Posted By Greg McFadden on 01 Nov 2011 12:14 PM
Since it is all single phase gas, as soon as you pull gas out, the pressure drops. the remaining pressure in the scuba tank will be proportional to the ratio of the volume of the tank itself to the total volume of the scuba tank + tank you are filling....
Correct.
There's a point that needs to be made in using SCUBA tanks as an HPA fill source lakeside. Once the SCUBA tank pressure drops below 3000psi, each subsequent fill will be less than the one before. The pressure drop is dependent on the size of HPA tank being filled and number of fills going on. You could stretch the available full pressure fills somewhat by using a 4400psi SCUBA tank. One forum posting I reviewed stated that ten fills of a 68ci paintball tank will drop an 80cf SCUBA tank down to 1000psi.
So basically, an HPA fill using a SCUBA tank will give ever lower pressure fills over the day. A CO2 fill station will give consistent fills during the day.