Filter Beta Ratio

Table of Contents

The topic of filter testing and filter ratings is critical in lubricant contamination control. Effective filters prevent the accumulation of dust particles which are the major contaminants in lubricants and lubricating systems. These contaminants can also affect the sensitive parts of machines and equipment, decreasing the effectiveness of the facility’s lubrication program. Understanding filter beta ratio and other filter ratings help maintenance managers decide on suitable filter types for filtration systems. Filter testing results in a rating called the filter beta ratio, which identifies their effectiveness against different sizes and types of particle contaminants. 

What is Filter Beta Ratio?

Filter Beta Ratio is considered the best and most commonly used rating for liquid filters. A filter’s beta ratio describes its performance and is tested using the ISO (International Organization for Standardization) 16889:1999 or the Multipass Method for Evaluating Filtration Performance of a Fine Filter Element. This ISO method lists eight beta ratios used in reporting filter performance, namely 2, 10, 20, 75, 100, 200, 1000, and 2000.

To understand these numbers, we first look at the testing method. This method involves using test fluids with a known quantity and size of particles that are allowed to pass through the test filter. Particle counters determine the number of particles in the fluid before and after the filter. 

From the particle counts, the filter efficiency can be calculated as the ratio of the pre-filter particle count to the post-filter particle count. The filter efficiency provides the filter beta ratio. A filter efficiency of 50% means that 1 out of 2 particles passed through the filter, corresponding to a beta ratio of 2. A filter efficiency of 90% means that 1 out of 10 particles passed through and the beta ratio is 10, and so on. 

Nominal Rating and Absolute Rating

Before the ISO 16889:1999 standard testing, the commonly used rating methods were nominal rating and absolute rating.

Nominal rating is the rating of the filter manufacturer that indicates the maximum size of particles that will pass through a filter. A nominal rating of 10 microns suggests that the filter should prevent the passage of anything larger than this size. But testing and actual use have proven that nominal ratings are not accurate.

Absolute rating is much more helpful and accurate than a nominal rating. Absolute rating indicates the largest spherical particle that can pass through the filter and is based on the largest pore or opening on the filter. But, there are no available tests to prove the actual effectiveness of the pore from keeping out larger particles.

Evaluating Filter Beta Ratio

With the development of standard filter testing, evaluating filters is as easy as comparing beta ratios. The rule here is the higher the beta ratio, the higher the efficiency of the filter in keeping out the particles. Also, a filter can have different beta ratios for different size particles. A filter with a beta ratio of 100 at 1 micron (1 micron = 1 millionth of a meter) will not have the same beta ratio at 2 microns. When comparing filters, the beta ratios under comparison must be based on the same particle sizes, i.e., microns.

To illustrate, if filter A has a beta ratio of 2 at 5 microns and B has a beta ratio of 200 at 5 microns, B is a much better choice. It will be difficult to identify which is better between A and B if both have the same beta ratio of 10, but A is tested using 10 microns while B used 5 microns. One can argue that B is more effective because it can trap smaller particles at the same rates as A, but this inference is not 100% correct until tested.

Factors Affecting Filter Beta Ratio

When evaluating filter performance based on the filter beta ratio, we recommend considering the following factors.

Fluid Surges or Rate of Flow

The performance of a filter can change with the rate of flow or the presence of fluid surges. The faster flow rate causes particles to push through the filter and result in more contaminants. Surges can also damage filters and similarly increase the contaminants.

Fluid Temperature

The fluid temperature affects the flow rate and, thus, the beta ratio and filter performance. Low temperatures result in slower flow, and high temperatures result in faster fluid flow.

Dirt-holding Capacity

The ISO 16889:1999 standard testing does not account for the number of dirt particles the filter can hold. A filter’s dirt-holding capacity can affect its performance over time.

The Importance of Filter Beta Ratio in Lubricating Systems

Filter beta ratios help you decide on the best filter for the best lubricating results. And with the right lubrication management software, you can track the actual performance of whatever filter you decide. The combination helps take the guesswork out of filter and lubrication performance.

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