












I
needed to determine what the maximum pressure the pressure transducer would
experience in flight, so I could buy the right sensor for


airspeed
sensing. The pressure in the ram
section of a pitot tube is comprised of two components, the dynamic and the
static. Because


most
pressure transducers sense the difference between some input and static
(gauge pressure), we only need to look at the dynamic


pressure
exerted by the moving air.



Dynamic
fluid pressure is defined as:
P(dynamic) = 0.5 (r) (v^{2}) , where v = velocity of
fluid (air), r = density of fluid
(air)



r(air)
@ sea level, incompressible (low Mach number) = 1.229 kg/(m^{3})



We
will assume a max velocity of 50 m/s (111 mph). So we get: Pmax=.5
(1.229 kg/(m^{3})) (50 m/s)^{2} = 1536.25 kg/(m s^{2})



We
need to convert this to PSI. To do
that, we need to convert kg to pounds(force), which is different from
pounds(mass). Remember


the
Mars Observer satellite? It went
splat because NASA forgot to convert pounds(force) to pounds(mass).



1
pound(mass) = .4535 kg 1
pound(force) = 32.174 pound(mass) ft/sec^{2} (multiplied by gravity at sea level)


1 ft
= .3048 meter 1 ft^{2} =
144 in^{2}



After
all these numbers are put in the equation, we get:



P(dynamic,
air) = 32 pound(force) / ft^{2} @ 111 mph = .22 pound(force) / in^{2} @ 111 mph = .22 psi @
111 mph



So,
to measure airspeed up to 111 mph, we need a pressure transducer that can
read at least .22 psi. I have three
Motorola MPX2010G


pressure
transducers that are rated at 1.4 psi.
They should work up to 318 m/s or 705 mph (in an incompressible
flow, which at 705 mph

is
not true, but anyway...) No problem.

