I was talking to one of the engineers in my office about the relationship between pressure and volume of a gas. I also think that Styrene posted information similar to this last year.
I wanted to calculate the actual flow of air through one of my airbrushes. This can be done using the pressure gauge on my compressor. The expanded volume of a gas is related to the mass and density of the gas, the volume, pressure, and temperature. In this case the temperature stayed constant so I negated that. Also, since both calculations are based on air I negated the gas density and mass. What it boils down to is that the expanded volume is simply the pressure times the compressed volume of the gas. Read the pressure gauge, spray for a measured number of minutes, read the pressure gauge again, and you can calculate the flow through the airbrush. (actually the pressure needs to be corrected for atmospheric pressure (14.7 psi standard) but since both calculations are done using the same gauge this can also be negated as well).
The formula I used is simply Pg x V1 = V2 where Pg = PSI Gauge, V1 = the volume of the compressor tank (in cubic feet) and V2 = the expanded volume of the air in cubic feet. Some of you other engineers and designers correct me if I’m wrong.
So ……
My 15 gallon compressor contains 2.01 cubic feet of volume (7.4805 gallons per cubic foot). Last night I started at 121 psi, used a rubber band to hold the trigger on my Omni 4000 down for exactly 30 minutes with the regulator set at 12 psi, then read the pressure gauge again. In 30 minutes the pressure had dropped from 121 psig to 71 psig. At 121 psig the initial volume was 272.10 standard cubic feet and at 71 psig the volume was 171.84 standard cubic feet, so in 30 minutes I used 100.26 standard cubic feet of air volume. This equates to a flow of 3.34 cfm (cubic feet per minute) at 12 psi (higher pressure at the airbrush would cause a higher flow rate, lower pressure would cause a lower flow rate). 3.34 cfm is MUCH higher than I expected to see, my initial guess was around 0.5 cfm. The Omni has a big nozzle so I may try it again on my Badger 100 over the weekend.
So …. What’s the point?
I see questions posted here from time to time where someone is asking how long they will be able to spray from a 5-gallon air tank that they pressurize at the local gas station. Assuming that:
1) The gas station’s air system runs at 90 psig (which is the maximum rated value of some air chucks)
2) The person then runs the tank down to 20 psi before he refills it (20 psi is very low!)
3) The person sprays at the same volume as I did (which in turn is affected by the particular airbrush and the pressure that is used for spraying)
The answer is: About 14 minutes of actual spraying time before they head for the gas station again.
This would be an interesting experiment to do using different airbrushes. I’d be interested to know how mine compare to some of the Azteks and other brands around. I really think that 3.34 cfm is on the high side.