[Diy_efi] WB free air calibration

[bcroe at juno.com]

Rob, 

Glad you got it working.

On Wed, 29 May 2002 09:57:36 -0700 "rob files" 
<rncfiles at hotmail.com> writes:
> Hi all,
> 
> After replacing some blown out parts, I got my diy-WB 
> sensor working last night.  Thanks to all the REAL 
> contributors to the project.
> 
> My free air reading is something like 3.91V.  I was 
> wondering, does this mean all of my readings will be 
> offset by 0.09 volts? 

You should first check the voltage from J12 (gnd) and 
J9.  This ideally will be 2.500 volts.  It should be within 
1% of that value.  Certainly if it measures .090 volt low, 
then all your readings at J8 will contain a ".09V low" 
error component.  

This is within the tolerance of the design.  If it bothers 
you, I suggest just adjusting your translation of volts 
to A/F that amount.  If you use an LED bar graph display, 
this error component will be cancelled by the differential
connection to J8 and J9.

> I was thinking about adding a trim pot to the calibration 
> resistor and adjusting Vout to be the 4.0V free air value.  
> Any reason why this would be a bad idea?

The first thing to understand is that multi digit accuracy of 
absolute readings is neither the design objective or a useful 
feature in actual applications.  Listen to Bruce P.  Next, any 
error at J9 will NOT be correctly compensated by adjustments
to Rcal.  

Third, the sensor was factory calibrated with Rcal to be 
accurate in the operating range.  I suspect free air, while 
a good sanity check, would NOT be considered an important 
operating point.  Adjusting this to a particular point would 
likely increase your error in the useful tuning range.  

The function the circuit depends on is 
       Rcal + 2210
(   ________________   ) 0.3028 Ip + 2.5 V = Vout 
       Rcal + 3580

If you are bent on the most accurate circuit possible, it would 
be necessary to adjust the ratios of R29, R30, R20, & R10, 
and then the absolute values of R21, R22, and R28 to well
beyond the specified 1% accuracy, and hope the temp 
coefficient is good enough to hold the new accuracy.  Bridge 
circuits can be built to do this matching, but there are much 
better ways to spend your time.  

Bruce Roe

> Rob

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