We are currently measuring air flow by pressure difference. (ie) by creating a ventury in the path of the air to create a pressure difference. The problem that we are facing with this approcah is that if the flow is very less, then we need to create a very small ventury,this creates excessive pressure drops for large flows. So to restate the problem in other words, the ventury created for a small flow is a problem in the case of large flow. So how do we attack this problem. Any suggestions?

Is there no other way you can measure air-flow? e.g. some kind of low-mass turbine or fan?
I can't think of anything else at the moment, but I will have a think, please feel free to post your own thoughts :)