Reference Manual
748384-C
September 2003
Rosemount Analytical Inc. A Division of Emerson Process Management
Introduction 1-21
NGA2000 Reference
1-9 LINEARIZATION
TOUCH-UP
NGA analyzers have two forms of linearization
available. One method is to "tweak" them. This
involves selecting up to three mid range values,
and forcing the reading to correspond to the gas
values at these points. The analyzer will then
interpolate intermediate readings appropriately.
This feature is useful when attempting to corre-
late the analyzer's performance against some
standard gases of dubious accuracy, but where
the gases must be believed. It can also be used
to adjust the normal linearity adjustment for the
particular idiosyncrasies of a gas divider, which
itself may have linearity problems. In this case,
perform the normal fourth order polynomial lin-
earization, and then "tweak" the curve in using
this tweak function. A preferable solution, inci-
dentally, is to correct the gas divider or calibra-
tion gases.
a. Basic
Instructions
Select the operating range desired.
Zero and span the analyzer in the normal
way.
Select
Expert controls and setup
Select
Analyzer module setup.
Select
Gas measurement parameters...
Select
Linearization functions...
Select
Midpoint correction setup...
You arrive at the setup for range 1, cycle
through the other ranges by pressing
"RANGE 2", "RANGE 3" and “RANGE 4" as
desired.
Set
Correction:
to enabled.
Setpoint being measured:
to point 1.
Flow the appropriate value of gas (typically
30% of scale).
Edit
Point 1 gas concentration
: to the
named value on the bottle.
When the reading has stabilized, press
“SET”.
Setpoint being measured
: to point 2.
Flow the appropriate value of gas (typically
50% of scale).
When the reading has stabilized, press
"SET” .
Setpoint being measured
: to point 3.
Flow the appropriate value of gas (typically
75% of scale).
When the reading has stabilized, press
"SET” .
Now do the same procedure for any other
ranges you want to tweak.
b. Details
This algorithm applies a segmented
straight-line approximation to the signal af-
ter the polynomial linearization has been
done. It forces the reading through the
points measured, and interpolates between
the points on a straight line basis. You
should only use this with care, to have to
use it essentially admits that there is some-
thing wrong with the normal linearization
setup, either a fault in the analyzer some-
how or a fault in the test gas setup. The
most usual reason for using this is to “cor-
rect" (actually, un-correct the analyzer) for
inaccurate mid-point check gases in an EPA
CEMS system. However if you are trying to
linearize an analyzer to 2% of point at 2% of
scale, this feature may save you when all
else has failed.
Network variables: MID_GASA,
MID_GASB, MIDPOINTA, MIDPOINTB,
SPAN_THEN, TWEAK
c. Troubleshooting
The normal reason for failure with this pro-
cedure is an error in the setup. Verify you
are editing the point that you have selected,
or that you have selected the point you are
