<VV> Re: MAP sensor - utlimately for a Corvair

[Jim Simpson]

Hi Chuck -- the electronic pressure sensors come in several varieties 
but in direct answer to your question, they can be either absolute 
pressure or relative pressure.

The absolute pressure ones are sealed during the manufacturing 
process and do NOT have an external reference port, just the single 
pressure sensing port.  You can use them a barometers if you'd 
like.  The relative pressure ones do have a second external port.  If 
you want to sense relative to atmospheric, just leave it open or if 
you want measure differential pressures (say between the right and 
left cylinder bank), hook a line to each port.  (Some relative 
pressure sensors don't have a second connection port, just a vent 
hole.  Hooking them up for differential pressure is a bit trickery 
but can be done.)

For details, google the Motorola MPX series pressure sensors.  They 
come in a variety of pressure ranges, sizes, and case styles.

Jim Simpson
Group Corvair (Instrumented '66 turbo w/ a Motorola absolute pressure sensor.)