Date:         Thu, 30 May 1996 17:08:31 -0400
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Ryan75@aol.com
Subject:      Re: O2 sensor is a thermistor (longish)

In a message dated 96-05-29 21:10:13 EDT, you write:

>The principle of the oxygen sensor in the exhaust stream is that of a 
>thermistor which is yes, a heater.  A controlled amount of current is 
>supplied to the thermistor and carefully measured on a continual basis. 
>If the amount of current consumed by the thermistor decreases, then it 
>is obvious that the thermistor is being subjected to heat from a source 
>other than the supplied current.  In simple terms, the O2 sensor is 
>measuring the exhaust gas temperature. 

Not really temperature, but really the amount of oxygen in the exhaust.  I
quote from the Bosch Automotive Handbook, 2nd Edition (an invaluable tool):

"One part of the ceramic body is located in the path of the exhaust, and the
other part is in contact with the ambient air.  The surface of the ceramic
body (zirconium dioxide) is provided with electrodes made of a thin
gas-permeable platinum layer.  The ceramic material used in the sensor begins
to conduct oxygen ions at a temperature of appproximately 300 degrees C.  If
the oxygen proportions at the two ends of the sensor differ, a voltage is
generated between the two electrodes due to the special properties of the
material used.  This voltage is a measure of the difference in the
proportions of oxygen at the two ends of the sensor.  The exhaust gas of
internal-combustuion engines still contains residual proprtions of oxygen,
even when combustion takes place with excess fuel.  The residual oxgen
content is largely dependnt upon  the composition of the air-fuel mixture
supplied to the engine for combustion.  This relationship makes it possible
to use the oxygen content in the exhaust gas as a measure of the excess air
factor.  In general, the sensor ceramic is heated by the exhaust gas.  In
order, in the case of cold installation positions, to also quickly reach the
working temperature, there are sensors whose ceramic can be heated by an
internal heating element." (all grammatical errors and bad german to english
translations are solely the responsibility of Bosch).

  So, like the oil pressure/temp senders, the contact between the device
(sender, O2 sensor) is responsible for the connection to ground.  In the case
of the single wire O2 sensor, the wire provides a very minute voltage to the
computer based on the content of unused oxygen present in the exhaust.  This
voltage is generated by the sensor itself, based on an electrochemical
reaction between the materials used in the sensor and the amounts of oxygen
at the two ends of the sensor (one end being exposed to ambient air, the
other to the exhaust gas).

My apologies if anyone was bored to tears by this admittedly long-winded
explanation...

Ryan