To determine whether the supply air ceiling filter (with the core being HEPA/ULPA high-efficiency filters, and some including pre-filters for primary and medium-efficiency filters) needs to be replaced, it is necessary to combine three core dimensions: differential pressure monitoring, performance testing, and intuitive observation. At the same time, the usage environment and time period should also be taken into account. The following are the directly applicable judgment criteria and practical operation methods It is completely in line with the daily operation and maintenance scenarios of cleanrooms
I. Core Judgment Basis: Differential Pressure Monitoring (the most direct and commonly used)
The pressure difference change of the filter is a core indicator reflecting the degree of clogging. It needs to be monitored in real time through the pressure difference gauge (or an independently installed pressure difference transmitter) that comes with the equipment. The specific judgment criteria are as follows:
1. Pressure difference judgment of high-efficiency filters (HEPA/ULPA)
Basic standard: When the measured pressure difference reaches “twice the initial pressure difference”, it must be replaced.
For example, the initial pressure difference of the new HEPA filter is 100Pa. If the pressure difference rises to 200Pa or more after operation, it indicates that the filter material is severely clogged and the air flow resistance is too large. Continuing to use it will cause the fan to overload, the air supply efficiency to decline, and even affect the cleanliness.
Design upper limit standard: If the pressure difference reaches the upper limit of the pressure difference designed for the equipment (industry norm: HEPA filter ≤250Pa, ULPA filter ≤300Pa), it must be replaced immediately regardless of whether it has reached twice the initial pressure difference.
(Note: Some high-end equipment is equipped with a differential pressure alarm threshold. Once the alarm is triggered, it is necessary to check and replace it immediately.)
Key points of practical operation
The pressure difference monitoring should be read after the fan has been running stably for 10 minutes (to avoid the interference of instantaneous pressure difference).
If the pressure difference suddenly drops (for example, from 180Pa to 50Pa), it may be due to the damage of the filter material or the failure of the installation seal. Other methods should be combined for verification instead of simply waiting for the pressure difference to rise.
2. Pressure difference judgment of the pre-filter (primary/medium)
The air intake side of the supply air ceiling is usually equipped with primary (G3/G4) or medium (F5/F8) pre-filters, whose function is to protect HEPA/ULPA. The replacement standard is more lenient:
Primary filter: Replace when the pressure difference reaches 50-80Pa (or twice the initial pressure difference), with a recommended cycle of 1-3 months.
Medium-efficiency filter: Replace when the pressure difference reaches 100-150Pa (or twice the initial pressure difference), with a recommended cycle of 6-12 months.
If the pre-filter is not replaced for a long time, it will cause the HEPA/ULPA pressure difference to rise rapidly, shortening its service life. Therefore, it is necessary to monitor the pressure difference of the pre-filter simultaneously.
Ii. Key Verification: Performance Testing (Confirming Filter Failure/Contamination)
When the pressure difference approaches the threshold or an abnormality occurs, it is necessary to verify through performance testing whether the filter really needs to be replaced to avoid misjudgment.
1. Cleanliness detection (particle concentration exceeding the standard)
The particle concentration is detected 1 meter below the supply air ceiling using a laser particle counter (points are arranged according to the ISO 14644 grid method). If the following conditions are met, it indicates that the filter has failed:
Class A/Class 100 clean area: Particle concentration of ≥0.5μm > 3520 /m³, or particle concentration of ≥5.0μm > 29 /m³;
Class B/Class 1000 Clean area: Particle concentration of ≥0.5μm > 35,200 /m³, or particle concentration of ≥5.0μm > 290 /m³;
(Other cleanliness grades refer to the corresponding ISO 14644-1 standard.)
The core reasons for the excessive particle concentration are: clogging of the filter material (insufficient airflow leads to a decline in purification capacity), damage to the filter material (failure to intercept particles), and poor installation and sealing (air leakage).
2. Filter leak detection (PAO aerosol method)
Applicable scenarios: When the pressure difference frequently decreases, the particle concentration exceeds the standard, and there is suspicion that the filter is damaged or the seal has failed;
Operation method: Use the PAO aerosol generator to release PAO smoke to the air intake side of the supply air ceiling. Scan and detect on the air outlet side of the filter (or below the supply air panel) with a photometer. If the leakage rate is greater than 0.01% (for HEPA filters) or greater than 0.001% (for ULPA filters), it indicates that the filter is damaged and needs to be replaced immediately.
3. Abnormal air flow velocity
If the average air velocity on the supply ceiling is lower than 80% of the designed value (for example, the designed air velocity in the operating room is 0.25-0.35m/s, and the actual measured air velocity is lower than 0.2m/s), and still fails to meet the standard after frequency conversion regulation of the fan, and the pressure difference is close to the threshold, it indicates that the filter is severely clogged and needs to be replaced.
Iii. Auxiliary Judgment: Intuitive Observation and Usage Cycle
1. Intuitive observation (Quick troubleshooting
After disassembling the air supply panel, observe the surface of the filter: if the filter material is obviously black or yellow, or if there is dust accumulation and lumps, it is recommended to replace it in advance even if the pressure difference has not reached the threshold (especially in scenarios with high hygiene requirements such as pharmaceuticals and food).
Check the sealing rubber strip of the filter frame: If the rubber strip ages, cracks or falls off, causing a gap between the filter and the installation frame, air leakage will occur. The filter needs to be replaced simultaneously (or the rubber strip can be replaced separately, but it is recommended to replace the entire filter first to ensure the sealing effect).
2. Service life reference (catch-all standard
Even if the pressure difference and performance tests are normal, the filter material should be replaced regularly according to the service cycle to avoid aging and failure.
HEPA filter: Replace it every 1-2 years under normal use. For high-frequency usage and high-pollution scenarios such as operating rooms and biosafety laboratories, the period has been shortened to 6 to 8 months.
ULPA filter: Replace every 1.5 to 2.5 years under normal use.
Pre-filter (primary/medium efficiency) : Replace strictly every 1-3 months (primary) and 6-12 months (medium efficiency), without waiting for the pressure difference to meet the standard.
Iv. Common Misjudgment Scenarios and Avoidance Methods

Summary: Filter Replacement Judgment Process (Practical Steps)
Daily monitoring: Check the differential pressure gauge every day and record the initial differential pressure. When it reaches “twice the initial” or “the design upper limit”, mark it as ready for replacement.
Performance verification: Use a particle counter to detect the cleanliness. If it exceeds the standard, conduct PAO leak detection simultaneously.
Visual confirmation: Disassemble the panel to observe the contamination on the filter surface and the condition of the rubber strip.
Cycle guarantee: Even if the pressure difference and performance are normal, it is still necessary to force replacement at the prescribed cycle (HEPA not exceeding 2 years).
Through the above judgment logic of “pressure difference as the main, performance as the secondary, and cycle as the bottom line”, the timing of filter replacement can be precisely controlled. This not only avoids premature replacement causing waste but also prevents late replacement from leading to substandard performance of the clean room and equipment overload damage.