Two comments:

   When the carbon gets worn enough to cause noticeable drivability
problems, the fraction of its length that is worn thru is still rather
small; it is far from a continuous gap in the resistive surface.  Thus
it may not disrupt the behaviour of a nearby portion of the surface
very much.

   The important property of the device is that it behave as a linear
(I think) potentiometer.  If you verify that it does so after shifting
the ceramic board a little, there should be no problem functionally.

Larry A.

On Sat, Mar 6, 2010 at 10:26 AM, Tom Hargrave <thargrav@hiwaay.net> wrote:
> OK, now we are getting into my long term manufacturing history. We used to
> manufacture thick film hybrids for GTE and the resistors on the hybrids were
> made the exact same way - with printed on ink, then we would trim the
> resistors up to value with a laser.
>
> We used ink manufactured to different values, for example 100 ohm ink was
> 100 ohm when printed in a square regardless of the size of the square. But
> if you were to print a rectangle twice as long as it was wide the value
> would be 200 ohm. Now let me explain.
>
> A square of 100 ohm resistor ink 1cm X 1cm will measure 100 ohms from one
> connection edge to another.
>
> Now put two of these resistors in series, or print a patch 1cm wide by 2cm
> long, and the resistance is now 200 ohms from connection edge to connection
> edge.
>
> Now put two of these in parallel, or print a patch 2cm wide by 1cm long, and
> the resistance is now 50 ohms from connection edge to connection edge.
>
> Now put four of these resistors in a series parallel circuit, or print a
> patch 2cm wide by 2cm long, and the resistance is back to 100 ohms from
> connection edge to connection edge.
>
> Now apply this to the carbon resistor in the AFM. The track worn across the
> resistor disconnects part of the parallel resistance from the circuit and
> the readings have to change!
>
> Thanks,
> Tom Hargrave
> 256-656-1924
>
> Our Web Sites:
> www.kegkits.com
> www.stir-plate.com
> www.andyshotsauce.com
>
>
> -----Original Message-----
> From: David Beierl [mailto:dbeierl@gmail.com] On Behalf Of David Beierl
> Sent: Saturday, March 06, 2010 10:12 AM
> To: Tom Hargrave
> Cc: vanagon@GERRY.VANAGON.COM
> Subject: RE: AIR FLOW METER
>
> At 10:40 AM 3/6/2010, Tom Hargrave wrote:
>>The fact that the carbon resistor is laser trimmed is what I was eluding to
>>when I stated that relocating the wiper isn't a 100% fix. The problem is
>>that the missing carbon track across the carbon resistor changes the value
>>all the way across the sweep even when you relocate the wiper.
>
> Dear Tom,
>
> I'd like to see that tested. �There has to be some reason that they
> made the thing so elaborate, and I strongly suspect that it's to
> reduce the sensitivity of the device to the precise tracking radius
> of the wiper. �Remember that it's acting as a potentiometer and its
> output is a smoothly varying voltage; but if you measure
> wiper-to-ground resistance while moving the vane it varies all over
> the place. �I wish I had one to do some measurements on. �My
> impression is that the large carbon surface contributes comparatively
> little to the resistance.
>
> Yours,
> David
>
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