USTER HVI 900 - Page 82

USTER® HVI 900 Instruction Manual
Uster Technologies, Inc. Chapter 5
Version 4.0 (1.99) Calibration - Page 5-8
Adjusting the values is accomplished by two-point regression analysis of
individual test data points. The mathematics of the simple regression
will show relationships defined by “slopes” and “offsets.” If, by chance,
the unadjusted values of the instrument tests agree exactly with the
designated values of the two-calibration cottons, the software “slope”
will be 1.0 and “offset” will be zero. However, this perfect agreement
will rarely ever exist. Therefore, in order for measured values to agree
with designated values of calibration cottons, slopes and offsets are
calculated from tests performed during calibration, and comparisons are
made with the 1.0 slope/zero offset conditions. This results in
corresponding adjustments to obtain agreement between measured
values and designated values of calibration cottons.
If nothing changes over an extended period of time—calibration cotton
samples do not change, operator techniques and procedures do not
change, fiber moisture content (temperature and relative humidity of
laboratory air) does not change, or the instrument does not change—the
slopes and offsets that were obtained from the initial calibration do not
need to be changed. The long-term stability in testing will exist.
The objective of calibration is to have slopes and offsets remain constant
for long periods of time. This is achieved through management of
laboratory conditions, following accepted procedures and techniques,
and performing routine instrument maintenance.
When a "Fail" condition occurs during a calibration, changes are made
to the parameters. However, ONLY those parameters that are critical
for calibration to be brought to a "Pass" condition will change. In other
words, no changes are made to those parameters that pass calibration
originally.
For example, assume that length and uniformity pass the original
calibration, but there is a "Fail" condition for strength (perhaps as a
result of a change in the relative humidity). During the next calibration
procedure, the slope and offset for strength are all that would be
changed. The slopes and offsets for length and uniformity would remain
at their original values.
Calibration for Short and Long Cotton are performed in the same
manner using the Cotton Calibration Check screen. Two different
checks are performed one at a time.