Ok, here is how all the welding shops and speed shops that I have been dealing with for 10 years fill by bottle.
They put the bottle on a scale. Then they pump N2O into the bottle until the total weight = tare weight + 20#.
So I have 20# of liquid and vapor in the bottle. Taking the density chart, at 20C gas is about 18% by mass and liquid is 82% by mass. So I can expect to get 0.82*20 = ~16# of liquid N2O out of the bottle. In the motor tank I can then expect this to separate into vapor and liquid by the same ratio in the bottle.
Correct?
Everything is correct except for the last sentence as will be discussed below.
this fits exactly to my painfull experience of the past years. Take content in kg / 1.2 x 0,8 and you have really usable liter content for our hobby, as a simple rule of thumb.
When the sum of the volume of the motors fired with said same bottle achives the usable volume calculated above, change the bottle, do not even think more, especially for motors at J and more impulse. No way you can get the tank full next time. this means too you shall track what happens with the bottle, what motors you filled and what you plan next meeting if the bottle is not empty. just avoiding surprises.
one thing i would like to know: do we know a safe method to fill from a large tank into a small tank, using common plumbing and hardware? and how? typically I prefer to carry small bottles on the field but only pay one big, where content is half priced. my vendor is not really hot on telling me for commercial reasons of course....
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The pros use a high pressure liquid pump., but you don't need it for filling smaller bottles. Read below.
At NYPOWER a few years back we did tank transfers from K cylinders to chilled small 20# cylinders. We used a stainless steel braided line rated for the pressure to connect the tanks. This method got us about 75-80% of the small bottle filled 15# in a 20# tank. To get anymore than that requires a pump.
You do not need a pump, but you must fill with liquid.
1.) You must feed nitrous from a source tank with an educator tube or from an inverted cylinder to insure you are feeding liquid nitrous to the smaller transfer tank.
2.) If you do not have a liquid pump you should chill the smaller tank a few degrees below the source tank to insure a >10 PSI pressure differential between the source tank and the smaller tank.
3.) When you open the valve between the 2 tanks, the liquid will flow from the high pressure source tank to the lower pressure transfer tank. Since the transfer tank is colder than the source tank, any gas will be condensed into a liquid by higher pressure and lower temperature in the transfer tank. If the transfer tank is completely chilled (including the valving) there will be no gas within the tank, only liquid, and you could put 23 to 24 pounds of liquid nitrous into a tank rated for 20 pounds but you shouldn't. If you weigh the tank after filing and have more than the rate weight, you should vent the excess until the content is reduced to the rated weight to insure having sufficient ullage volume in the smaller tank.
One should never transport a liquefied gas in a tank without ullage (headspace) volume. If the liquid flashes to gas over the critical temperature, the tank could fracture.... Not good.
When you fill a vented tank with nitrous, you can also get 100% liquid into the vented tank. This is because some of the liquid evaporates as the tank is filled, reducing the temperature of the liquid. As you fill the vented tank, the gaseous nitrous is vented and the liquid gets cooler, until you eventually fill the tank completely with liquid. The tank will completely fill and the excess liquid will vaporize as it leave the vent, resulting in the atmospheric moisture created white plume you see from a filled vented hybrid tank.
Bob