Particle counter

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Particle counters are devices for detecting the size and number of particles in liquids or gases. A distinction is made between single particle counters and particle counters that take measurements on the collective. The following list provides an overview of some common methods for particle detection:

  • Extinction measurement (light shading)
  • Scattered light method
  • Laser diffraction
  • microscopic counting methods
  • Field disturbance method

and other.

Absorbance measurement

This is a single particle measurement method. The basic structure of an extinction particle counter consists of a light source, a measuring cell through which the particles to be measured move and a detection unit. If there is no particle in the measuring cell, this measured value is the reference. If a particle is now in the measuring cell, part of the light coming from the light source is shaded. The detector measures a different value than without particles. Depending on the size of the particles, a signal of different strength is measured.

Scattered light method

With this method, the scattered light of individual particles is detected. The basic structure of a scattered light particle counter is similar to that of the extinction particle counter and thus consists of a light source, a measuring cell and a detector. Since the detector may have to detect the smallest signals with the scattered light method, z. B. a photomultiplier can be used. A scattered light sensor is usually constructed in such a way that no light reaches the detector when there is no particle in the measuring cell. This is z. B. achieved by a light trap or by placing the detector at a 90 ° angle to the direction of illumination. If there is a particle in the measuring cell, the incident light is scattered on it. The underlying physical effects of this change in direction of light are diffraction, refraction and reflection. The signal at the detector changes depending on the size of the particles. In principle, a distinction is made between different scatter ranges:

where α = π · d / λ with particle diameter d and wavelength λ.

Laser diffraction

The method of static laser diffraction analysis for measuring particle size distributions is not a method in which individual particles are counted. The particle size distributions are output as a volume-based distribution by default and not as a number-based distribution, as is the case with particle counting. In the static laser diffraction analysis, the diffraction pattern of a collective of particles is recorded by measurement. By applying mathematical methods, the particle size distribution can be calculated from the detected diffraction pattern.

Particle size distribution measurement with the method of static laser diffraction is an optical method in which the scattered light emanating from the particle collective is detected in a certain solid angle. The evaluation theory is based on spherical particles. The laser light scattered on the particles is detected at different angles. Large particles bend the laser light with high intensity and small particles provide scattering contributions with lower intensity. The recorded diffraction and scattered light patterns are converted into volume-based particle size distributions using mathematical algorithms. For powder samples with larger particles, the evaluation can be carried out according to Fraunhofer theory. It is not necessary to enter or submit the optical parameters of the material to be measured. That is why the Fraunhofer theory is also used for powder mixtures of which the optical properties of refractive index and absorption are not known at the laser light wavelength to be used. For finer particles, a special case from Fraunhofer can be expected, the so-called Mie theory. This requires the entry of the refractive index and the absorption of the material at the irradiated wavelength. The analyzes are usually carried out based on or based on ISO 13320.

The optical measurement technology of the different manufacturers of laser flexor instruments differs in terms of light sources, beam guidance, optical bench and detection. In practice, the measuring range is in the submicron and micron range, i.e. between 40 nm and 2000 µm depending on the device manufacturer. The sample preparation as dry measurement of the powder or wet measurement, i.e. H. Particle size distribution measurement in a wet dispersion unit. In this case, the particles are dispersed in a suitable liquid. An optimal area of ​​application for powder is between a particle size of 100 nm to 100 µm. Laser diffraction analysis is a universally applicable powder analysis method for powder mixtures and pure substances. The output are volume-distributed particle size distributions.

calibration

The calibration is used to clearly assign a measurement signal to a specific particle size. If necessary, other parameters such as B. the coincidence error of the sensor can be determined during calibration.

Particles in liquids

When calibrating particle counters for analyzing the size and number of particles in liquids, different standards are used depending on the application. Particle counters can e.g. B. for water applications in accordance with the ISO 21501 standard with monodisperse latex particles of known size.

For measurements on liquid hydrocarbons, e.g. B. hydraulic fluids, lubricating oils or fuels, the particle counter is calibrated according to the ISO 11171: 2016 (E) standard . The calibration particles used here are not necessarily spherical, but rather have a rather irregular shape. So z. B. With the extinction particle counter, the signal of a particle is assigned to the particle diameter which corresponds to the circle with the same projection area.

Particles in air

The ISO 21501-4: 2018 standard is primarily used here. Monodisperse latex particles in air are used.

Sampling

Sampling plays a crucial role in particle counting. The particle counter can be located directly in the product flow, it can be installed in a bypass (online measurement) or a single sample is taken, which is then analyzed by the particle counter (offline measurement). So if a sample division z. B. carried out by taking an individual sample, it must be ensured that this is representative. For this it must be known whether temporal or spatial fluctuations occur in the initial liquid. If necessary, a homogeneous distribution must be ensured through appropriate processing. Furthermore, the individual sample must be chosen large enough to obtain a representative result. The container, transport, storage and preparation of the individual sample must also be such that, for B. no particle entry can take place from the outside.

Areas of application

In order to meet all areas of application, particle counters are available as laboratory devices, permanently installed for online measurements or as portable devices.

Particle counter for liquids

Typical areas of application for particle counters for liquids are found in the filter and oil industry: To define the performance of a filter, two particle counters are used; one analyzes the liquid before and one after the filter. To avoid failures caused by contamination of the operating fluid, particle counters are used in oil and hydraulic applications. Particle counters according to the VDA19 standard are also used in the automotive industry to test technical cleanliness. Particle counters are also used in water management and, in accordance with the USP <788> standard, in the pharmaceutical industry for determining the purity of injection solutions.

Particle counter for gases

A common application is to measure the number and size of particles in clean rooms . Air particle counters are also used in the field of environmental protection and air pollution or in the pharmaceutical environment.

literature

  • ISO 21501-2: 2019 (E) Determination of particle size distribution - Single particle light interaction methods - Part2: Light scattering liquid-borne particle counter , ISO 2019
  • ISO 21501-3: 2019 (E) Determination of particle size distribution - Single particle light interaction methods - Part3: Light extinction liquid-borne particle counter , ISO 2019
  • ISO 21501-4: 2018 (E) Determination of particle size distribution - Single particle light interaction methods - Part4: Light scattering airborne particle counter for clean spaces , ISO 2018
  • ISO 11171: 2016 (E) Hydraulic fluid power - Calibration of automatic particle counters for liquids , ISO 2016
  • J. König-Birk, Filter testing: Particle counters for hydraulic and lubricating oils. In: Filtration + Separation. Volume 48, Issue 3, 2011, pp. 33-36, online
  • HG Barth, RB Flippen, Particle Size Analysis. In: Analytical Chemistry. Volume 67, No. 12, 1995, pp. 257R-272R

Individual evidence

  1. ↑ a b Karl Schwister (eds.): Handbook of Chemical Engineering , 4th Edition, Carl Hanser Verlag, Munich 2010, ISBN 978-3-446-42435-7
  2. ^ Matthias Stieß: Mechanische Verfahrenstechnik - Partikeltechnologie 1 , 3rd edition, Springer-Verlag, Berlin Heidelberg 2009, ISBN 978-3-540-32551-2
  3. ↑ Kurt Leschonski, Fundamentals and modern methods of particle measurement technology
  4. ↑ ISO 13320 2009-10-01: Particle size analysis-Laser diffraction methods
  5. ↑ VDA19: Testing of technical cleanliness - particle contamination of functionally relevant automotive parts - , 1st edition, Verband der Automobilindustrie eV (VDA), Oberursel 2008
  6. ↑ USP <788>: Particulate Matter in Injections , 2010, archived copy ( memento of the original dated November 30, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.usp.org
  7. ^ Pramod Kulkarni: Aerosol Measurement: Principles, Techniques, and Applications , 3rd edition, Wiley-Verlag, New York 2011

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