Re: O2 sensors

[Brendan Rudack <rudack@umcc.ais.org>]

On Wed, 4 Nov 1998, John Larson wrote:

> Good morning. Orin, your reply makes more sense than Brendon's, but the first

Sure....what or who got confused there????   ;-)


> With regard to the 5 wire unit, of which I was not aware,

These things are pretty cool. They can measure lambda in the rich and
lean
regions(they aren't limited to the very small window around stoich.).  
These sensors are amperometric and measure the actual flow of oxygen ions
(current) rather than getting an output voltage from oxygen partial
pressures on the exhaust and reference side of the element...
By measuring current we get a more linear response form the sensor which
allows measurement of the rich and lean areas.  The typical sensor has a
logarithmic response (remember the Nernst equation from chemistry? If not
I'm sure Bob Myers would be happy to give us a chemistry lecture!) which
makes the sensor output rise and fall sharply from the stoichiometric
point.  
 
> you might be interested to
> know that Toyota and Jeep use a sensor in some applications that operates on a
> resistance principle rather than a voltage signal.

I must admit I haven't seen any of these. I still feel obligated to talk
about them though.... :-)  The sensing elememt in these
guys is made up of materials (such as Titanium dioxide)that have a change
in resistance when
exposed to different oxygen ion  concentrations. These things perform
comparable to the galvanic sensor we are all familiar with.  A good thing
is that these sensors do NOT require an air reference (A few current
sensors are being made breathable and other companies are working on
making their sensors breathable yet waterproof as we speak).  The quirk
is that
resistive devices are temperature sensitive, therefore these sensors must
be heated in order to stabilize the temp.  Furthermore, the galvanic
sensors creates a voltage but the resistive sensor needs a voltage to
operate...

A few points about visual differences of sensors as far as the lower
shield goes.  This is the shield that sits in the exhaust stream.  
If you decided to put a heated sensor into a car which does'nt operate the
heater circuit, the sensor may not be getting up to temperature.  Heated
sensors
often have smaller louvers because they do not require as much exhaust
flow across the element ot keep it at temp.  Therefore, if you have a
1 or 2 wire sensor in your car, and are looking for a cheap replacement,
find another 1 or 2 wire sensor, not a heated 3 or 4 wire,  make sense?
Smaller louvers also help to
shield the element from exhaust gas contamination.  For this reason,
heated sensor may be less prone to poisoning...


So, we can see that there are 2 basic types of oxygen sensors.  A
switching type (galvanic and resistive) and wide range.  However, we are
all stuck with the switching type of sensor... Our ECU's won't recognice
the wide range signal...  Even if you could use a wide range I don't think
you would want to, the cost big $$$$$.  So, sense we are all stuck in the
same game... choose your weapon! I hope everyhting is clearer now...
 

Brendan Rudack 
'98 A4tq 
'88 90q 2B tq pro-rally ride (will run a Delphi (AC) O2 sensor...)
Fenton, Michigan
USA