Particle counter

Particle counters are devices for detecting the size and number of particles in liquids or gases. We distinguish single-particle counters and particle counters that take measurements of the collective. The following list provides an overview of some common methods for particle detection:

  • Absorbance ( light shading )
  • Light scattering method
  • Laser diffraction
  • Microscopic counting method
  • Field perturbation method

And others.

  • 6.1 Particle counter for liquids
  • 6.2 Particle Counter for gases

Extinction

This is a method of single-particle measurement. The basic structure of absorbance particle counter comprises a light source, a measuring cell, through which passes the moving particles to be measured and a detection unit. No particle is in the measuring cell, so this reading is the reference. If, now, a particle in the measuring cell, thus a part of the coming of the light source is shaded. Thus, the detector measures a different value than without particles. Depending on the size of the particles varies a strong signal is measured.

Light scattering method

In this method, the scattered light of individual particles is detected. The basic structure of a light scattering particle counter, similar to the particle counter of the absorbance and thus consists of a light source, a measuring cell and a detector. Since the detector must detect small signals in the scattered light method may, for example, may here a photomultiplier are used. Usually a stray light sensor is configured such that just no light reaches the detector, when no particle is located in the measuring cell. This is, for example, by a light trap or by placing the detector at a 90 ° angle to reach the illumination direction. A particle located in the measurement cell, the incident light is diffused thereon. The underlying physical effects of this change in direction of light are diffraction, refraction and reflection. Depending on the size of the particles, the signal change at the detector. In principle, various scattering regions can be distinguished:

Wherein α = π · d / λ of particle diameter d and wavelength λ.

Laser diffraction

Here, the diffraction pattern of a particle collective is detected by measurement. By applying mathematical methods allows the particle size distribution from the detected diffraction pattern calculated.

Calibration

The calibration is used to uniquely assign a measurement signal to a specific particle size. If necessary, additional parameters such as the alignment error of the sensor can be determined during calibration.

Particles in liquids

In the calibration of particle counters for analyzing the size and number of particles in liquids, depending on the application, different standards are used. Thus, for example, particle counter 21501 to be calibrated with monodisperse latex particles of known size for water applications in accordance with ISO standard.

For measurements in the hydraulic fluids of the particle counter according to ISO 11171:2010 (E) is calibrated. The Kalibrierpartikel used here are not necessarily spherical, but have a more irregular shape. Thus, for example, Extinktionspartikelzähler the signal of a particle associated with the particle diameter corresponding to the projection same circle.

Particles in air

This is mainly the ISO 21501-4:2007 standard for use. It can be used in air monodisperse latex particles.

Sampling

When particle counting sampling plays a crucial role. The particle counter can be located directly in the product stream, it can be installed in a bypass (online measurement) or it is drawn a single sample which is then analyzed by the particle counter (off-line measurement). Therefore, if a sample division, for example, made by drawing a single sample, it must be ensured that this is representative. But it must be known whether temporal or spatial fluctuations in the source liquid. If necessary, a homogeneous distribution by a corresponding processing must be ensured. Next, the individual sample must be large enough to obtain a representative result. The container, transportation, storage and processing of the incremental sample must be such that, for example, there can be no particle entry from the outside.

Applications

To meet all application areas meet, there are particle counter as laboratory equipment, fixed for online measurements or as portable devices.

Particle counters for liquids

Applications for particle counters for liquids can be found in the filter and oil industry: To define a filter's performance, two particle counters are used; one analyzed before the liquid and after the filter. In order to avoid losses caused by the contamination of the working fluid, particle counters are used in oil and hydraulic applications. In the automotive sector particle counters are used in accordance with standard VDA19 to test the technical cleanliness. Next find particle counters also used in water management and in accordance with standard USP < 788 > in the pharmaceutical industry for determining the purity of injection solutions.

Particle counter for gases

A common application is the measurement of the number of particles and particle size in clean rooms. In the area of ​​environmental protection and air pollution or in pharmaceutical environment air particle counters are used.

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