USTER HVI 1000 - Cotton Calibration

USTER® HVI 1000 Instruction Manual
Uster Technologies, Inc. Chapter 5
(3/08) Calibration - Page 5-11
3.1.4 Cotton Calibration
In order to better understand the theory behind the calibration
procedures it is important to understand the instrument/software
principles of calibration.
Calibration cottons are used for the USTER® HVI 1000 system.
Calibration is performed following engineering principles using
hardware devices. Adjustments are then made to "within instrument"
unadjusted raw values through software manipulations that cause the
test values to agree with designated values of laboratory cotton
samples. In this sense, the word "calibration" refers to the adjustment
of values rather than to true calibration.
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, but this perfect agreement
will rarely ever exist.
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 condition. This results in corresponding adjustments to
obtain agreement between measured values and designated values of
calibration cottons.
If, over a period of time, nothing changes--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 there should be no
change in the slopes and offsets obtained from the initial calibration and
long term stability in testing will exist.
The objective is to have slopes and offsets remain constant for long
periods of time. This is achieved through good management of
laboratory conditions, following good procedures and techniques, and
by performing routine instrument maintenance.