O2 sensor output vs temp vs impedance

[Frank F Parker]

On Tue, 8 Jul 1997, Fred Miranda wrote:
> I thought I heard though that the accepted way to measure the temp was via
> the sensor's
> output impedance. I've got info somewhere on a O2 interface chip that looks
> at output
> impedance to determine when the sensor is warmed up enough to go on line.
> 
Fred,

Would be interested in that IC chip #. I know that NAtional makes some
o2 sensor interface chips.

The answer on whether to control sensor temp for accurate a/f measurements
is an emphatic YES- you must do that. I have a enginering graph from
Bosch for their LSM-11 sensor which is specially made for rich field a/f
measurements. It is their Motorsports sensor. It does , however, have
about the same output curve as a std production O2 sensor. This graph
shows internal resistance of sensor changes  as follows:
 	SENSOR INTERNAL RESISTANCE       	TEMP

		10 ohms				900 deg C
		35 ohms				750 deg C
		120 ohms			650 deg C

This results in the following change in a/f for a particular output
voltage, say 0.78 volts.

		VOLT		A/F		TEMP

		0.78		13.23		900 deg C
		0.78		14.11		750 deg C
		0.78		14.41		650 deg C

Thus you can see that there is over a 1 a/f # change if you do not
control the temp. Bosch does in their high $ LA-2 meter and so does
Bob Bailey in his $1000 O2 meter that uses the LSM-11. I have one of these
and have had it checked on a dyno against both a MOTEC a/f meter( $5000)
and the measured air and fuel flows. It was very close to the measured
values. $1000 is not cheap, but way less than any other ACCURATE meter
and mandatory if you are to do any serious tuning with a DFI or major
mods of a production efi.

Hope this helps.

Frank Parker
fparker at umich.edu

PS: Very good book on automotive sensors (including O2) is "Automotive
Electronics Handbook" by Jurgen, ISBN:0-07-033189-8

fp