Monitor Particles On-Site with Low-Cost Patch Microscopy

Drew Troyer

The very best oil analysis programs incorporate some degree of on-site analysis. For most plants, mills and mines, particle monitoring is the most productive on-site oil analysis activity available. Studies show that particle contamination is responsible for up to 80 percent of lubrication-related machinery failures. Hard particles can enter the system through seals, breathers, new oil changes, top-offs and internal generation. Together, they abrade, fatigue and erode component surfaces - robbing it of precious life (Figure 1).

Because particles cause so many problems, they should be monitored regularly and controlled by filtering the oil or changing as required. Particles are also generated when a machine fails. Monitoring for increased generation of wear debris alerts the technician of an impending failure so that corrective action can be planned. The simple patch test enables the technician to quickly identify abnormal levels of contamination and wear debris so that corrective actions can be scheduled accordingly.

While several excellent automatic particle-monitoring devices (particle counters and wear debris meters) are available, it is possible to try on-site particle monitoring without spending a great deal of money using low-cost patch microscopy. This helps determine if on-site particle monitoring suits your needs. It also provides needed experience with particle levels so you can determine if a more sophisticated particle-monitoring device is needed, and if so, to buy the one that best suits your needs. The procedure for performing low-cost patch microscopy follows.

You will need a few supplies for this test:

  • Patch-making funnel (Table 1)
  • Oil sampling vacuum pump
  • 25 mm filter disc
  • Tweezers
  • Filtered mineral spirits or reagent-grade kerosene (other solvents may be suitable too)
  • 100X or better microscope. The one shown in this article can be purchased at Radio Shack for less than $20. Other models are also available at a low cost.
Table 1. Resources for Patch-Making Funnel
EasyVac Inc., Knoxville, TN, 423-691-7510
Gorman Industries, Cedar Hill, TX, 972-723-8318
Gelman Sciences, Ann Arbor, MI, 800-521-1520

The Patch Test Kit for Particle Contamination Procedure

Step 1
Prepare a filter patch for viewing under a microscope.
A. Insert patch-making funnel into the top of the vacuum pump. Tighten the seal by tightening the knurled fitting.   F. Draw the diluted sample through the filter by activating the vacuum pump until the funnel is empty.
B. Remove funnel from white base by turning it counter- clockwise.  

G. Fill the funnel to the 25 ml mark with mineral spirits dispensed through a filter installed on the bottle.

Draw the rinse fluid through the filter by activating the vacuum pump until the funnel is empty.

C. Using tweezers, carefully place 25 mm filter disc onto the screened surface of the funnel’s interior. Return funnel to the white base.  

H. Using the tweezers, remove the patch and place it on a clean surface to dry for 10 minutes.

Rinse the interior of the funnel with filtered mineral spirits. Dry and carefully put it away for future use.

D. After aggressive agitation of the sample, pour oil into the funnel precisely to the 25 ml mark.  

Step 2 Remove the base from the 100X microscope.

Step 3 Switch the microscope’s light to the ON position.

E. Fill the funnel to the 50 ml mark with mineral spirits dispensed through a filter installed on the bottle.   Step 4 View the dry filter patch under the microscope and focus the image.
  Step 5 Compare the sample patch to standard visual and microscopic images (Figures 2 through 4). It will be necessary to create standard images that reflect your specific target cleanliness requirements. Then determine condition using the Particle Contaminant Diagnosis Table.  

Patch Comparison Images

The example images below are an application-specific representation of caution, severe and critical ratings and don’t apply to all machinery.

Figure 2. Caution - This patch shows a generally high level of contamination. The presence of the shiny particles suggests some wear is occurring. This machine requires attention to avoid a severe condition that could lead to poor operation or failure.
Figure 3. Severe - This heavily loaded patch indicates a very high dirt load. The presence of numerous shiny particles suggests a concerning wear trend. Action should be taken immediately before catastrophic failure. The machine may already be showing signs of operational failure.
Figure 4. Critical - This dense patch is loaded with dirt. The heavy load of shiny particle suggests a critical situation. Operational failure is probably occurring and catastrophic failure is likely.

Table 2. Particle Contaminant Diagnosis
Sample patch has fewer particles than the standard reference patch marked “Caution”
Sample patch has about as many particles as the reference patch marked “Caution,” but fewer than patch marked “Severe”
Sample patch has about as many particles as the reference patch marked “Severe,” but fewer
particles than the reference patch marked “Critical”
Sample patch looks like the reference patch marked “Critical” or worse

Step 6
If the contamination is in a caution, severe or critical condition, diagnose according to the particle contamination analysis flow chart.

Step 7
Record findings, comments and recommended actions on the oil analysis summary report.
On-site oil analysis and particle monitoring are both critical to oil analysis success. This simple test enables you to try both with little investment so you can prove the concept before upgrading your program with more expense equipment. The technique is easy, effective and economical. Give it a go!

Click Here to See Flow Model for Patch Test Diagnostics Patch123 patch123 patch 123

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