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I
needed to determine what the maximum pressure the pressure transducer would
experience in flight, so I could buy the right sensor for
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airspeed
sensing. The pressure in the ram
section of a pitot tube is comprised of two components, the dynamic and the
static. Because
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most
pressure transducers sense the difference between some input and static
(gauge pressure), we only need to look at the dynamic
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pressure
exerted by the moving air.
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Dynamic
fluid pressure is defined as:
P(dynamic) = 0.5 (r) (v2) , where v = velocity of
fluid (air), r = density of fluid
(air)
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r(air)
@ sea level, incompressible (low Mach number) = 1.229 kg/(m3)
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We
will assume a max velocity of 50 m/s (111 mph). So we get: Pmax=.5
(1.229 kg/(m3)) (50 m/s)2 = 1536.25 kg/(m s2)
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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
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the
Mars Observer satellite? It went
splat because NASA forgot to convert pounds(force) to pounds(mass).
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1
pound(mass) = .4535 kg 1
pound(force) = 32.174 pound(mass) ft/sec2 (multiplied by gravity at sea level)
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1 ft
= .3048 meter 1 ft2 =
144 in2
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After
all these numbers are put in the equation, we get:
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P(dynamic,
air) = 32 pound(force) / ft2 @ 111 mph = .22 pound(force) / in2 @ 111 mph = .22 psi @
111 mph
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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
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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
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is
not true, but anyway...) No problem.
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