Flow sensor

Dear raghuvarrier:

Rotameter, also known as a "variable area flow meter". Flow will need to be vertical upwards. The float is smaller than your requirement, and can be used to block a light beam or operate a hall effect switch.

David A. Smith

Here's another thought, off the top. Late last century, "fluid logic" enjoyed a brief fame. Briefly air blowing through carefully shaped passages could switch from one output leg to another, depending a brief pressure pulse from a control port. If you have a little blow availlable, this would be a possibility. They were suitably small devices. Another bulkier method along the same lines.... a venturi tube gets variable suction in a side pipe depending on main flow. That suction could be led to e.g. a "Bellofram" valve (a pneumatic diaphragm valve) to mechanically actuate a shutter.

Brian Whatcott Altus OK