Technical Description
Activaero’s Sedimentation Chamber is manufactured out of brass with a convection free sedimentation tube of a diameter of 6 mm. Aerosol particles introduced into the Sedimentation Chamber come to rest as a sediment in this tube with a velocity that is proportional to the square of their geometric diameter. In order to visualize the particles, they are illuminated by a laser diode emitting light with a wave length of 780 nm. The scattered light from the particles is detected by a miniature video camera built in to the chamber.
Fig. 1: Software after a particle size measurement
The video signal from the camera is acquired and digitalized by a video data acquisition card. A sequence of digitalized grey scale pictures is then on a computer where it is then analyzed using the specialized software programme: LabVIEW® (see Fig. 1). Through pattern recognition, particles are detected and particle coordinates (x/y) are calculated for each particle.
See a sample video of settling particles. >>
A comparison of the particle coordinates on the different pictures makes it possible to sort the particles in a way that the settling velocity can be calculated (see Fig. 2). The aerodynamic equivalence diameter can then be determined using this settling velocity.
Fig. 2: Schema of particle settling velocity measurement
The settling velocity vs is calculated from the ratio of the settled vector Dx to the settling time Dt. The aerodynamic equivalence diameter dae is calculated by:
nL: dynamic air viscosity
g: Acceleration of Fall
CC: Cunningham-Slip Correction
Performance
The particle size range of the particles analyzed by the Sedimentation Chamber could be between 0.5 µm und 10 µm. Figure 3 shows the high accuracy and selectivity of the system.
Fig. 3: Particle size distribution of a multimodal styrol particle aerosol
In a straight comparison between the Sedimentation Chamber and an Andersen impactor, the sedimentation cell shows a significantly lower standard deviation (see Fig. 4). This lower standard deviation could be due to the variability of particle separation in the impaction method used by the Anderson impactor. The Mass Median Aerodynamic Diameter (MMAD) parameters were nearly similar for both.
Fig. 4: Comparison of the sedimentation chamber versus Andersen impactor.
conclusion
The Sedimentation Chamber is an accurate and selective particle measurement method that is useful for monodisperse and polydisperse aerosols. The Sedimentation Chamber is an especially useful method for the analysis of dry powder applications with a high variability of aerosol density. And rather than the long laborious analysis needed for previous methods, the Sedimentation Chamber allows for measurement to be completed within only a few minutes.