[Diy_efi] wide band O2's

[Bruce A Bowling]

The WB02 subject is a very interesting one. I spent about 7 solid months 
researching the subject and performing numerous test. There are so many 
aspects to this whole subject, circuit configuration is just one of them.

Build a gas flowbench and flow some calibration gas over the sensor, this 
is a real eye-opener. I did not test a lot of meters (but several I did 
test, I will not post the results because it was not a real detailed test, 
people should do this themselves), but the one that was the most accurate 
was the ETAS LA-3, maybe this is why it costs $4K. But there is no where 
near $4K of parts in it, just a simple '332 processor but real good analog 
circuitry for the pump, etc. And they use a switcher power supply for the 
heater (high frequency w/ averaging), this yields a nice clean heater 
supply voltage without PWM switching noise being injected in the 
pump/nernst. Heater PWM noise getting into the pump/nerst servo is a real 
problem, one has to be careful to sample on the same point of the PWM and 
not during switching.

Garfield Willis did some testing years ago on these, check the archives for 
his name and see his comments. He held a lot of info to himself, but 
everything he alluded to I was able to verify independently with my 
testing, and someone else I corresponded with also came up with the same 
results. In a nutshell: the sensor (each) needs gas calibration - they tend 
to vary all over the place, enough that the cal resistor is not enough 
(concluded from bench testing), the standard calibration curves assume one 
gas mixture type (this is an issue with nitrous, alcohol, E85, propane, 
etc), the heater control is absolutely critical (especially when the 
battery voltage drops low enough not to provide enough voltage even at 100% 
PWM, this is why both ETAS and the PWC use a switcher supply for the 
heater), there are significant pump current offsets on both sides of stoich 
(and they are different, and depend on the particular sensor). And there 
are things like barometric partial pressure correction, exhaust 
backpressure compensation, etc.

It all depends on how accurate you want the meter, and under what 
conditions. The only way to determine if a particular meter is accurate is 
to measure it with a known gas source. And run the meter with varying 
supply voltage (like from 8 volts to 16 volts, particularly the low end). 
The PWC site gives detailed info on how to make a gas flowbench and all 
other pertinent info (like baro correction and Brettschneider 
gas-composition PC applications, etc) in order to perform a study of WB02 
meters.

- Bruce


At 12:38 AM 7/6/2005, you wrote:
>Adam
>Here is an interesting article on  WBO2 calibration from the Bruce & Al at 
>Megasquirt
>http://www.msefi.com/msinfo/PWC/
>Regards
>WOT