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7 Explanations Volume 5
7.4 Calibration Modes
The Uni-Probe LB 491 offers five calibration modes:
1. One-point calibration
This type of calibration can be used in many applications. You
have to enter or measure the absorption coefficient, the length
of the absorption path and one calibration point. You should
chose this calibration mode if
– a certain product density has to be kept constant for the
process during operation
– only one calibration point (value pair with count rate and
density or concentration value) is available
– only provisional calibration is to be performed.
2. Linear curve fit
The product density is measured or entered for two or more
calibration points (two-point or multi-point calibration). You
should chose this calibration mode if
– accuracy over the entire measuring range is required
– only two calibration points are available.
You should prefer this calibration mode also over a square or
cubic curve fit, if
– several calibration points are available which are very
closely adjacent, so that not the entire measuring range is
covered by samples. The calibration curve may be signifi-
cantly falsified by minor errors occurring during sampling or
in the lab. In this case it may even be better to do only a
one-point calibration.
– sampling can only be done very inaccurately, so that the
samples are subject to errors.
3. Curve fit with linear and square coefficients
This type of calibration is used only rarely and requires visual
assessment of the calibration points on a y(x) graph!
With this calibration mode the product density is measured or
entered for at least three calibration points. You should chose
this calibration mode only if
– the entire measuring range has been covered evenly by
several calibration points
– the measurement is run in % concentration, °Bx or another
unit which is not in linear proportion to the density.
4. Curve fit with linear, square and cubic coefficients
This type of calibration is used only very rarely and requires not
only careful visual assessment of the calibration points on a
y(x) graph, but also a subsequent exact check of the measured
value reading using a test generator!
With this calibration mode the product density is measured or
entered for at least four calibration points. You should select
this calibration mode if it becomes apparent during operation
that measured value deviations occur with the third calibration
mode in partial areas.
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