How would you calculate the time it would take to drain down a cyclinder that is 250mm diameter and 10 meters long with an orifice .025mm. Just using atmospheric pressure. Then I would like to know how much of an increase in flow would I get if I introduce some extra pressure. i.e compressed air of 2 bar. Thank you. Could you please work this out for me using the numbers given, as I just cant get my head around it. I would be very grateful. And thanks very much for your help on this so far Alan. I'm just terrible at maths and I cant get the right answer's when I fill in the numbers.