Hi, I'm a final year Biomed. Eng. student currently working with a team on a project investigating Threshold Responses to Inspiratory loads, and their relation to sleep apnea. The key to this study is a box which provides the loads. It is a rectangular box containing 6 screens or 'stages', each designed to apply a specific load, such as 0.5cm.H2O, 1cm.H2O etc as a flow of 60L/min is drawn from one end of the box. These are 'switched on' and off using balloon valves along the top of the box. These loads will combine in series to form a total load of 6cm.H2O when air is being drawn through all 6 screens. Now the problem is calculating the correct area of the screen (a sheet of laser drilled stainless steel) to expose at each stage to get the correct pressure drops. I calculated them from a sample I had of the material, but found when I tested them finally in reality, they were giving incorrect pressure drops. All I can think of is that by enlarging the area of the screen exposed, the air now has more area to flow through, meaning it is passing slower through the screen, giving me lower pressure drops than it should. If I'm right then all future screens I've calculated to be 'right', will end up being too low once put into practice, so I'm hoping someone can help get me to grips with the correct calculations and factors involved. I hope that's the least bit understandable! Please let me know if you need more information, and I'm extremely grateful in advance for any help anyone can give me!