Why do you need to perform DOP leak test on ultra-clean bench high efficiency filter?

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Why is it necessary to perform a DOP leak test on an ultra-clean bench's high-efficiency filter?

1. Overview

If the high-efficiency filter or its installation has defects such as small holes in the filter itself or cracks formed during installation, the intended purification effect may not be achieved. Therefore, after installing or replacing a high-efficiency filter, it's essential to conduct a leak test on both the filter and its mounting connections.

2. Purpose of Leak Detection

The primary purpose of leak detection is to identify any leaks in the high-efficiency filter and its installation. This allows for timely corrective actions to be taken to maintain the integrity of the filtration system.

3. Scope of Leak Detection

Leak detection is typically performed in clean areas, laminar flow benches, and on high-efficiency filters installed on equipment. It ensures that the entire system remains free from contamination risks.

4. Leak Detection Methods

One commonly used method is the DOP (Di-Octyl Phthalate) leak test, which involves using DOP as a dust source and measuring the aerosol concentration with a photometer. Another method is the particle counter scanning technique, which uses ambient dust and measures it with a particle counter. However, the particle counter provides cumulative readings, making it less efficient for real-time scanning. Additionally, the low atmospheric dust concentration on the upstream side of the filter can make detection difficult unless smoke is introduced. This is why the DOP method is widely preferred in many countries due to its accuracy and efficiency.

5. DOP Leak Test Procedure

5.1. Working Principle

In the DOP method, DOP aerosol is used as a dust source on the upstream side of the high-efficiency filter. The air is then collected by a photometer on the downstream side. As the aerosol passes through the photometer’s diffusion chamber, the scattered light is converted into an electrical signal via the photoelectric effect and amplified linearly. This signal is displayed on a micro-ampere meter, showing the relative concentration of the aerosol. The DOP test essentially measures the penetration rate of the high-efficiency filter.

5.2. Materials and Instruments

- DOP solvent as the dust source - DOP generator to produce aerosol smoke under pressure or heat - Aerosol photometer to measure and display aerosol concentration (must be calibrated and within validity period) - Nitrogen gas - Epoxy resin silica gel for sealing - Spare high-efficiency filter of the required model

5.3. Leak Detection Steps

5.3.1. Preparation

Ensure all testing equipment is ready, and have the layout of the air ducts in the area to be tested. Coordinate with the HVAC company to handle tasks like sealing or replacing the high-efficiency filter on the day of the test.

5.3.2. Testing Process

1. Check the DOP level in the generator; add more if needed. 2. Connect the nitrogen bottle and turn on the temperature control until the indicator changes from red to green (around 390–420°C). 3. Attach one end of the test hose to the upstream port of the photometer and the other to the inlet of the high-efficiency filter. For system testing, introduce DOP directly into the air conditioning unit, avoiding contact with internal filters. If the generator is too small, place it in the static pressure box or fan inlet. 4. Turn on the nitrogen valve, set the pressure between 0.05–0.15 MPa, and slowly open the aerosol valve. Adjust the photometer reading to 10–20 once stable. 5. Move the sampling head across the downstream side of the filter and its frame, keeping a distance of about 3–5 cm. Scan at a speed below 5 cm/s, covering the filter media, seams, and seal areas. 6. If the penetration rate exceeds the acceptable limit, use epoxy resin silica gel to seal leaks or tighten bolts. Repeat the scan until the reading is within acceptable limits. 7. Document all findings and mark the sealing points clearly.

5.3.3. Acceptance Criteria

- For a 100,000-class clean room: filter efficiency must be >95% (photometer reading <5) - For a 10,000-class clean room: filter efficiency must be >99.97% (photometer reading <0.03)

6. Notes

6.1. DOP leak tests should be conducted after installation or replacement of high-efficiency filters. For clean areas above 10,000 classes, annual checks are typical, while septic areas require checks every six months. If there are significant abnormalities in dust levels, wind speed, or bacterial counts, a leak test should also be performed. 6.2. Although DOP contains a benzene ring and is suspected to be carcinogenic, alternatives like Emery 3004 are now available. The testing process remains similar, though the dust source differs.

This procedure is specifically designed for ultra-clean workbench high-efficiency filters. After thorough inspection, the filter can be safely put into service, ensuring strict control during production and testing.