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11
Ask Bob / Re: Tank sizes
« Last post by bnowlin on February 18, 2018, 08:22:56 PM »
Thanks Bob,
That means to me that some of the sellers are over advertising on some of their big tanks and smaller also just to sell them especially if they are the same size like 88 and 98.  The fire and diving, etc  people do it by minutes. for safety reasons and leave 15 minutes or so in them.
Appreciate ya. 
BoBn
12
Ask Bob / Re: Tank sizes
« Last post by rsterne on February 18, 2018, 07:29:21 PM »
To convert from the inside "water volume", usually in cc or CI.... to the amount of air it will hold in CF.... you need to know the fill pressure (in bar).... If the tank is listed in cc, convert to CI by dividing by 16.4 (cc/CI).... If you know the pressure in psi, convert to bar by dividing by 14.5 (psi/bar)....

First multiply the water volume by the fill pressure, to give the number of CI or air inside the tank at that pressure.... Then divide by 1,728 to convert CI to CF....

For example if the inside volume is 144 cc, that is (144 / 16.4) = 8.78 CI.... If the fill pressure is 3,000 psi, that is (3,000 /14.5) = 207 bar.... When filled to 207 bar, that tank will hold (8.78 x 207) = 1,817 CI of air at 1 bar.... Converted to CF, that is (1,817 / 1,728) = 1.05 CF....

The above is according to Boyle's Law for ideal gasses, and works up to about 3,000 psi.... It gets a bit more complicated when the tank is filled to more than 3,000 psi because of the VanDerWaals effect.... If we use the above calculation for a standard 1 hour SCBA tank (which is 550 CI water volume) we get the following at 4500 psi (310 bar)....

(550 x 310) = 170,500 CI.... (170,500 / 1728) = 98.7 CF (according to Boyle's Law).... However, that tank only holds 88 CF of air at 4,500 psi.... about 11% less than you calculated.... Up to 3,000 psi, you can use the basic Boyle's calculation as I detailed it.... At 4,500 psi, you need to reduce that by 11% because of the VanDerWaals effect.... At 6,000 psi the effect is much greater, about 18% compared to at 3000 psi.... At 10,000 psi a tank only holds 64% as much air as the simplified calculations tell you.... Sorry, but there is no easy formula to use once the pressure goes over 3,000 psi....

Bob
13
Ask Bob / Tank sizes
« Last post by bnowlin on February 18, 2018, 06:45:43 PM »
Bob,
I am sure you have answered this many many times, but how do you calculate what size tank, in cu ft , you have if you only have the ccs ie 105 cc 144 cc etc.  in 4500 psi SCBA tanks?
Thanks the other Bob
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PCP, C02, and Helium Powered Airguns / Re: Any Interest in 6mm ? - Build Thread
« Last post by rsterne on February 18, 2018, 06:14:42 PM »
I held it in a 5C collet mounted in a Hex block in my milling attachment in the lathe, oriented so that the beveled edge of the weight was a 90 deg. to the axis of the lathe.... and used a 1/16" end mill (actually a tiny router bit) in the chuck.... I advanced the work towards the chuck in 0.005" increments, making two passes for a depth of 0.010" for the marks.... The crossfeed of the carriage governed the length of the marks.... The six faces of the hex block gave me half the marks, then I rotated the weight 90 deg. in the hex block and did the other six.... Sounds easy, but it took me all afternoon to do the 24 index marks....

Bob
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PCP, C02, and Helium Powered Airguns / Re: Any Interest in 6mm ? - Build Thread
« Last post by Christopher on February 18, 2018, 05:25:57 PM »
That looks great. How did you tool the "clock" mark indicators on the adjuster?

Chris
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PCP, C02, and Helium Powered Airguns / Re: Any Interest in 6mm ? - Build Thread
« Last post by rsterne on February 18, 2018, 04:02:56 PM »
Here are the completed harmonic barrel tuners.... The muzzle of the barrel is about 3/8" inside the front of the mount, about where the outside threads start....



The scale is like a clock face, with the quarters having longer index marks.... with 12 o'clock being where the setscrew is.... Each mark equals 0.003" of movement on the tuning weight.... By starting with the weight against the shoulder at the front, you can record the position using "hours" for the turns and "minutes" for the index marks, so 9-1/3 turns I would record as 9:20.... and 15-3/4 turns as 15:45 etc.etc.... I'm really pleased with the way they turned out....

Bob

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Airgun Classifieds / Looking for a FX Impact
« Last post by HighAltAir on February 18, 2018, 01:57:38 PM »
Hi Fellow Shooters
Looking for a used, reasonable priced FX Impact, for project. Not looking for a bunch of add ons. Don’t need scope, rings, air tanks, cobra mounts just the gun 25 cal, mag and maybe LDC if it is worth while.
Thanks
CH
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Airgun Hunting / Re: Rattus Rattus FallOffUs
« Last post by steveoh on February 18, 2018, 09:01:31 AM »
Some still images...
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Contests, Give aways and more. / Re: Benjamin Fortitude Contest / Giveaway!
« Last post by squirrel hunter on February 18, 2018, 06:17:27 AM »
  236.81
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Contests, Give aways and more. / Re: Benjamin Fortitude Contest / Giveaway!
« Last post by toro-eye on February 18, 2018, 05:36:59 AM »
$13.13

Thanks
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