Limitations of long-path averaging instruments

Long-path averaging instruments measure the average velocity or concentrati on of a substance or substances over an averaging path. These measurements are then often used for calculation of the average concentration and mass n ow rate of the substance. The purpose of this paper is to describe some of the limitations of these instruments and to suggest ways in which these lim itations can be minimized. Two limitations were examined: measuring concent ration in a single dimension (e.g., ignoring the variation in concentration over the width of the sample plane), and deriving an average concentration without considering velocity effects. The resultant errors will be applica tion-specific. Estimates of the second source of error can be obtained from the covariance of concentration and velocity profiles over the path length. Unfortunately , suitable field data were not available, and to illustrate the method, est imates of the error were obtained for a range of possible concentration and velocity profiles. Errors of 50% or greater in the mass flow were incurred far the concentration and velocity profiles considered. This error was red uced to a negligible level by segmenting the averaging path length. It is r ecommended that velocity and concentration profiles be obtained for a broad range of applications to enable the importance of covariance errors to be better assessed.