Real-Time Non-invasive Measurements of Aerosol Flow in the Laboratory

JULIE PONGETTI, Nick Collings, Jonathan Symonds, Chris Nickolaus, Cambustion Ltd

     Abstract Number: 581
     Working Group: Instrumentation and Methods

Abstract
Almost all aerosol experiments include a need to measure gas flows accurately, either to check that a measurement instrument is operating correctly, or to directly correct a measured parameter. Many general laboratory methods exist for measurement of gas flow – including, rotameters, soap bubble, orifice, and more. However, these are usually not ideal for aerosol laboratory use as they may variously be damaged by aerosol particles, have the flow reading affected by the aerosol particles, affect the aerosol itself. Here we focus on the principle of pressure drop through an orifice and investigate the challenges involved in adapting this well-established flow measuring technique to aerosol flows.

The mass flow of a gas of known properties can be related to its pressure drop through an orifice of known geometry, based on local temperature and pressure conditions. This relationship remains valid for dilute aerosol flows, but over time particles accumulate on the orifice, affecting its discharge coefficient and hence the calibration curve.

The geometry upstream of the orifice plays a role – amongst others – in determining the location and rate of the particle accumulation. Restoration of the orifice initial performance is possible upon cleaning of the flow duct, where mechanical removal of the accumulated particles by use of a brush was found to be most effective.

These initial results were successfully used to develop a commercial instrument, the Cambustion AF10 Aerosol Flowmeter. The latest tests carried out on the new product show that a one-point empirical calibration adjustment to the physical model for each orifice is sufficient to achieve a 2% accuracy on the whole range of flow rates covered. Endurance tests with highly loaded aerosols also show the effectiveness of the integrated cleaning brush, which allows performance to be guaranteed over long experiments while causing minimal disruption (< 1 s).