286 Vindication

[Mark Boxsell]

At 06:52 PM 19/02/96 EST, you wrote:

>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).
>

That's what people do however you have to stop the engine and restart it
at the top of the mountain before it corrects!
Two sensors is the best method. Two absolute or one absolute and one
differential is does not matter. (If you want to change altitude).

>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?
>

You are referencing it against barometric and if it does not change what's
the difference!
You can calculate a correction by picking any point on the line, the issue
is that it remains by definition a constant.

>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.

This is my point. Pressure change due to weather has no effect (because they
are so small) as far as the engine is concerned so if you don't change your
altitude no problem.

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

Yes, they sure can.

My final point, if the pressure does not change then you don't have to
correct for it.
ie. there is nothing to correct.

>
>- Bruce
>
>--
>-----------------------------------------------------
><<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>-----------------------------------------------------
>               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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>

                regards,
                                  Mark Boxsell
                                  MRB Design.