Weighed some tanks at various PSIs and got unexpected results.

[CaveSloth]

On a pair of doubled LP85s:

92.4 lbs at 3750 psi
86 lbs at 2350 psi
82 lbs at 1450 psi
75.3 lbs empty (15 psi)

On one AL80:

39.2 lbs at 3100 psi
35 lbs at 900 psi

On another AL80:

39.2 lbs at 3100 psi
36 lbs at 1375 psi

I calculate that the LP85s contain around 75.8 cf of air at their service pressure of 2640, and the AL80s contain around 70.5 cf at their service pressure of 3000 psi.

 

[OceanEyes]

You might want to rethink some of your math.

According to the Huron Scuba chart the actual volume of the cylinders you mention when filled to their rated capacity are as follows:
Luxfer 80- 77.4 cubic feet
Luxfer S80- 78.2 cubic feet
Catalina S80- 77.4 cubic feet
OMS 85- 85 cubic feet
As you did not specify which aluminum 80 you weighed, I've listed the three that are currently available in the USA that have a working pressure of 3,000 psi.

 

[CaveSloth]

You might want to rethink some of your math.

According to the Huron Scuba chart the actual volume of the cylinders you mention when filled to their rated capacity are as follows:
Luxfer 80- 77.4 cubic feet
Luxfer S80- 78.2 cubic feet
Catalina S80- 77.4 cubic feet
OMS 85- 85 cubic feet
As you did not specify which aluminum 80 you weighed, I've listed the three that are currently available in the USA that have a working pressure of 3,000 psi.

Those may be the manufacturer published capacities, but I am showing the actual recorded weights.

Can you show me where my math is off? Did you calculate the capacities yourself based on the psi and lbs I recorded and come up with different numbers?

 

[FreeFlyFreak]

What was the temperature of the tank when weighed?

Was it the same temperature when weighed full as when weighed empty or low?

 

[rhwestfall]

Need to show your math - As you said "75.3 pounds empty (15 psi)", is the 92.4 pounds at 3750 the gauge reading or gauge +15?

 

[CaveSloth]

What was the temperature of the tank when weighed?

Was it the same temperature when weighed full as when weighed empty or low?

Temperature was before and after diving, maybe 20 degrees or so different. Could have an effect possibly.

Before diving would have been a higher temperature. The before figure would have been the one at highest pressure which was taken just before a dive. The fact that I get very similar cf per psi at all measurements though tells me that temperature had little effect.

 

[CaveSloth]

How do you arrive at that number? I get somewhere in the 84-85 range.
At 3750, I got 107 cu ft not accounting for ideal vs real gas laws.

I would love to see how you got 84-85.

Here is my math:

92.4 - 75.2 = 17.2 lbs of air at 3750 psi
86 - 75.2 = 10.8 lbs of air at 2350 psi
82 - 75.2 = 6.8 lbs of air at 1450 psi

17.2 / .08 = 215 cf @ 3750
10.8 / .08 = 135 cf @ 2350
6.8 / .08 = 85 cf @ 1450

based on the numbers above we can calculate that the tanks contain:

17.4 psi / cf
17.4 psi / cf
17.0 psi / cf

1 / 17.4 = .05747 cf per psi

2640 x .05747 = 151.7

divide by 2 since they are doubled:

Each tank holds 75.86 cf at 2640 psi.

 

[FreeFlyFreak]

Temperature was before and after diving, maybe 20 degrees or so different. Could have an effect possibly.

Before diving would have been a higher temperature. The before figure would have been the one at highest pressure which was taken just before a dive. The fact that I get very similar cf per psi at all measurements though tells me that temperature had little effect.

Without a doubt it would.

Try this thread I haven't dug into it yet.

Tank volume calculations..what was the temp used?

Pressure in a tank varies quite a bit with temperature.
Same volume of gas but higher temp gives higher pressure inside tank.

 

[CaveSloth]

Without a doubt it would.

Try this thread I haven't dug into it yet.

Tank volume calculations..what was the temp used?

Pressure in a tank varies quite a bit with temperature.
Same volume of gas but higher temp gives higher pressure inside tank.

Weird though that I got a consistent ~17 psi per cf or air in the tanks at all measurements, regardless of the temperature variation.

 

[RayfromTX]

I posted based on my weights on my lp85s. Running the numbers on yours I get 77. I use .08 lbs per cu ft for the weight of air. I ignore real gas laws.