A single stage regulator is accurate to about +/-2%.....so, when the bottle goes from 3000 psi to 1000 psi, the regulator wil spit out within +/-40 psi of your setting. A two stage regulator regulates the pressure again so the setting is much more accurate.....now the second stage outputs +/-2% of 80 psi = +/-1.6psi. Old trick to improve accuracy of regulator. When you weld, are you good enough to notice the pressure change? If so, get the double, if not use the single.