The Cambustion DMS500 is a type of instrument known as a Differential Mobility Spectrometer. The TSI EEPS (Engine Exhaust Particle Sizer) is another example. Electrical mobility is a measure of the ease in which a charged particle will be deflected by an electric field, and is a function of both the charge on the particle and its aerodynamic drag. Both of these parameters are in turn functions of particle diameter, so by knowing a particle's electrical mobility (which the DMS500 directly measures), its size can be calculated.
Particles enter the DMS500 via a cyclone which removes particles bigger than the maximum measurable particle size of the instrument (1000 or 2500 nm). They are then electrically charged by collision with air ions, in a process known as diffusion charging. The charged particles enter the classification column and are carried along it by a sheath of clean air. Within the column they are subjected to a radial electric field from a central high voltage electrode which repels the particles towards the outside of the column. Particles which are of low electrical mobility will be repelled to the outside slower than highly mobile particles, and hence will travel further down the column before impinging upon one of a series of electrometer rings along the length of the column. These measure the electrical current from the arrival of charged particles, and thus their mobility.
A number of problems with this approach have been overcome with the DMS500. Firstly, electrostatic theory states that a charged particle passing past a metal surface will induce an image charge within that surface. The implication of this is that larger (less mobile) particles which eventually arrive on electrometer at the bottom of the column would induce such image charges upon electrometers at the top of the column, and thus false small particle measurements would be seen. To overcome this problem, the Cambustion DMS500 uses a patented screening grid in close proximity to the electrometer rings which ensures the rings just detect particles which have passed through the grid. This simple but effective physical solution is known as the Space Charge Guard.
Electrical mobility is proportional to charge, and inversely proportional to particle diameter. However, the mean level of charge applied to a particle by diffusion charging is also a positive function of particle size. These two competing effects of size upon charge and drag lead to the electrical mobility having a minimum as a function of size, after which is starts to rise again. This inversion point in the mobility function means that for every mobility value there are two possible corresponding particle sizes. The inlet cyclone of the DMS500 removes particles bigger than the inversion point, thus removing this problem. However, this places a limit on the maximum measurable particle size of around 500 nm. To increase the range of the instrument, the DMS500 operates at sub-atmospheric pressure which moves the mobility inversion point to larger particle sizes and thus increases the range. It has been shown that operation at 250 mbar does not cause evaporation of volatile particles. By operating at a lower pressure the DMS is now offered with a maximum particle size of 2500 nm.
The DMS500 calculates the particle size spectrum from the electrometer currents using a computer modelled transfer function. This takes into account the amount of charge given to each size of particle and the precise geometry, flows and electrical characteristics of the instrument. An example of such a transfer function is plotted. There are 22 electrometer channels, and as standard the DMS500 provides 38 logarithmically spaced channels of size spectral data. The extra information required to get 38 channels from 22 measurements is provided by assuming that all spectra will be continuous and relatively smooth. No assumption is made about the precise form of peaks in the size spectrum in the continuous spectrum. In addition, multiple lognormal modes can be identified and parameterised for easy data analysis, improved spectral resolution and to allow separate mass calculation of each mode (see application note DMS06 for details). Before dispatch, all DMS500 instruments are finely calibrated using NIST traceable solid nanospheres (PSL) of known size (>70 nm), and by comparison with an SMPS (Scanning Mobility Particle Sizer) for steady-state sodium chloride and sulphuric acid aerosols ( <70 nm) - see this for more details.
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