Model 4500 Premier Dust and Opacity Monitor89C1-3 Optical Pathlength Ratio (OPLR)The last item that we need to address is the fact that opacity monitors may not always be mounted atthe exit of a stack. Since the Ringelmann observer always views opacity at the exit of a stack, wemust also introduce a factor to correct for the change in pathlengths between the measurement andexit diameters of the stack. It should also be pointed out at this time that the Land Model 4500Premier Dust and Opacity Monitor is a double pass instrument. This means that the light beamcrosses 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 ratiobetween the diameter of the stack exit and the diameter of the stack at the point where the instrumentis installed. For a parallel stack, the PLCF is 1.0. For a tapering stack which is narrower at the exitthan 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 thedistance travelled through the stack gases by the beam. For a double-pass instrument like the Model4500 Premier, the OPLR is equal to half the PLCF i.e.OPLR = PLCF2The parameter used in the Model 4500 Premier is the OPLR, and it has a default value of 0.5, correspondingto a parallel–sided duct.12345Key1. Light source l o 4. Reflector2. Detector 5. Beam Splitter3. Distance light beam passes through medium (L)Double Pass System
