To quantify a filter’s efficiency—its ability to retain contaminants—one may indicate the diameter of the largest rigid, spherical particle capable of passing through the filter, specifying the experimental conditions under which the filtration test was performed (flow rate, pressure conditions, etc.). To describe the filter’s performance, this particle size is then expressed in micrometres, a unit suited to the size of common industrial contaminants.
However, rather than providing a single cutoff value, manufacturers often report retention rates for different particle sizes, which are far more practical for real-world use.
The Beta ratio
The β ratio is calculated by comparing the number of particles of a given size present at the filter inlet (per unit volume) with the number of particles of the same size that pass through the filter:
β = number of upstream particles / number of downstream particles
Thus, if the filter retains one out of every three particles of size ≥ 10 µm, the filter’s β ratio will be 3. If only one out of every 300 particles passes through the filter, its β ratio will be 300. The higher the filter’s β ratio, the more efficient the filter.
Expert advice
Be aware that some manufacturers calculate this ratio not for a specific particle size, but for all particles larger than a given size, which inevitably results in a higher value.
Filtration efficiency
Rather than referring to the β ratio, many manufacturers directly indicate the retention efficiency of the filter. This value, expressed as a percentage, represents the proportion of particles retained by the filter. It is derived from the β ratio as follows:
E = ((β – 1) / β) × 100
For example, consider a filter receiving a liquid containing 5000 particles of size 2 µm per mL. After filtration, only 50 particles per mL remain. The β ratio is therefore 5000 / 50 = 100. Its efficiency at 2 µm is:
(4950 / 5000) × 100 = 99%
Here is a summary table:
| β ratio (pour une taille de particule donnée) | Proportion of particles passing through the filter | Efficiency (%) |
|---|---|---|
| 2 | 1 particule sur 2 | 50 |
| 10 | 1 particule sur 10 | 90 |
| 20 | 1 particule sur 20 | 95 |
| 75 | 1 particule sur 75 | 98.7 |
| 100 | 1 particule sur 100 | 99 |
| 200 | 1 particule sur 200 | 99.5 |
| 1000 | 1 particule sur 1000 | 99.9 |
Expert advice
Attention, certains fabricants calculent ce ratio non pas pour une taille de particule donnée, mais pour toutes les particules de diamètre supérieur à une taille donnée, ce qui conduit nécessairement à une valeur plus élevée.
A filter’s efficiency curve
The filter retains more than 99.9% of particles larger than 70 µm. It can therefore be used, for example, to achieve near-total filtration of particles with a diameter of 80 µm.
Let us consider the filter whose efficiency curve is shown below. It is clear that the filter’s efficiency depends on the particle size: the larger the particles, the more easily the filter retains them.
The filter can also be used to retain particles with a diameter of 50 µm, but in that case, the purification quality will be lower.
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