Technical Description
The sedimentation chamber itself is manufactured out of brass with a convection free sedimentation tube with a diameter of 6 mm. In this tube the particles settle down with a velocity, that is proportional to the square of their geometric diameter. To visualize the particles the particles 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 video camera.
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 stored in the computer RAM. This picture sequence is then analysed by using LabVIEW®. By 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. Out of the settling velocity the aerodynamic equivalence diameter determined.
Schema of particle settling velocity measurement
The settling velocity vs is calculated out of the ratio of the settled way ?x and the settling time ?t. 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 could be between 0,5 µm und 10 µm. The following figure shows the high accuracy and selectivity of the system.
Particle size distribution of a multimodal styrol particle aersosol
The sedimentation cell has shown a significant lower standard deviation as the Andersen impactor. This lower standard deviation could be founded in the variability of particle separation of the impaction method. The MMAD parameters were nearly similar.
Comparison of the sedimentation chamber versus Andersen impactor.
conclusion
The sedimentation chamber is a accurate and selective particle measurement method that is useful for monodisperse and polydisperse aerosols. Especially for dry powder application with a high variability of aerosol density the sedimentation chamber is a useful method. A measurement is done within a few minutes.