Reducing Bubble Contamination in Particle Count Results Using Pressure

In fluid analysis programs, particle count data plays an important role. Wrong counts obtained from bubbles can result in downtime and increase costs. A particle counter provides data, which can be utilized to indicate the quality of hydraulic fluid employed in aircraft and to detect maintenance intervals for bulky instruments. Factors like reduced production time, human safety, and expensive physical assets make it necessary to have reliable and consistent data.

Drawbacks of Sampling Protocol

In order to ensure quality data, a sample prep SOP for specific particle counting applications, such as collection, degas, sample, and agitation, must be produced. However, all fluids contain bubbles, which can add counts to the data and counteract the efforts of getting reliable and consistent information. This holds true for even the most robust sampling protocols. Studies have shown that when a sample fluid contains bubbles, the data displays an irregular count distribution over a large number of channels.

During sample preparation, bubbles can be produced when the fluid is excited to convert particles into suspension. The particle counter itself can create bubbles. To further complicate the process, many instruments pull fluid via the wetted path and produce cavitation. This in turn produces small bubbles and results in false data.

Traditionally, an ultrasonic bath is used to remove bubbles from fluids during the sample preparation procedure, but variations in fluid viscosity affect the time taken to introduce the samples into the ultrasonic bath. Even pressure has the same effect on bubbles in suspension.

Liquid Particle Counters

The HIAC 8011+ liquid particle counter provides a suitable solution to overcome these issues. The Instrument not only enhances the sample prep SOP, but also improves the time needed to get reliable results. The HIAC 8011+ comes with a user defined setting that pressurizes the sample chamber to a required level and utilizes that pressure to push the sample via the particle counter sensor.

This unique process offers a number of benefits. For instance, this technique uses pressure as the sample delivery mechanism and prevents cavitation; when pressure is used, sample handling is reduced and results are obtained that are similar to the preferred method of employing an ultrasonic bath. After the required pressure is realized in the sample chamber, sampling starts automatically as per the sample recipe produced by the user.

Sample Preparation

8011+ system setup:

• Recipe name: Bubble Evaluation
• Number of samples: 3
• Sample volume: 5ml
• Reporting standard: Counts/ml
• Tare: 1.8ml
• Initial sample pressure: 40psi
• System pressure: 80psi
• Channel sizes: 2,3,4,5,7,12,14,21,25

To prepare a water sample, clean dry air is first passed through an unused sample bottle for 5 seconds. This bottle is filled with water till the shoulder and placed into the 8011+ instrument, and finally the "Bubble Evaluation" recipe is run. This baseline run is later saved to a USB drive.

A "bubble" sample is then prepared to run at 50psi sample pressure; however care must be taken to ensure that sample pressure is set to 50psi. A bottle of sample fluid is prepared by shaking it manually for 1 minute and then placing it into the 8011+ sample chamber. Next, the "Bubble Evaluation" recipe is run. This bubble run is later saved to the USB drive. The sample is again agitated for 1 minute, followed by degassing for 25 to 35 seconds. After placing the sample into the 8011+ sample chamber, the "Bubble Evaluation" recipe is run again. This degassed run is again saved to the USB drive. The procedure is repeated for 60, 70 and 80psi.

Results and Discussion

Results showed that increased pressure in the sample chamber resulted in decreased particle counts across all channel sizes (Figure 1). In case of samples where the ultrasonic bath was utilized during sample preparation process, the count data improved significantly (Figure 2). In particular, test data revealed that the application of ultrasonic bath had a major impact on count data across smaller channels (2,3,4µm).

Figure 1. Impact on particle count data with increasing chamber pressure. Image credit: Beckman Coulter

Figure 2. Impact on particle count data combining chamber pressure and ultrasonic bath. Image credit: Beckman Coulter

Comparison of the data given in Tables 1 and 2 showed that ultrasonic bath had no major impact on count data at channels greater than 5µm, than that of the data using pressure alone. The difference in count between these two techniques on the larger channel sizes led to an average differential of <10 counts/ml across these channels.

Table 1. Chamber pressure and ultrasonic bath count data.

Table 2. Chamber pressure count data.

As shown in Table 1, count difference was observed when pressure was raised from 50 to 80psi. When an ultrasonic bath was utilized, count reduced by 31.7% across all channels. Likewise, a sample was shaken and placed into the sampler. When pressure increased to 80psi, counts reduced by an average of 27.7%.

Conclusion

Micro-bubbles usually occur in samples after excitation which can considerably impact count data. In order to reduce the effects of bubbles, the HIAC was specifically designed to allow configurable levels of pressure in the sample chamber. Thus, this instrument can be used to obtain reliable and consistent data, irrespective of sample condition.

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.