Compressed Air
You can use compressed air... However it IS more expensive to start up. U can't just go filling 20oz CO2 bottles with air. You need a dedicates air system on each marker. These are normally rated at 3000 or 4500psi and most have adjustable output pressures. The reason they're more expensive is that that system has to step down the pressure in the bottle to a pressure that the marker can handle. Also the bottles have to be stronger that CO2 bottles to bear these high pressures.
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