We use beta backscattering technology to measure the thickness of gold
on plated fingers.  The instrument is calibrated at 0, 15, 50 and
"infinity" microinches.  In the past we have been plating at 50
microinches.  We recently starting plating at 30 microinches.  The
instrument is calibrated at the beginning of each shift.  Gold thickness
on product
is checked during plating after every 10 panels.  Calibration is checked
every two hours using the 15 and 50 microinch standard.

Recently we plated several hundred panels and the gold was subsequently
found to be consistently low (18-22 microinches or one half the measured
thickness) on every panel.  When the calibration was checked,  the 15
and 50 microinch stantards still read correctly.  The instrument was
recalibrated using a different calibration channel.
Then the fingers on the plated panels read 18-22 microinches.  When we
go
back to the original calibration channel, the problem is still there;
the plated fingers read 36 to 42 microinches.

Since the problem is consistantly repeatable, we have ruled out a
defective probe.  We have tried measurements holding the probe upside
down and beating it with a wood pencil.  The probe is as steady as a
rock.

Has anyone experience a similar problem?  If so, have you been able to
find a root cause for the problem.  Our gold salt supplier said he has
heard of this problem several times before.

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