Breaking Down the Technical Cleanliness Workflow Part 3: Particle Size Classification and Particle Count Extrapolation and Normalization

Particle size classes (differential and cumulative) and particle counts

In the third post of this six-part series, we look at particle size classification and particle count extrapolation and normalization. Here is where classification, extrapolation, and normalization fit into the overall technical cleanliness inspection process:

• Preparation
  • Extraction
  • Filtration
  • Drying and weighing
• Inspection
  • Image acquisition
  • Particle detection
  • Particle size measurement and classification
  • Particle count extrapolation and normalization
  • Contamination level calculation
  • Cleanliness code definition
  • Maximum approval check
  • Separation of reflective and nonreflective particles
  • Fiber identification
  • Results review
  • Reporting

Particle Size Classification

The outcome of the particle detection we covered in part 2 of this series is a sheet with the results of each detected particle. The size (typically the maximum Feret diameter) of each particle is listed. All particles are grouped into different size classes. This makes the subsequent report much shorter and enables a better comparison of measurements.

You can define the size classes. The classification parameters and how the classes should be divided are defined in various international standards. There are two major groups of size classes:

Differential classes: Size classes are defined by a minimum and a maximum particle size. Each particle is counted in only one class.

Cumulative classes: Size classes are defined by a minimum particle size. As a result, it’s possible that particles will be counted in more than one class.

Particle Size Classification

A defined area on the filter is scanned and checked for particles. Different filter areas (Fig. 1) are defined below.

Figure 1: Filter areas used in particle count extrapolation.

Filter size: A standard-size filter has a diameter of 47 mm, which results in a total filter area of 1735 mm².

Flow-through area: The filter is not completely covered with particles. Particles can only be in the area where the washing rinse went through the filter during the filtration process. This flow-through area can be set by the operator and must be a central circular area with a diameter less than 42 mm.

Maximum scan area: The maximum scan area has a diameter of 42 mm, which results in a total maximum scan area of 1385 mm².

Inspection area: The actual scan area can be defined by the user. Typically, the maximum possible scan area is used for the scan, but the inspection area can also be smaller. A smaller inspection area leads to fewer images and, thus, speeds up the time needed to inspect the filter.

All particles are detected when the flow-through area is completely inside the inspection area. If the inspection area is smaller than the flow-through area, the system needs to extrapolate the number of particles detected. The flow-through area must be set in your inspection software and will be used for particle count normalization.

Particle Count Normalization

The absolute or extrapolated count of particles must be normalized to a reference value.

Depending on the standard used and the filter tested, the measured number of particles is normalized to a comparison value. This enables you to compare multiple measurements, even if the examined samples are not the same size.

Depending on the method, a different value for normalization is used:

Washed area: Normalization on a washed surface area is used when the detected particles were washed from a sample surface. The resulting particle count is normalized to an area of 1000 cm².

Washed volume: Normalization on a washed sample volume is used when the detected particles were washed from a larger structured sample. The resulting particle count is normalized to an area of 100 cm³.

Washed parts: Normalization on washed sample parts is used when the detected particles were washed from a number of similar samples. The resulting particle count is normalized on a single sample part.

Filtered fluid: If the filtered fluid itself is analyzed and the detected particles are not washed from a sample, normalization has to be done on the amount of filtered fluid. The resulting particle count is normalized to a filtered fluid of 1 ml or 100 ml.

Note that the unit “cm³” is used for the washed volume and the unit “ml” is used for the filtered fluid. The different units are used to avoid mixing the values of washed sample volume and filtered fluid.

Following particle size classification and particle count extrapolation and normalization, contamination levels are checked for each particle size class. Check back for "Contamination Level Calculation," part four of six in our "Breaking Down the Technical Cleanliness Workflow" blog series.

Related Content

Breaking Down the Technical Cleanliness Workflow Part 1: Preparation

Breaking Down the Technical Cleanliness Workflow Part 2: Image Acquisition and Measurement

The Value of a Turnkey Cleanliness Inspection System