Importance

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Abstract

The present invention relates to an apparatus which can be inserted between the stack and a transceiver of a transmissometer to provide zero and span measurements using the primary light source from the transceiver. The apparatus includes a calibration device mounted between the transceiver and one side of the stack and within the path of the light beam. A zero reflector is mounted within the calibration device for movement back and forth between an inactive position and an active position within the path of the light beam to reflect the same amount of light back into the transceiver as the retro-reflector would across the stack when the stack is clear of smoke. In addition, means is connected to the zero reflector for accomplishing this movement. A span filter is also provided within the calibration device which is movable between an active position in the light beam path to provide an upscale reference calibration check. The invention further contemplates a method of using the apparatus to establish zero and upscale references during initial set-up, followed by periodic establishment of new values indicative of soiling of the transceiver windows.

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First Claim