Ametek Land 4500 MKIII, User Manual

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C - 3
Model 4500 MkIII Opacity and Dust Monitor
User Guide
C3 Pathlength Correction Factor (PLCF)
The last item that we need to address is the fact 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 at this time
that the Land Model 4500 MkIII Dust and Opacity Monitor is a double pass
instrument. This means that the light beam crosses the medium twice and
hence experiences twice the amount of absorption as illustrated below. 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 which 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 Model 4500 Premier, the OPLR is equal
to half the PLCF i.e.
OPLR = PLCF
1
2
3
4
5
2
Key
1. Light source l
o
4. Reflector
2. Detector 5. Beam Splitter
3. Distance light beam passes through medium (L)
Double Pass System