What are the initial resistance and final resistance used to judge?

There are usually two resistance data on filter samples: initial resistance and final resistance. The initial resistance describes the pressure drop of the new filter under the specified air volume, and the final resistance is used to judge maintenance or replacement during the agreed use period. Many sites only remember the final resistance number and regard it as the only replacement standard. However, in actual maintenance, the filter life is not only determined by the pressure difference, but also affected by damage, moisture, process risks and actual air supply capacity.

The following will start with the initial resistance purpose, final resistance threshold, pressure difference monitoring, and abnormal judgment.

Initial resistance helps check the equipment working point

The initial resistance is the pressure drop of the new filter under the specified air volume, and is an important input for fan selection and initial commissioning. When selecting, the initial resistance of the filter at the target air volume should be confirmed and included in the calculation of the total resistance of the system.

If the initial resistance data is inconsistent with the actual air volume of the project, you should check the resistance curve at the corresponding air volume instead of using the sample value directly. For specific methods, please refer to How does the filter's rated air volume match the actual operating air volume? and Why should we look at the complete performance curve when selecting FFU?。

When comparing different models, air volume, efficiency level and test caliber should be unified. If you only compare a resistance number without looking at the corresponding working conditions, you may choose an unsuitable model. For the relationship between filter thickness and resistance, please refer to Does increasing filter thickness definitely reduce resistance?。

The final resistance is used to judge whether to agree on maintenance or replacement.

The final resistance is the pressure drop when the filter is used to an agreed stage, usually recommended by the manufacturer or agreed upon by the project. When the final resistance is reached, replacement or maintenance is generally considered. However, the final resistance is not the only life standard, and the actual judgment must also be combined with:

• Whether the filter is damaged or damp;

• Whether the seal has failed;

• Whether the downstream particle concentration is abnormal;

• Whether the system air volume still meets the requirements;

• Are there any special risks associated with the process?

If the filter has not reached the final resistance, but has become damp, damaged, or failed leak detection, it cannot continue to be used because the pressure difference is still low. For related troubleshooting, please refer to What should be checked when cleaning equipment is accepted?。

The final resistance threshold needs to be connected with the system capability

The replacement threshold should refer to the manufacturer's information and project requirements to confirm that effective air supply can still be maintained when approaching this value. If the system has insufficient fan margin, the air volume may have dropped to an unacceptable level before the final resistance is reached. At this time, you should replace it in advance, or choose a model with lower initial resistance and larger dust holding capacity.

When selecting the model, the operating points at the initial resistance and final resistance should be checked to ensure that the air volume meets the requirements during the entire use cycle. For details, please refer to How does insufficient filter area affect fan selection? and What will be the impact if the wind speed facing the filter is too high?。

Location and methods of differential pressure monitoring

Differential pressure monitoring is the main means to judge the status of the filter. When measuring, please note:

• The pressure sensing position should reflect the true pressure difference before and after the filter;

• Record the air volume and speed to avoid comparison under different working conditions;

• After the new filter is installed, record the initial pressure difference as a baseline;

• Record the pressure difference trend regularly instead of just looking at a single value;

• Multiple filters are recorded separately to avoid average masking of anomalies.

If the pressure difference suddenly drops, check for damage or bypass; if the pressure difference rises rapidly, check the pollution load or pre-filtration status. Related analysis can refer to What particle problems can airflow short circuits cause?。

A sudden drop in differential pressure is not a performance improvement

A sudden reduction in pressure difference during operation does not necessarily mean that the filter becomes more breathable. Common causes include damaged filter material, failed frame seal, loose installation interface, or open bypass. At this time, unfiltered air may directly enter the clean side, and the particles on site will increase instead.

If the on-site particles are abnormal but the filter efficiency report is qualified, the sealing and damage should be investigated first. For details, please refer to Does higher filtration efficiency necessarily lead to improved site ratings? and How to read the test particle size in the filter efficiency report?。

The relationship between initial resistance and final resistance and operating cost

Filter resistance affects fan power. A filter with low initial resistance and slow arrival of final resistance may have lower energy consumption in long-term operation. But when choosing, you should not just look at the resistance, but also check the efficiency, dust holding capacity, sealing method and replacement cost.

When comparing operating costs, factor in electricity costs, filter costs, replacement labor and downtime arrangements. Insufficient maintenance channels will increase subsequent expenditures. This is reflected in what later expenses will be increased due to insufficient maintenance channels . There is a more detailed analysis of

Differences in resistance between different filtration levels

The initial resistance and final resistance of H13, H14 and U15 may be different under the same air volume. The selection should be based on fan capacity and replacement cycle. For details, please refer to How to choose between H13, H14 and U15? and How do HEPA and ULPA match cleanliness goals?。

If the on-site particles are abnormal but the filter efficiency is acceptable, the sealing and airflow should be prioritized instead of continuing to upgrade the filter. Related analysis can refer to Does higher filtration efficiency necessarily lead to improved site ratings?。

Equipment selection and terminal matching

Filter selection should correspond to target particle size and actual resistance.The HEPA and ULPA filter pages have parameters for different levels of products. When selecting, check the efficiency of the specific model at the target air volume.

Please refer to the room background air supply FFU clean air supply products , please refer to the local protection of the equipment side. EFU equipment side fan filter unit . The logic of division of labor between the two lies in The division of labor between device-side EFU and room FFU is explained in detail.

If multiple devices require partition duty and troubleshooting, they can be combined FFU group control system provides unified management.

Project judgment example

The filter of a certain project did not reach the final resistance, but the particles on site continued to be high. The investigation found that the frame sealing strip was aging and the bypass leaked. Adjustment method: Replace the sealing strip and recheck for leaks instead of waiting for the pressure difference to reach the final resistance. On-site particle restoration after adjustments.

In another project, the filter pressure difference rose rapidly and exceeded the final resistance in advance. The investigation found that the upstream pre-filtration failed and a large amount of dust entered the high-efficiency filter. Adjustment method: Replace the pre-filter and optimize the maintenance cycle, and check whether the high-efficiency filter is damaged. After adjustment, the pressure difference trend resumed.

These two situations illustrate that: the initial resistance and the final resistance are used for different judgments. The final resistance is not the only replacement standard. The pressure difference trend, sealing status and on-site particle results need to be looked at together.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

Initial resistance and final resistance are not just two sample numbers, they are related to fan selection, filter replacement cycle and long-term operating energy consumption. Aili Industrial has collaborative capabilities in air filter manufacturing, independent research and development of fans, control systems and complete machine assembly. According to the project target air volume and allowable resistance range, it can provide working point data of different models under initial resistance and final resistance, combined with pressure difference monitoring and group control records, to help determine whether the replacement timing and fan margin match. Specific filter parameters can be viewed HEPA and ULPA filters , the complete machine package can be viewed FFU clean air supply products and EFU equipment side fan filter unit。