291
3000*.0291=87.42

nitpicking, but that 2% matters in SAC calculations, this is why I will re-emphasize the point of knowing your ACTUAL tank capacity, not you NOMINAL tank capacity. The math was done correctly in post #2, along with a link to the vast majority of tanks on the market. Many of which have very different actual capacities than their nominals.
 
winner!!!!!

---------- Post added January 7th, 2015 at 09:56 AM ----------

[(rated capacity)/(rated pressure)] x (fill pressure) = what you got
 
Can I throw a curve out there...... does temperature change the volume? Will you get more breathing time from a cold tank as oppose to a hot one?
 
Sort of. The flow rate is the essentially the same regardless of temperature. The first stages work off of pressure differentials, so while you are moving more volume per breath, you can't utilize that in your body, so you're breathing more dense air, but it doesn't really matter because you can't use it. Same thing happens when you go deeper, you are breathing the same volume of air to fill your lungs, but because of the increased density of the water, you are using more air out of the tank for each breath.

If the tank changes temperature your pressure will change per Boyle's law.

I.e. Tank is filled to 3500psi at 75F, but you're diving in 50F water, the pressure will drop to 3350 once the tank equalizes temperature which doesn't take particularly long. This is why when you are diving in cold water it is important to mark your pressures once the tanks have hit the water.
Conversely another common one is the tanks will be at 50f when the shop fills them in cave country in the winter to 3600, and once they hit the nice warm spring water, they will rise to 3800. Much less of a concern in recreational diving, but in technical diving it's a big concern.

The "rule of thumb" is 5psi per *F, but it's VERY loose and only good for large temp swings, 20F+. It's quite a bit more when the tanks are at lower delta T's, but those generally don't matter much, and also is only good for low pressure tanks.

I.e.
LP steel filled to working pressure at 50F has about a 6psi/*F delta when it hits 70F.
A HP steel filled to working pressure under the same conditions, has about an 8psi delta

Formula for that is
(460+Temp final)*(PSI init+14.7)/(460+Temp init)=PSI Final
460 is because you have to work in the Rankine scale to get it to work which has 0 as absolute zero, but the degrees have the same scale as Fahrenheit.

So to answer your question, yes but the opposite of what you said. Because your SAC is independent of gas density, and your tank volume is really only dependent on the tank pressure, if the tank gets warmer you get more bottom time, but if it gets colder, because the pressure drops, you have less cubic footage inside of the tank to breathe, so you have less time. The only temperature that matters is the water temperature you'll be diving at, so ideally you want the tanks to always be colder than the water when you check them for pressure. That means making sure the tank is room temperature before you check the pressure and ask for a top off if they short filled you. In NFL most of the shops will fill to 3700psi or so to let the tanks cool to 3600. Allows the tanks to hit about 90F during filling and cool to 75F room temperature.
 
Can I throw a curve out there...... does temperature change the volume? Will you get more breathing time from a cold tank as oppose to a hot one?

short answer is yes. A cold tank with 3000psi has more gas in it than a hot tank with 3000psi in it.
 

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