Land 4500 MKII Premier, Operation And Maintenance Manual

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Model 4500 Premier Dust and Opacity Monitor
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C1-3 Optical Pathlength Ratio (OPLR)
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
Premier 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
2
The parameter used in the Model 4500 Premier is the OPLR, and it has a default value of 0.5, corresponding
to a parallel–sided duct.
1
2
3
4
5
Key
1.
Light source l
o
4. Reflector
2. Detector 5. Beam Splitter
3. Distance light beam passes through medium (L)
Double Pass System