Wasted air from bleeding hose assembly

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lamarkeiko

Member
Jun 22, 2017
129
2
Oregon, United States
I have had my first SCBA tank and hose assembly for only a few months now. I have been recharging my rifle after shooting about 10 shots each time. There seems like there is a lot of wasted air in the micro-bore hose each time the bleeder is opened. The hose assembly I have is 40" long, which seems excessive if it is wasting a lot of air. The Air Venturi micro-bore hose is only 24" long, which seems plenty long for me. Would there be much of an air loss saving for me if I changed out to the shorter hose? Are there other options to keep from wasting air if it is excessive, such as a valve on the end of the hose assembly? Thanks for any replies back.

Lamar

 
Microbore hoses don't have much volume, I expect there is a different culprit.

Back of the envelope.. Microbore on the AV website says DN2, which I believe is 2mm. Cross sectional area is pi r squared, so 3.14mm2, which is .00000314m2. 40" is about 1m, so that is .00000314 m3. There are 1000 liters in a cubic meter, so this is .00314 liters inside the hose. 

If you have a 6.8L or 9L SCBA tank, the microbore hose length is not a major factor..

I welcome folks to correct any assumptions I made.

Bill 
 
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Thanks Bill for those calculations. That is the type of information I'm looking for so I can try to figure out my air waste while bleeding. If your figures are correct, I still need some more information so I can calculate for myself what the waste is.
Using the Airgun Calculators by Lloyd Sikes, I have a 45 minute/66 cu ft SCBA tank that I get filled to 4500 psi. Charging my 180 cc gun to 200 bar/2900 psi, shooting to 160 bar/2320 psi, I can get 91 fills from my full SCBA tank. Using your figure of .00314 liters inside my hose, .00314 X 91 calculates to .286 liters of 2900 psi wasted air. Now I need to know how many liters are in my 66 cu ft tank. You mentioned a 6.8L and 9L tank, but I don't know what that computes to in cu ft.
I also calculated that .286 liters = 286 cc, which is about 1.5 full cylinders in mt gun. That does not seem like very much wasted air if I'm looking at it correctly.
Could I get some follow up on these figures?

Lamar
 
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Our sport does a great job of confusing people....

"Water Volume" is what I like to use. It is the actual volume of the cylinders. Liters can convert to cubic ft, but then you add a multiplier for pressure, and then a fudge factor for 'residual air'. Very confusing.

We use SCBA cylinders used by first responders. They rate them by "minutes". A 45 minute cyliinder is 6.8L. A 60 minute cyliinder is 9L.

Looking at Airtanksforsale, for example, they offer a "97 Cu Ft (550 Cu In) " and a "71 Cu Ft (410 Cu In)". Using a conversion calculator, 550 cu in equals 9L and 410 cu in is 6.7L. But 97 cu ft is "expanded".. For example, 97 cu ft is 2747L (a lot more than 9L). So.. divide 2747 by 9, you get 305. Using metric to make it easier, 305 bar is just under 4500 psi.

So while everyone offers the same SCBA 45 and 60 minute cylinders, folks end up rating them differently. I suspect they are all right, they just use different assumptions.

Clarity? Not sure why we make it so confusing...
 
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I missed your comment on "45 minute/66 cu ft SCBA tank". Given the 45 minute tank is 6.8L, why 66 cu ft where someone else claims the same tank is 71 or 74 cu ft? It could be that the 66 cu ft assumes a minimum usable pressure. In other words, if it assumed the 4500 psi tank was only able to be used down to 500 psi, then 1/9 of its capacity is "stranded", making it only a 66 cu ft usable air (if you're breathing the air, the difference would be important). 
 
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