[Dan Semko]

Thomas,
Your formula is indeed correct provided that the fitting is not reduced at the impact wrench. When the fitting diameter is smaller than the production source, the pressure is increased. The demand remains constant but the initial volume changes inversely to the pressure and the length which from 3/8 to 1/2 then back to 3/8ths would be marginal. Using your example, if we take a swizzle straw and a regular straw of equal lengths with the same exit opening (port) and measure the restrictive pressure, it will remain constant in both straws exit end.