This sympton is pointing most likely that the cell windows are dirty. They should be replaced or cleaned.
Gary is the Manager for the Multiwave Photometer, Reid Vapor Pressure Analyzer and Dynamic Reflux Sampler. He recied a BS in Chemistry from Fairmont State Unitversity. He joined ABB in 1989 as an Application Engineer for the Multiwave Phtometer and later moved into product management.
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