What should I do if the air volume is unstable during ULPA leak detection?

Halfway through the leak detection, the anemometer reading kept jumping, and the readings at each location where the probe was scanned were different. At this time, I had no choice but to scan down, and the scanned data was useless.

If the air volume is unstable, there is no basis for leak detection. The leakage signal and operating condition fluctuations are mixed together, and it is unclear whether it is a filter element problem or a system disturbance. In this case, the correct approach is not to 'scan it and then talk about it', but to stabilize the working conditions first and then re-test. For the specific acceptance sequence, please refer to the relevant instructions on whether to adjust the air volume first or check for leaks first.

Where do common airflow disturbances come from?

The reasons for unstable air volume generally fall into the following categories at the scene.

The automatic speed control system operates frequently. The unit with constant air volume control will repeatedly adjust the speed when the pressure difference fluctuates. If the control parameter settings are unreasonable, or the resistance of the system itself is unstable, the rotation speed will continue to swing back and forth within a certain range, and the air volume will also change accordingly.

Valve or damper action. If there are multiple devices sharing the air duct in the system, the valve action of a certain device will affect the air volume distribution of the entire pipeline. If other equipment is running or switching during leak detection, it will be difficult for the reading to be stable.

Upstream static pressure changes. In a centralized air supply system, changes in the status of the air conditioning box or other units will affect the terminal static pressure. The static pressure is unstable, and the terminal air volume also changes.

Doors open or people move around. The door opening and closing of the clean room and the entry and exit of personnel will disturb the local air flow. Device-side applications such as EFU equipment-end fan filter unit , the opening and closing of the equipment door panel has a more direct impact on the internal air flow.

Problems with the instrument itself. The anemometer response speed, sampling frequency and reading method will also affect the observation results. If the instrument itself responds slowly or the readings fluctuate greatly, it may be mistakenly judged as an unstable system.

How to operate under stable working conditions

Once the source of the disturbance is found, corresponding measures can be taken.

Temporarily fix control parameters. If the automatic speed control system operates frequently during leak detection, the speed can be fixed near the design conditions in test mode. Return to normal control after leak detection is completed. This operation needs to be confirmed not to affect other functions of the device.

Pause operation or switching of adjacent equipment. If there are other devices in the system that may interfere with airflow, try to keep them in a stable state during the leak test. Necessary switching operations can be completed before and after leak detection.

Wait for the system to stabilize before starting. After the air volume is adjusted, let the system run for enough time to confirm that the readings no longer drift. Systems with large fluctuations in air volume may need to wait longer.

Confirm door and passage status. During the leak detection period, try to keep the clean room door closed and personnel activities reduced. The equipment-side application must confirm that the equipment door panels and inspection openings are normally closed.

Use appropriate instruments and methods. The response speed and sampling method of the anemometer must match the measurement purpose. If the readings fluctuate greatly, you can use the average value or a measurement method with a longer stabilization time. For specific measuring point layout, please refer to the relevant instructions for differential pressure measuring points.

How to retest after stabilization

After the working conditions stabilize, rescan under the same conditions as the initial inspection.

Record the working conditions during inspection. Air volume, rotation speed, valve opening, and operating mode must be recorded in the leak detection record. During subsequent re-inspections or comparisons, this information is the basis for judging whether the conditions are consistent.

Cover the affected area. Locations with abnormal readings due to unstable air volume need to be rescanned. If the air volume keeps fluctuating during the entire testing process, it is recommended to retest the entire unit or section.

Keep the original data. Do not delete the readings during the fluctuation period. Note the measurement conditions as a reference. A low reading during a period of fluctuation cannot be directly regarded as qualified evidence.

Make a decision after completing the retest. Only the results measured under stable working conditions have the basis for judgment. involving For HEPA/ULPA air filter products of different grades, the test methods and judgment standards are implemented in accordance with the corresponding requirements. Supporting For the FFU fan filter unit system, the unit gear and air volume records must be kept together with the leak detection records.

Debugging and formal testing should be separated

The equipment debugging stage and the formal acceptance stage have different requirements for air volume stability.

A certain degree of fluctuation can be allowed during the debugging phase , with the purpose of finding suitable operating parameters and troubleshooting system problems. The leak detection data at this time is used as a reference and not as a basis for final judgment.

Formal testing is required to be conducted under working conditions that comply with the plan. Conditions such as air volume, temperature, and upstream concentration must be stable for the test results to be valid for judgment.

The two cannot be mixed. Submitting data during debugging as formal test results, or using formal test requirements to constrain the debugging process will cause unnecessary trouble. The nature and working conditions of each inspection should be clearly marked in the records.

Project judgment example

A clean room was undergoing ULPA on-site leak detection. Halfway through the scan, it was found that the anemometer reading had been fluctuating between 160 and 210 m³/h. During the shutdown investigation, it was found that the valve of another device on the same pipeline was automatically adjusting, causing the air volume in the pipeline to continue to change.

Treatment method: Pause the automatic adjustment of adjacent equipment, switch the system to a stable operating state, and wait until the air volume stabilizes at around 190 m³/h before starting leak detection. After rescanning, the readings were stable, and two interface leaks were found. After treatment, the retest passed.

If the air volume fluctuations are not dealt with at that time and the sweep is completed directly, the two leaking points may be covered up by the fluctuations, or the problem may not be discovered until retesting. involving ULPA ultra high efficiency filter or For leak detection of HEPA high-efficiency filters , the importance of stable working conditions is the same.

Selection and Acceptance Checklist

About Aili Industrial’s leak detection working condition cooperation

Whether leak detection can be performed smoothly is directly related to the system control method and equipment operating status. If these conditions are considered clearly at the planning stage, there will be fewer detours on site.

Aili Industry has independent manufacturing capabilities in filters, fans, control systems and sheet metal processing, and can provide ULPA ultra high efficiency filter、HEPA high-efficiency filter and supporting air supply equipment. When the filter is matched with equipment such as FFU and EFU, the air volume target, control method and interface conditions can be uniformly confirmed during the planning stage. For projects that require on-site leak detection, we can assist in clarifying test conditions and recording requirements to reduce repeated testing caused by air flow fluctuations.

Explaining the acceptance method, control method and operating conditions during the inquiry will help to consider the test conditions together during the planning stage. The specific testing methods and judgment standards are subject to the project plan.