envea STACK 710, Руководство пользователя

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6 Theory of Operation and Application STACK 710
50
(PC-000878-MA Issue 7, December 2021)
ENVEA UK Ltd
6.3 Pathlength Correction Factor (PLCF)
The last item that we need to address is that opacity monitors may not always be
mounted at the exit of a stack. Since the Ringelmann observer always views
opacity at the exit of a stack, we must also introduce a factor to correct for the
change in pathlengths between the measurement and exit diameters of the stack.
It should also be pointed out that the STACK 710 Dust (opacity monitor) is a
double-pass instrument. This means that the light beam crosses the medium twice
and experiences twice the amount of absorption, as illustrated in
Figure 8
. This
must also be corrected in our system.
The Pathlength Correction Factor (PLCF) is only relevant for Opacity
measurements. It is the ratio between the diameter of the stack exit and the
diameter of the stack at the point where the instrument is installed. For a parallel
stack, the PLCF is 1.0. For a tapering stack that is narrower at the exit than the
base, the PLCF is less than 1.0.
The Optical Pathlength Ratio (OPLR) is the ratio between the diameter of the stack
exit and the distance travelled through the stack gases by the beam.
For a
double-pass instrument like the STACK 710, the OPLR is equal to half the PLCF,
for example,
OPLR = PLCF
2
Key
1. Light source lo 4. Reflector
2. Detector 5. Beam Splitter
3. Distance light beam passes through a medium (L)
Figure 18: Double pass system.
We can use the previous equations to calculate the opacity at the stack exit, if we
define the pathlength correction factor (PLCF):