Particle Size Analysis

Particles, Their Size And Their Distribution

When talking about a dust and its size it is important to understand that, with very few exceptions, we are talking about a distribution of different sized particles. It is equally important to understand how the distribution is presented (typically by volume % but occasionally by number %) and how it was measured (for example, by using laser diffraction or by sieve analysis). Different particle size analysis methods will yield different results for the same material due to their limitations and the physical principles employed with each method.

Particle Technology Limited offer particle size analysis of a sample using various methods detailed below. We can offer this service on various forms of sample from bulk to particulate present on a filter generated from activities such as stack emissions monitoring. Restrictions will apply depending on the sample type and the information required.

 

 

Laser Diffraction

Particle Technology has two laser diffractometers, the Horiba LA-950 and the Microtrac S3500. Both instruments employ the laser diffraction technique, with each machine having certain advantages/features to meet different requirements. This technique can be used to quickly provide detailed measurements of samples in the nominal size range of 0.1 microns up to 3000 microns. The data is generally given in volume %, both differential (the amount at each size) and cumulative undersize (the amount below the stated size).

As with most particle sizing techniques, laser diffraction makes the following assumptions: 1) all particles are spherical; 2) all particles are of the same density. In most cases the particles being measured are not spherical and often the particles being measured are not all of the same material and therefore do not have the same density. Particle size from laser diffraction is reported as equivalent spherical diameter – this is the diameter of a sphere that diffracts light in the same way as the particle being measured.

Coulter Counter/Electrozone Sensing

Particle Technology has a Beckman Coulter Multisizer 3 which employs the coulter counter/electrozone sensing method. This method can both size and count particles in the nominal size range of 1 – 200 microns.

The electrozone sensing method works by passing a small current between two electrodes submersed in electrolyte and separated by a small orifice (typically 30 to 400 microns diameter). The sample is added to the electrolyte solution and as the measurement is carried out particles are drawn through the orifice. As each particle passes through it displaces the electrolyte and this disturbance changes the resistance between the electrodes, which is measured and is a function of the volume of the particle passing through the orifice. Using this information for each of the (typically thousands of) particles the instrument calculates the size distribution of the sample and presents it in either volume or number base.

The electrosensing zone technique also makes the same two assumptions as laser diffraction (see above). However, the technique is directly measuring the volume of the particle; therefore, the particle size can be represented as equivalent spherical diameter, based on volume – this is the diameter of a sphere that has the same volume of the particle being measured.

Automated Particle Counting (APC) of a sample of oil to determine its ISO 4406 contamination level

HIAC 8000A

The HIAC 8000A is used primarily for testing the cleanliness of hydraulic fluid to ISO Standard 4406 and has an overall size range from 1.2µm to 100µm.

Microscope analysis to determine the number of particles extracted

Image Analysis Microscopy

Image analysis allows us to identify the shape of a particle in addition to counting and sizing them, as well as allowing us to characterise material that previously presented difficulties, such as fibres and needles.

Sieve Analysis

A relatively basic but effective particle size analysis method is by sieve analysis. Particle Technology own a full range of ASTM standardised test sieves and automated sieve shakers. The sieves are selected based on sizes of interest ranging nominally from 20 microns to ~3mm. They are placed with the smallest sieve at the bottom and the sample is added to the top sieve. As the sieve shaker vibrates the sieves at high frequency, the smaller particles pass through the apertures of the sieve onto the next size down, eventually being retained or collected as undersized material. This mass data is used to present the particle size distribution.

High purity silica sand retained on a 500micron stainless steel woven wire mesh sieve.

Advantages & Disadvantages

Advantages-Repeatable/no loss of sample. Robust method. Effective for coarser samples.

Disadvantages – Typically sizes particles based on their smallest dimension and so a long particle with a small diameter would be classed as smaller than it would if it was analysed by laser diffraction.

If you require information on the particle size distribution of a dust sample, please get in touch. We can carry out analysis on samples ranging from samples taken of bulk materials for general QC/QA purposes to samples taken during environmental monitoring activities such as stack emissions testing where we can remove and analyse the particulate from the filter provided.

Please contact us for further information.

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