Date:         Thu, 30 May 1996 11:15:15 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         wabbott@mtest.teradyne.com (William Abbott)
Subject:      O2 sensor is...

	Not a termistor.
	A thermistor is a device like a resistor which varies
as a direct function of temperature. 

	An O2 sensor is an entirely different beast- it produces
a voltage indicating the presence/absense of O2 (Can't remember which
way at the moment, its in the Bosch FI book). The voltage produces by
the O2 sensor is an electrochemical phenomina. At the high temp of 
a warmed-up car, the free oxygen (if any) reacts with the metal(s) of
the sensor to produce a voltage, just like a electrical cell (which
is corrupted in usage to 'battery'. What's in your car is a battery of 
cells, but a 'D' cell is not a battery, its just one cell.)

	So your basic O2 sensor has one wire, the voltage out, which is
relative to vehicle ground (the exhaust pipe). Since this cell consists
of a small electrode and exhaust gas, it doesn't make much current and
therefore must be amplified and thats why 02 sensors are only found
with 'electronics'- you can'd drive a relay with the output of the
sensor. Were it a thermistor, you could, and the world would be a
simpler place.
	The fancy sensors with three wires have a heater which brings 
them up to temp faster so the FI system can control emissions quicker.

	The controlled-current-through-a-termistor-sensing <something>
you describe sounds a lot like the hot-wire mass-flow sensor Bosch (&VW?)
use in 80s and later FI- replacing that moving flap from D-Jet and
the can-lid in the venturi from K-Jet CSI.

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