Emerson Rosemount CCO 5500, Reference Manual

Page: 114 / 124

A.7 Principles and modes of operation
A.7.1 Calculation of gas concentration
The analzyer determines gas levels by measuring the absoption of infrared radiation
transmitted through the flue gas. The wave bands from the infrared radiation are sensitive
to absorption by the measurement gas. The Rosemount
™
CCO 5500 Analyzer has two
detectors: one measures the radiation directly to provide a live output, sensitive to the
measurement gas. The second detector measures the radiation after passing through a
gas cell fitted with pure reference gas (CO). This provides a reference measurement
completely unaffected by the measurement gas.
The basic expression from which the gas concentration in the gas is determined is:
Y = G - K(D2/D1)
Where: D1 = the reference output from the detector
D2 = the live output from the detector
G = a scaling factor (16000)
K = a constant, known as the zero correction factor, set so that when there is zero
measurement gas in the duct, Y = 0
thereby, K = G(D1
0
/D2
0
)
The analyzer then smooths, linearizes, and compensates the parameter Y for effects of
path length and flue gas temperature to produce a measurement of CO gas concentration
in the flue gas.
A.7.2 Error compensation
The accurate determination of gas concentration depends on the measurement of the
radiation levels received by the detectors. Any error in the measurement caused by
detector drift will produce errors in the calculation of the gas level. In order to maintain
accuracy, it is necessary to compensate for such drifts. The Rosemount
™
CCO 5500
Analyzer uses a technique of repeated calibration adjustments.
The operating cycle of the instrument is in two parts. First, the instrument obtains
measurements from the two detector outputs, D1 and D2. It then positions the calibration
cell containing pure CO in the sight path and measures the two detector outputs again to
give readings E1 and E2.
From the basic scale shape equation:
Y = G - K (D1/D2)
and from the calibration equation:
Y
0
= G - K (E2/E1)
or K = (G - Y
0
)(E1/E2)
Theory of operation
106 Rosemount CCO 5500