Understanding the Different Particle Measurement Techniques

An important part of any machine conditioning program is particle counting. There are a number of existing tools to monitor and track the quantity and severity of the contamination, whether it is a result of machine wear or external contamination. The specific application and particle type will usually dictate what is the best particle counting method for the job at hand.

For example, the continuous cleanliness of a hydraulic system is extremely critical and even extremely low levels of dirt ingress can clog valves and actuators leading to premature failure. On the other hand, gear and transmission systems with multiple moving parts coming together will be able to withstand a lot more wear particles than a clean hydraulic system.

Available Methods – Direct Imaging Particle Counters

In order to create a direct imaging particle counter as shown in the illustration below, direct imaging systems incorporate a solid-state laser configured with a CCD array. The laser illuminates the sample, and the laser light is magnified by an optical lens. A CCD video camera gathers the images of the sample and records them.

The recorded images are analyzed for shape and size. For each image, an equivalent circular diameter or ECD is calculated and particle count and size distribution is established in addition to ISO codes. Further to particle shape morphology, direct imaging systems supply other particle counting output formats but ISO 4406 is the most common.

The capability of direct imaging systems to capture the actual wear particle silhouette permits an ‘Automated Ferrography’ capability for wear particle classification. Every particle which is larger than 20 μ is classified by a neural network in the categories of fatigue, free water, cutting, severe sliding, non-metallic, and fibers.

Identifying the type of wear particle and supplying particle size distribution, count, and severity of each of the abnormal wear mechanisms complements information supplied by other instrumentation methods like analytical ferrography and ferrous monitoring. This capability is executed on the Spectro LNF Q220 and Q230 instruments.

Imaging systems can distinguish between water droplets, solid particles, and air bubbles in oil for all particles over 20 μ. Water and air bubble counts are taken away from the measured particle count to establish a true net particle count.

Advantages

• Accurate to 1 um resolution
• No coincidence effects – below 5 million p/ml
• Additional shape classification
• No calibration needed (intrinsically correct)

Disadvantages

• Sooted oils >2% need dilution
• Interference from additives and water below the classification threshold (20 um)
• Heavy oils >320 cst need dilution

LaserNet Fines Q230 Series

Available Methods – Pore Blockage

Pore blockage particle counters are employed as on-site particle counters for in-service machinery oils, they use a fine mesh which particulate collects on. These particle counters are based upon either a constant pressure or constant flow design. The constant pressure designs calculate the alteration in flow rate whilst maintaining a constant pressure.

Constant flow instruments calculate the pressure drop over the mesh whilst keeping constant flow. The particle count distribution is gauged by extrapolation in both cases. Typically, a pore blockage mesh design yields one or two ISO codes. Pore blockage particle counters are seldom employed by commercial laboratories because of the limited data produced, but can be of great value where interference from soot, water, or additives in the samples is highly prevalent.

Advantages

• No degassing to remove air bubbles
• No interference from soot
• No interference from additives or water

Disadvantages

• Very distribution dependent at the pore size being measured
• Pore density is not one size fits all
• Only measures a single size channel

Rockwell digital CONTAM-ALERT

Available Methods – Light Blockage Particle Counters

The traditional instruments employed for in-service oil analysis are optical particle counters (OPC’s), or laser light blocking particle counters. The working principle of traditional light blockage particle counters is shown below.

A light source, usually a laser, passes through a sample. The light is partly obstructed by particles so less light arrives at the photodetector array, leading to a change in voltage which is proportional to the area of the particles. The photo detector technology is the same principle which is applied in garage door openers.

Traditional light blockage particle counters possess a number of inherent design limitations. The photo detector results have measurement errors which are a result of the presence of air bubbles and water within the oil sample. Preparing a sample properly by utilizing ultrasonic agitation helps to decrease the impact of air bubbles on particle count.

It is usual to require ‘water stripping’ solvents to attain a more accurate count for water containing samples (an oil sample that is ‘milky’ contains water). The presence of water leads to a significant error in the particle count reported.

Advantages

• Portable
• Easy to automate
• Accurate for contamination control

Disadvantages

• Periodic calibrations required
• Coincidence errors
• Easily affected by water and soot

HIAC ROYCO 8011 OPC

Summary

As discussed, there are three main methods in the field of particle analysis; direct imaging, pore blockage, and optical light blocking. Each one possesses its own unique pros and cons. It depends on the specific application being monitored as to which method will provide the best value for the end user.

Particle analysis is a crucial part of machine condition monitoring and the type of tool you select will depend upon whether its intended use is basic particle counting cleanliness control tasks or machine specific wear particle analysis. The severity and concentration of interfering objects: additives, water, and soot should also be considered and will impact the choice.

This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.