Sample Flow Effect on Photometric Response

Reduction of the photometer sample flow to a level outside the tolerance limits, while using the internal reference settings for the 100% baseline response, results in a moderate increase in displayed % Leakage value. This higher value is the result of velocity changes of the aerosol passing through the light scattering chamber’s area of observation. The reduced velocity results in a more conservative evaluation of the filter under test. If actual upstream aerosol samples are used to establish a 100% baseline response (ratiometric mode), the unit’s aerosol response is self-correcting for non-conforming flow rates and standard accuracy conditions maintained. (FAQ013)

An increase in sample flow, however uncommon, results in the opposite effect. Increased velocity through the detection chamber decreases the displayed leakage value resulting in a liberal assessment of the filter performance and potential for passing a leaking filter.

These effects are one of the reasons for the ±10% limits paced upon the sample flow requirements in aerosol photometer testing protocols.