Particle Counting - Peril or Prize?

Ashley Mayer
Tags: particle counting, oil analysis

Is particle counting worth it? The answer is most certainly yes. But it comes with a stop press: only if it is done properly.

Recent years have seen a dramatic increase in the deployment of particle counting as a must-have condition monitoring technique. Because of this, there is a heightened awareness of the importance of clean oils, a mushrooming of on-site laboratories, and the decrease in the cost of particle counters (particularly automatic optical particle counters). But add this all up and the accompanying negative consequences have been an increase in the occurrence of incorrectly performed particle counts, and the frustrations that accompany them.

Some type of particle counting capability is an absolute must for an analysis program, be it an on-site or commercial laboratory-based analysis program. But there are various options available when it comes to selecting a particle counting method. Choosing the method most applicable to your situation is vital in achieving success and avoiding disappointment.

There are three methods that should be investigated: optical particle counting, pore-blockage particle counting, and microscopic evaluation. Choosing the wrong option, just because it sounds popular, is not in itself going to improve one’s contamination control program – it will only re-enforce the notion that particle counting is more of a peril than a prize.

Under the Microscope
Let’s start with the last-mentioned option first. In situations where extremely dirty oils are the norm, or where an on-site capability is being developed and budgets are tight, consider microscopic evaluation. Using this method, a fraction of sample is diluted in solvent and used to prepare a patch, or filtergram. Patch makers range from the portable, manual vacuum pump-type to the more sophisticated (and only slightly less inexpensive) bench-mounted setup with an electric vacuum pump.

Filter media can be laboratory-quality filter paper, or for dirty oils, consider using a piece of blotting paper which traps less “noise” and highlights the more important data. The resulting patch is then examined under a microscope. Inexpensive commercial imaging packages are available which allow one to estimate a particle count by comparing the test image against one with a known count.

Microscopic evaluation is a good option for the oil analysis practitioner new to the game. It is also a good option where heavy water contamination makes an optical particle count difficult or even impossible.

A simple patch maker, microscope and imaging package will cost no more than a few hundred dollars. Purchase a microscope with the ability to capture images on a computer, then trend the images. There are some good microscopes available which do not use optics, but instead plug into a USB port on a PC and the images are viewed on the screen. Be aware that when you upgrade to an updated system of particle counting, your investment in patch making will not be lost – the patch maker and microscope are still a necessity for wear debris analysis.

Pore Blockage
The second option to consider is the pore-blockage type particle counter. These particle counters determine a flow or pressure profile through a particle-collecting device to estimate a particle count. These machines are transparent to water or air bubble contamination, either of which can be an issue in most oils. Depending on design, they may also assume a normal particle distribution, which can be affected by changing filtration. Consider using this technique for oils where water contamination exists, or dark-colored oils (such as engine or other types of opaque oils) are monitored.

Optical Particle
The third option is the automatic optical particle counter. This option is for the serious practitioner, and can be used with very clean and dirty oils alike. If a stroll through your plant reveals an overabundance of gooseneck breathers and strainers instead of filters, reread the previous two paragraphs.

Optical particle counters come in two varieties, the light-blockage or the laser-scattering type. More sophisticated machines have the ability to perform image recognition on the contaminant particles, classifying them into different wear modes and even differentiating water droplets and air bubbles. It is probably fair to say that optical particle counters are the source of most of the frustration with particle counting. An optical particle counter will set an on-site laboratory back by roughly $15,000, and is absolutely money well spent, but only where it can be used as designed.

There are a few points to observe and rigorously adhere to. Firstly, samples must be well agitated. Hand-shaking is not enough – purchase a mechanical paint shaker to do the job, and perform the particle count immediately after agitation. Secondly, emulsified water in the oil will destroy a particle count - for some fluids as little as 0.03 percent water is enough to prevent an optical particle count from being performed at all, never mind allowing the rendering of any useful results. If you operate an on-site laboratory, perform water-masking techniques. If you use a commercial laboratory ask them to do the same, and do not quibble at paying a little extra – it’s more expense for them and the results will be worth it to you. Thirdly, expose the sample to an ultrasonic bath followed by a vacuum to remove air bubbles from the sample. Another hint: for viscous oils consider heating the sample in a laboratory oven. This will improve agitation and helps to remove air bubbles.

Diluting the sample with a dry super clean low-viscosity oil is another option. This practice also minimizes problems associated with emulsified water and sensor saturation. Don’t make the mistake of choosing the wrong option initially, as failure is virtually guaranteed along with the resulting mistrust and abandonment of this absolutely essential technique. Choose the best tool for the job, avoid the perils, and win the prizes.


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