I
NITIAL
S
TARTUP AND
C
ALIBRATION
3-14
October 1999 Rosemount Analytical 748250-L
Model 880A Non-Dispersive Infrared Analyzer
3.10.2 N
ON
-S
TANDARD
R
ANGES AND
C
OEFFICIENTS
When ordered, special linearization coefficients for non-standard fullscale ranges are
entered in the appropriate range(s) at the factory. If one set of linearization coefficients
has been ordered and a range has not been specified, these coefficients will be for
Range 3.
The user may want the display to read in engineering units (ppm). This response is
linear over the operating range.
The following coefficients will make no correction to the straight line response, but will
cause the analyzer to display engineering units:
A
0
= 0.00000
A
1
= 1.00000
A
2
= 0.00000
A
3
= 0.00000
A
4
= 0.00000
To calculate linearization coefficients other than those installed at the factory, either
11 or 21 data points should be taken. (If urgent, a curve can be created with as few
as 4 points. This should be considered temporary and a more accurate curve made
with more points should be created as soon as possible.)
These data points can be obtained with an accurate gas divider or other flow mixing
device. Before calculating coefficients, the data must be normalized to ranges of 0 to 1
units for both % and concentration readings. Then the axis must be reversed as
illustrated in Figures 3-9 and 3-10. A multiple linear regression is then used to
calculate coefficients. (For example: If the range is 0 to 5000 ppm and readings are 0
to 100%, then divide all of the concentrations by 5000 and the readings by 100. Put
the normalized concentrations on the Y-axis and the normalized readings on the
X-axis.
These data points can be entered into any program capable of computing a fourth
order polynomial curve. This curve will be the mirror image of the curve on the
application sheet at the back of the manual, however the linearization coefficients will
be different. Use the coefficients calculated with the curve for linearization coefficients.
Use these coefficients to solve the following equation:
Y = A
0
+ A
1
X + A
2
X
2
+ A
3
X
3
+ A
4
X
4
.
After taking the data points, the user may determine coefficients for user-specific gas
by either using any program capable of calculating a fourth order curve fit or calling
the factory to have the specific coefficients calculated.
When entering user-determined coefficients, note that the microprocessor only
recognizes five significant figures to the right of the decimal point (i.e., 0.12345). Also,
the user should ignore the three non-significant digits as they may change value while
the significant digits are being changed. The values of the non-significant digits do not
affect the instrument's linearity.
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