Ensuring Accurate Particle Counting via Correct Sample Preparation

The key contributor to an inaccurate particle counter sampling result is incorrect sample preparation. Even though in industrial areas such as sample testing laboratories, this fact is accepted, the general population misunderstands it. When the sample is not prepared properly, clean fluid may seem contaminated or highly contaminated samples may look clean.

Impediments to Proper Sampling

The impediments to proper sampling are:

• Contaminated collection vessel – Accuracy is compromised when the vessel used for sample collection is contaminated. One may initially use a clean vessel, but subsequently may use the same vessel repeatedly.
• Improper agitation for suspension of resident particles in the sample – There is a difference in the time taken and method used based on the contamination type, mass, fluid type and the time period wherein the sample has been static.
• Inadequate Degassing – Little or no degassing is done to remove the entrapped air after proper agitation. Degassing can be done by vacuum or with an ultrasonic bath.
• Allowing degassed and agitated sample to stand for some time – The myth presently believed is that by allowing the sample to set for an extended time period after agitation, the sample will degas on its own. However this is not true as gravity is at work and settling down of particles will make the sample look cleaner than it is.
• Using a high-speed magnetic stirrer – In this case, little stirring is fine, however too much speed while stirring causes the particles to drive out of the suspension to the vessel’s inside wall removing them from the sampling path. Also, in case ferrous metals attach themselves to the stir bar, it must not be used for that sample fluid or the reported results will be skewed.

Basic Process for Most Petroleum Fluid Preparation Situations

The basic process to be followed is as follows:

• Collect the sample in a clean vessel with a clean cap. This is very important. After sampling, clean the collection bottle thoroughly before using it again for another sample or you can discard the first vessel after use and get a new vessel.
• The sample needs to be agitated ultrasonically to disintegrate agglomerated particles for 1min. The ultrasonic bath fluid level must be halfway up the sample bottle. 1min is sufficient for most sampling scenario, however if the fluid is viscous this time must be extended to meet fluid needs.
• The sample must then be hand-shaked for 2 minutes. In case the sample has been static for an extended period or is of a higher viscosity, add an additional time. ISO recommendations are minimum of 1min on a mechanical shaker (ISO 11171 Annex E, Section E4, 2010)
• Degas the solution in an ultrasonic bath. Again, the ultrasonic bath fluid level should be halfway up the sample bottle. Degassing is done for 25-35s for typical hydraulic fluids with a viscosity range of 10-50 cSt. For fluids with viscosities more than 50cSt, the degassed time may need to be extended in order to remove all entrapped air.
• Allow the sample to stand for 5 to 10s only to ensure that there are no visible bubbles and then immediately introduce to the counter and begin sampling to prevent settling of larger particles.
• Additionally, Particle Counting systems such as the HIAC™ PODS and HIAC™ 8011 employ pressure to move the sample through the flow control and counting elements. While this pressurization is not an alternative for degassing a sample, it does provide a secondary method to further remove any residual air bubbles from the sample.

Figure 1. HIAC PODS System. Image credit: Beckman Coulter

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Figure 2. HIAC 8011 System. Image credit: Beckman Coulter

Conclusion

To help keep the 50 µm or larger particles suspended especially during measurement of larger sample volumes, stir bars can be used. The stir bar must be essentially particle free and stirrer speed slow enough to prevent the centrifuge effect and yet rapid enough to suspend the contaminants to be measured. Similar to all sustainable and robust procedures, variables may need to be minimized. This article aims at enabling one to have all the needed steps for the process and following those steps in the right sequence.

This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.

For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.