Particle measurement
The term particle measurement is a group of measuring methods for qualification and quantification of "small bodies" ( particles of different) nature, this includes not only dusts and nanoparticles ( soot , aerosols , etc.), but also elementary particles , viruses and bacteria . Other organic and inorganic substances in the macroscopic range, such as skin flakes or fine crystal splinters , are also treated as "particles" in this context.
Particle measurements are used in almost all areas of the natural sciences ( physics , chemistry , biology ) and in technology and criminology . The type of measurement methodology therefore depends heavily on the type and nature of the particles as well as the question to be answered that preceded the measurement setup.
Finding the number of microscopic particles in the air
Areas of application
Areas of application This type of measurement is carried out - among other things - in the area of clean rooms. It serves to qualify the premises (see EN ISO 14644 ). When particle measurements are mentioned, they are often mistakenly seen only in direct connection with clean rooms in semiconductor technology . Another well-known field of application of this type of particle measurement is the determination of diesel soot in urban landscapes. This has increased in importance due to various EU regulations in recent years.
Measuring principle: scattered light particle measurement
When measuring scattered light particles, a defined amount of air (sample) is passed through a laser beam. The light from the laser beam is refracted by the particles or is absorbed by them . Photodiodes can measure these effects and convert them into an electrical signal. This signal is compared by a computer with a previously recorded signal in which latex spheres of a defined size were measured in order to create reference values . With this data it is possible to use statistical methods to determine the number of particles in a cubic meter of air or gas.
Determine the number of macroscopic particles in the air.
If it is necessary to detect particles in gases that are larger than 25 µm, a different measuring method is necessary. The Andersen cascade impactor ( Andersen collector ) is ideal for this case . Depending on the design, this can collect particles from 1 µm down to the macroscopic range. Another advantage is the ability to analyze the type of particle afterwards. The Andersen collector is used, among other things, in agriculture to qualify stable air. In the field of semiconductor technology, this measurement method is not used due to its complex measurement evaluation. - In addition, it is not necessary to measure particle sizes> 25 µm, as particle sizes> 5 µm already have a negative effect on production and are not permitted in the front-end.
Finding the number of particles on a surface
Detecting particles on a surface can be done by several methods:
- Particle precipitation photometer
- A Petri dish or a similar reference surface is placed on the surface to be measured and left there for a few days. The particles on the bowl are then counted under the microscope or automatically evaluated.
- Surface particle counter
- These are often used in the semiconductor industry. They are mainly used to analyze the surface of wafers . The measuring principle is similar to that of the scattered light particle measurement. A laser beam is guided over the wafer and the scattered light that is reflected from the wafer can be converted into an electrical signal by means of photodiodes. This is compared by a computer with previously stored reference values.
Finding the number of particles in liquids
There are various methods for doing this. A basic distinction must be made as to whether all particles of the liquid to be examined are recorded (volumetric measuring principle) or only a part (in-situ measuring cell) and the number of particles per unit volume is then deduced using mathematical methods. A distinction can also be made between the measurement of individual particles and the measurement on the collective (e.g. nephelometer ). The scattered light method and the extinction measurement (light shading) are particularly suitable for measuring individual particles with diameters in the µm range.
If individual particles are to be examined in a highly diluted sample, a volumetric measuring cell is preferable, since every particle is actually recorded there. In this case, an in-situ measuring cell provides an inaccurate result, since particles can flow through the measuring cell without being detected. Furthermore, the actual size of the particles is often not correctly recognized here, as particles are only partially recorded and thus mathematically evaluated as smaller particles. In the case of in-situ measuring cells, the correct specification of the number of particles per volume of liquid depends heavily on the mathematical method used.
Individual evidence
- ^ Walter Müller: Basic mechanical operations and their laws . Oldenbourg Wissenschaftsverlag GmbH, 2008.