Cheap O2 Sensors and reference air

[Walter Sherwin]

Your O2 sensor is a lot like a small DC battery that produces a voltage in
proportion to an oxygen molecule differential.

Take a look at your O2 sensor (or cut apart an old one).  The main body is a
closed ended tube of ZrO2 ceramic with a coating of platinum on the outside
only.  The platinum side is the side which "Sees" the oxygen concentration
of the exhaust gas stream.  The non coated inner side is the side which
"Sees" the oxygen concentration of the reference air.  The reference air
just happens to be the air that you and I breath.  So, how does the
reference air get to the inside without allowing water, dirt, contaminants,
and crap in along with it?.......

Simple.  The inner cavity is sealed from all access, with the exception of
small amounts of air that migrate through small gaps between the conductor
wire and its sheath.  Usually the other open end of the wire is located in
the passenger compartment, where there is a nice supply of clean comfy air.
This is why it is important to ensure that the terminal is attached only to
the wire via solder or crimping, and does not encase or crush the wire
sheath.

If you want more information on ZrO2 sensors, or even the more exotic
(accurate) UEGO sensors, then there are tons of books in your local library
that show all of the electrical and chemical equations.  Look for Internal
Combustion Engine Fundamentals by Heywood as a starter.

Last thing...........in order for the computer to measure the small voltage
produced by the ZrO2 sensor, it must be tied to the same ground plane as the
sensor.  If your sensor is a 4 wire (or more) unit,  then one of those wires
is probably a ground which ties back to the computer.  If your sensor is a 3
wire (or less) unit, then your sensor is probably grounding to the engine
block, via the exhaust components, and ultimately to the computer.  This is
normally not a problem, until you 1)experience a lot of corrosion 2)add a
custom exhaust system 3)introduce gaskets or sealers at the joints 4)you get
the picture................In such circumstances it is usually a good idea
to measure the resistance from the case of the sensor to the computer, and
as a safeguard I usually also affix a wire from the casing of the sensor to
the engine block.


Bye:
Walt.





-----Original Message-----
From: Chris Conlon <synchris at ricochet.net>
To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
Date: Friday, October 09, 1998 11:22 PM
Subject: RE: Cheap O2 Sensors and reference air


>
>> If the wires are properly crimped you will not loose the reference
atmospheric
>> air inside the wires.
>
>Ok, I'll bite. Could someone please explain to me how the reference air
>for an O2 sensor is inside the wire or comes from the wire? I had kind
>of figured there was an active layer (or layers), with thin conductive
>film electrodes on each side, which still allowed O2 to pass through,
>with the wires attached to the thin-film electrodes.
>
>Honestly I'm not trying to be difficult, or disagree with what anyone has
>said about how these sensors work, I'm just trying to *understand* it.
>
>   TIA,
>   Chris C.