Check my math

[malammon]

I'm not a math or physics major, but would someone please check this rather simple notion. Ignoring temp. and gas compressibility characteristics, if a OMS LP steel 108 holds 112 cuft @ 2640 psi then it holds 131.51 cuft @ 3100 psi right? Not that I would ever overfill the tank, but I was just wondering.

 

[JimC]

If you have a "LP 112" (112cu @ 2650psi) then yes, it holds 131.52 cu @ 3100psi.

However, if your tank is an "LP 108" then it holds 108 @ 2650psi, and 126.82cu @ 3100psi.

 

[Uncle Pug]

if the 108 contains 112 at 2640 and you fill it to 3100.

 

[malammon]

OMS Tank Specs for the LP 108 indicate the the capacity is 112 cuft. Why it is called a 108, I don't know.

Thanks Uncle.

 

[MikeS]

I think that your math is correct but your explanation is not, I think you have mass and volume confused.

The tank holds 0.624 cubic feet of air at 2640 psi which is the equivalent of 112 cubic feet at 14.7 psi. The volume of the tank is (112*14.7)/2640 or approximately 0.624 cubic feet.

The volume of the tank does not change; the mass of air contained in the tank varies with pressure.

At 3100 psi the tank holds a mass of air equivalent to 131.5 cubic feet at 14.7 psi (3100 * 0.624 / 14.7 = 131.5 ).

Mike

 

[JimC]

MikeS once bubbled...
I think that your math is correct but your explanation is not, I think you have mass and volume confused.

The tank holds 0.624 cubic feet of air at 2640 psi which is the equivalent of 112 cubic feet at 14.7 psi. The volume of the tank is (112*14.7)/2640 or approximately 0.624 cubic feet.

The volume of the tank does not change; the mass of air contained in the tank varies with pressure.

At 3100 psi the tank holds a mass of air equivalent to 131.5 cubic feet at 14.7 psi (3100 * 0.624 / 14.7 = 131.5 ).

Mike

Which makes me wish my LDS would weight my tanks to fill them instead of using a PSG. They NEVER fill them full, but atleast its free.

 

[omar]

since we are getting into the math end of this. The gauge pressures you are using are just that "gauge" and not absolute. so you need to add 14.696 for the atmosphere if you insist on using 3 digits in your air mass. also oms lists the actual internal volume so you should use that. In addition it it normally refered to as free air delivery

omar

 

[MikeS]

omar once bubbled...
oms lists the actual internal volume so you should use that.

omar

Omar,

If you are going to be picky perhaps you should actually do the math! The volume specifications on the OMS page are rounded to the nearest liter. Using that number would result in less accurate results.

17 liters * 0.03532 = 0.60044 cubic feet

0.60044 cubic feet * 2640 psi = 1585.162 / 14.696 = 107.8635
Perhaps this is where the 108 came from.

If this is correct how can they claim an air capacity of 112 cubic feet?

Also, if you want to throw in the issue of the relative versus absolute pressure gauges to show us how smart you are, maybe you should add in the effects of altitude and temperature and show us the calculations.

Mike

 

[omar]

All right.

Because you don't know the difference between absolute and gauge pressure the actual equation to compute the volume is:

[(2640 + 14.6959)/14.6959]*volume

17 liters is the equivalent of 0.600349 ft^3 (you rounded up wrong 0.035315 is not 0.03532)

So it is is actually 108.45 ft^3 a little closer to what they have labeled it. (gee, maybe the 108 that they use came from this)

Maybe you should pay attention before you put down in writing what a boob you are.

The free air delivery that OMS indicates takes into account the compressibility of air by using the van der Wall's EOS. This is a fair approximation but there are better ones such as the Beattie-Bridgeman EOS.

The compressibility factor (Z) using the B-B EOS for air at 2640 and 70 degF is 1.018. Can you figure out what to do with this? I doubt it.

The general barometric equation to determine pressure at altitude is:

Palt = 33 * exp (-0.038 * (alt/1000))

The actual equation is:

P = Po * e^-[(M*g*h)/(kB*T)]

Where M is the molecular mass of air
g is the acceleration due to gravity
h is the altitude
kB is the Boltzman constant
T is the absolute temperature

this will reduce to:

P = Po * e^ -0.0383*(h/1000) (for imperial units-feet)

So there you go clown boy.

omar

 

[MikeS]

Omar,

If we are going to digress to name calling perhaps you, Omar, should stick to making tents. You’ve spouted some formulas without demonstrating any practical applications, boy are you smart, I want to be like you when I grow up.

But let’s get back to your statement “oms lists the actual internal volume so you should use that.” Can you explain this?

Do you dispute the fact that the volume as listed on the OMS webpage is rounded to the nearest liter? If so explain the disparity between the statedhttp://www.omsdive.com/cyl_spec.html:

Air Capacity Cu Ft @ 2640 psi = 112 ft3
Liquid Capacity = 19

Let’s see your explanation, not a formula copied out of a book; that is if your not to busy making tents.

Mike