Reference Manual
748384-C
September 2003
1-18 Introduction
Rosemount Analytical Inc. A Division of Emerson Process Management
NGA2000 Reference
1-8 LINEARIZATION
Analyzers often have a non-linear response to
concentration changes in the sample, due to the
physical principles on which they work. In par-
ticular, NDIR analyzers operate more-or-less on
Beer's law, which states a logarithmic relation-
ship between optical absorption and concentra-
tion. It is more or less logarithmic because
practical gases are not ideal, particularly C0
2
at
higher concentrations. The normal form of lin-
earization used by NGA analyzers is the fourth
order polynomial approximation. This is used
for traditional and regulatory reasons. The theo-
retical solution would be to use a logarithmic
approximation, however in computer terms this
is merely a higher order polynomial, no different
from the fourth order in precision in most cir-
cumstances.
Although in V2.2 analyzers there are a series of
menus for automatic self-linearization, these are
not operational in this edition of software. In-
stead it is necessary to enter or download the
polynomial coefficients. These are determined
by an external calculation, flowing known gases
through the analyzer and recording its
un-linearized readings, and calculating the ap-
propriate corrections.
Frequent problems in doing this include errors in
the dilution system used to generate the points,
errors in the time allowed for the analyzer to
stabilize after entering a gas, and errors in the
calculation that determines the coefficients.
The NGA allows you to use up to four sets of
linearization coefficients, and to assign these to
any of the ranges, subject to certain limitations.
Each set of coefficients has a range over which
it is valid, and this has to be at least as great as
90% of the range to which it is supposed to ap-
ply. For each range you can select which set of
coefficients to use, and whether to use any such
set at all. If you select a set of coefficients that
are not allowable, the analyzer will automatically
select the next higher set, or the highest set if
those sets are themselves valid for this range.
a. Basic
Instructions
In the
Main menu
:
Select Expert setup and controls...
Select Analyzer module setup.
Select Gas measurement parameters...
Select Linearization parameters…
Select which set of coefficients to use for
each range.
Select Linearization coefficients...
Press "NEXT" if necessary until you are
dealing with the desired range.
Edit the linearization coefficients as desired.
Edit the linearization fullscale range as
appropriate.
b. Advanced
Instructions
Use a dilution system of known quality,
proven with an known linear analyzer such
as a FID.
Perform a zero and a span on the range
desired. Verify that you have allowed plenty
of time for the analyzer to stabilize.
Verify that the fullscale range setting is cor-
rect, and also the linearizer range settings
are correct.
Flow a number of dilution samples for long
enough to get stable readings, until you
have enough data to generate the curve.
This is normally between ten and sixteen
points. Verify that you have given the ana-
lyzer time enough to stabilize - you are look-
ing for an accuracy of reading of better than
0.1 % in order to generate a satisfactory
curve 1 You may find it helpful to set the re-
sponse time to a long value so that you
have a good reading, or if you are using a
computer for the data acquisition, make it
do a statistical analysis on the signal to ver-
ify that you have no indication of a slope by
the time you take the value.
