I saved this from a post here some years ago:

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Document AN 05-10-24 specifies the airspeed that should be indicated
by a given water column. There is an approximation formula that says that

Airspeed (Statute MPH) = sqrt (Water Pressure In Inches * 1980.0)

If you do the formula right, 100 MPH should come out about 5 inches of
water column.

end save

I use 30 feet of water for one atmosphere (approximately 15 psi). None of this
is exact but then again nothing is.

CW

"Ron" wrote:

Good idea! As I don't know how to calculated this, to get me in the
ballpark do you know/remember the pressure at 40 mph?

Ron



"Marc J. Zeitlin" wrote in message
...
NEW-USER wrote:

Hello all, can somebody suggest a good place to get an air pressure
switch to hook into the Pieto line of an aircraft which will then in turn
operate the hourmeter when the aircraft is flying.

I used a switch in the pitot line for exactly this purpose. Go to
"www.digikey.com" and search for "pressure switch". Determine what speed
you want the switch to come on at (I chose 40 mph) and then determine what
pressure will be created at that speed and choose the appropriate switch.
I don't remember which one I used.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2007