286 Vindication

[Bruce Bowling]

~ >Warning; is it a MPX2200DP? It reads differential pressures. You need absolute
~ >pressure. I had to use the MPX200AP thats absolute w/o temp compensation. Since
~ >Motorola did'nt have any compensated one year ago. I use turbo press up to 1,5
~ >bar but when calibrating the MPX200 it streched it's linear area 1 bar over the
~ >specified so it's still useable. The temp comp wont be any big problem i think.
~ >When i calculated on it it follows the compensation for airtemp vs density that
~ >you anyhow must do in your software (or maby in hardware is possible to of
~ >course), so its near to automatic, but maby a bit slow! That is if you  apply
~ >inucted airtemp to the MAP sensor of course (how that is done???). Tell me how
~ >you intend to dampen the MAP signal? Mecanical or electronical, thats one of my
~ >problems. 
~ >
~ >Keep up good work, please tell me if its a new AP on the market!
~ >
~ >Tommy Palm
~ >tommy.palm at oron.ds.sll.se
~ >Sweden 
~ >>
~ >>
~ >
~ If you are not going up and down mountains all day a differential sensor
~ will work fine.
~                                  regards,
~ 

I may not be absolutely correct on this, but I was under the impression that
you need *both* absolute and differential, to get a linear form Y = A + B*X,
so one knows the slope as well as the offset.  With two absolute pressure sensors,
one can obtain the above form, because both sensors are referenced to an
absolute pressure (thus yielding A), and the difference will yield B,
If one uses only one absolute sensor, then one can take a reading when
the engine is off (like when the key is on run but not yet to start) and
save this as atmospheric pressure.  Then when the car is running, the second reading
yields the same info as with two sensors.  That is if the atmospheric
pressure remains constant. (I know Electromotive uses the one sensor method).

With only a differential sensor, you can never reference this against
any absolute values.  This may be O.K., but how then does one
implement, for example, a timing advance lookup table with absolute
vacuum as one of the inputs?

I guess one can come up with an absolute pressure value and always
use this with the differential sensor.  But this may work only if
you do not go up an mountain, and if the barometric pressure remains
the same forever.

BTW, my environmental correction WWW page shows the effects
of altitude and barometer on engine HP/torque, and they can be dramatic.

- Bruce

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               Bruce A. Bowling
  Staff Scientist - Instrumentation and Controls
 The Continuous Electron Beam Accelerator Facility
    12000 Jefferson Ave - Newport News, VA 23602
                 (804) 249-7240
                bowling at cebaf.gov  
        http://devserve.cebaf.gov/~bowling
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