This calculator will help you work out the pitot pressures for different
speeds.
http://chrusion.com/BJ7/ASICalc.html
Aircraft Spruce sell a range of suitable pressure switches, calibrated to
your requirements, that can be firred to a pitot circuit..

I've flown several aircraft with Hobbs displaying time in the air - mainly
for maintenance purposes. Some used the aircraft's main pitot (ASI) -
others had a separate pitot for the Hobbs.


At 20:45 29 August 2011, S. Murry wrote:
On Mon, 29 Aug 2011 15:18:41 -0500, Bart wrote:



It does, indeed. But to be Hobbs equivalent it would have to be
"total" or "cumulative" time of flight.

Bart

True. Getting back to Paul's original question, how about using a pitot

tube to a pressure switch. There are pressure switches made for HVAC
ducts that are rated for pretty low pressure (quick search on Grainger
revealed some that actuate with as little as 0.02 in WC pressure). Quick

pitot calculation reveals that 20 MPH = 50 Pascals pitot pressure, or
about 0.2 " WC, so such a switch should work.
--
Stefan Murry