BS EN ISO 28439:2011 pdf download
BS EN ISO 28439:2011 pdf download.Workplace atmospheres ——Characterization of ultrafine aerosols/nanoaerosols-Determination of the size distribution and number concentration using differential electrical mobility analysing systems.
Smaller buffer volumes are peferred if the aerosol agglomerates by coagulation (high concentration of pnmary particles) (see 11.3.3).
NOTE The state of knowledge at the L*e of publication allows no recommendation br an upper volume to be made
6.3 Pre-separator
A pre-separator is required such that large particles above the desired measurement range are preopitated, This can be done, for example, by use of a suitable impactor or cyclone. The pre-separator shall be cleaned and, if necessay, greased regularly.
6.4 Particle charge conditioner
The aerosol is charged with free electrical charges by collisions with gas ions and electrons. The free electrical charges are usually produced by a radioactive source in the air stream, separated by a thin enclosure. Sources like Kr, 210Po or are used. The entire aerosol reaches a charge equihbnum of known distribution (see Reference (9)).
NOTE The charging of non-spheflcal particles differs from that of spheres. Therefore the distribution of electrical charges as a function of particle size employed m the inversion of the critical electrical rnobihty into a particle size interval is strictly valid only for sphencal particles
6.5 DEMC
The conditioned aerosol reaches the etectncal mobility classification section. A common DEMC compnses an inner and an outer electrode maintained at an electrical potential difference typically between 20 V and 10 000 V(see Figure 2).
The particles are transported in laminar flow along an annular region or tube along with a dean air sheath. The motion of the charged particles depends on their different mobilities. causing them to reach the electrode at different positions. Particles of a narrow electrical mobility range centred on the critical electrical mobility are sampled via a slit towards the end of the annular region and transported into the detection section.
6.6 Aerosol particle detector
The separated aerosol with the specified, critical, electncal mobility is led to a counter which determines the number of particles per unit volume, The most widely used counter is the CPC (CNC). In this device, the aerosol is brought into contact with supersaturated vapour (alcohol or water) which condenses on to the partides The particles grow rapidly to large droplets, typically several micrometres in diameter, and can then be detected using optical methods Another detector is the electrometer, which determines the net electrical current provided by the sampled particles.
After passing through the detector, further analysis of the particles is not possible. Additional samplers operated ii parallel to the DMAS can be used to collect samples for further analysis, e.g. an electrostatic or thermal precipitator.
8.2 Sampling
If worker exposure is to be estimated, the aerosol shall be sampled in the breathing zone of the worker. Al existing DMASs at the tine of publication are large static instruments. The sample should be taken at a representative position fri or very near the worker’s breathing zone A sampling line as specified in 6.2 can be applied. H the worker is not stationary, samples from different locations can be taken by moving either the entry of the sampling line or the complete DMAS
Diffusion losses in the line shall be taken into account (see Clause 11).
H the instrument perfomis a scannfrig or stepping voltage mode, the time for one measurement is a few minutes, typically 3 mm to 6 mm, During this time, the aerosol should not differ in size and number of particles.
H highly fluctuating concentrations, such as in welding workplaces, are to be measured, it is recommended that a buffer vessel of a few litre& volume be used ii the sampling Irne to avoid artificial peaks during a scan.
Another option is to average a series of samples if short-term emissions appear regularly
Some DMAS devices allow a selection of rstrument pnneters 11w sampling air flow rate, sheath air flow rate, scan times or even a change of the DEMC type. These settings affect the measurement range. It is recommended that the range be selected in such a way as to cover the whole size distribution of the ultrafine particies at the workplace. Practical experience shows that the maxima of the particle number size distributions lie between approximately 10 nm and 500 nm, depending on the occurrence of primary, aggregated and agglomerated ultraffrie particles.
The total number concentration integrated over the measurement range found at workplaces vanes between io particleslm3 and 1014 partlcles/m3 In clean air sections or urban background and welding plumes. respectively (see Reference [10)).
