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external influences are measured and corrected (calculated) out of the basis reading. Correctors, positive or
negative, are all calculated in basis weight units (gsm) added to the uncorrected basis weight reading. Being in
basis weight units allows easy comparison of the relative magnitudes. A brief description of these correctors
follows.
Dirt
Dirt Correction corrects for debris build up on the heads and for changes in air density due to air temperature, or
pressure changes. Dirt correction is based on the flag reading from the most recent standardize. If dirt build up
is significant between window cleanings, increasing the standardize frequency decreases inaccuracies due to dirt
build up.
Z
Z correction compensates for basis weight changes due to changes in the height (and thus basis weight) of the
air column. Head gap separation will also affect beam geometry. The Z correction will compensate for both. Z
correction is based on the on sheet (now) Z readings. (Z corrector requires the presence of a Z sensor.)
KCM
Corrects for any difference in absorption properties between the calibration standard and the customer product.
KCM is grade dependent. Typical KCM values are very close to 1.00.
Profile Correction
Corrects for any sensitivity of the sensor due to head misalignment in the machine direction or cross direction.
The Profile Correction must be built (measured) on site. It is best to build the profile correction under
conditions that are identical to normal machine conditions, particularly the temperature (that is, it is best to
build the Profile Correction immediately following a break or other shutdown while the scanner is still warmed
to operating temperature).
BWDO (Basis Weight Dynamic Offset)
Corrects for any change in the product in between the position that the sensor measures the product and the
position where the sample is taken for dynamic correlation. An example of this would be if the sheet were under
tension during the manufacturing process but was allowed to relax after taking a sheet as a dynamic sample. If
the sheet stretched online a dynamic offset would be added to account for this fact.
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