MAP system

[David M Parrish]

> From:          "Jim Staff"  <staffj at freenet.msp.mn.us>

>   The question all you great theorists are here to answer. In Map based system I
> measure pressure, however I need to put the unit on the outside of a pipe not to
> disturb air flow. However this placement gives me only the pressure exerted on 
> the walls of the pipe and not the pressure in the tube.

That's exactly what you want. (I think.) When the sensor is ported into
the wall, you're measuring static pressure, which should be nearly 
the same all through the volume. (Except for possible differences due 
to the viscosity of air near the surface. I'm not sure that effect is 
great enough to worry about.)

If the port is open into the air stream, you're measuring what's 
called dynamic pressure, which measures both the static pressure 
and the pressure exerted by the movement of the air. The air speed 
indicator in an airplane is a sensitive pressure guage with one side 
ported to static and the other to dynamic, so it measures only 
pressure created by the velocity of the air. I'd think the added factor
of air velocity would be more a problem than a solution in a MAP system.

---
David Parrish
Going with the flow...