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Aerosol Diluters for Filter Testing and High Particle Concentrations

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iq air laser particle counters experience and knowledge is backed by years of professional work in clean room and indoor air quality industry

Are You Worried Your Particle Counter May Be Reporting Inaccurately?

> Are you performing filter testing?

> Is your cleanroom ISO class 8 or ISO class 9?

> Are you using a cleanroom particle counter to perform sampling in an IAQ or non-cleanroom environment?

> Do you think you may be exceeding the concentration limit of your particle counter?

If you answered YES to ANY of these questions, you are at high risk of recording bad data due to coincidence counting!  Consult with an expert to find out if you need an aerosol diluter for your application.  Call 1-800-531-4889.

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Proper Filter Integrity Testing Requires Aerosol Dilution
During the 1980’s, miniaturization in the semiconductor industry required very clean, particle free cleanrooms to be developed. It was critical the HEPA filters and ULPA filters in the cleanrooms be free of pinhole defects that would allow contamination into the process. The traditional method to determine filter integrity required challenging the filters with a high... [More Information]

Aerosol Diluter by Milholland and Associates

Product Pros Cons Features Application MSRP
Aerosol Diluter Model 450 AD
Model 1000 AD

• Simple to operate

• Portable

• No Power Source Needed

• Comes with all necessary tubing

• Unattractive Casing • Particle Dilution of 450:1 for Model 450AD

• Particle Dilution of 1000:1 for Model 1000AD

• NIST Traceable Calibration

Filter testing

Aerosol Research

Cleanroom

Pharmaceutical

Aerospace

Semiconductor

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concentration of DOP (oil) droplets and then scanning the downstream filter face (surface) with an aerosol photometer. Leaks were determined to be any defect resulting in a reading greater than 0.01% of the challenge concentration.

As the semiconductor geometries decreased, airborne molecular compounds (AMC’s) became a contaminant. One source of AMC’s was the oil used as a challenge aerosol for filter integrity testing. In the late 1980’s a new process was developed that used polystyrene latex spheres (PSL) as a challenge aerosol and the particle counter as a detection device. This process used considerably less total aerosol challenge than the photometer method. With only 1x10-3 of the mass of the DOP aerosol and a more stable latex sphere, AMC’s from filter integrity testing were no longer a problem.

Need for Aerosol Dilution Device
A major obstacle that had to be overcome was measuring the PSL aerosol challenge concentration. Aerosol particle counters are designed to measure individual particles and are limited to approximately 5x104 to 1x105 particles per minute, depending on the counter design. Thus, direct sampling is limited to a maximum of up to 100,000 particles per cubic foot for a counter with a 1.0 cfm sample rate or 1,000,000 per cubic foot for a counter with a 0.1 cfm sample. Coincidence counting occurs when concentrations exceed these limits. The counting errors can be due to both optical and electronic coincidence. Simply stated, coincidence occurs when two or more small particles pass through the laser beam at the same instant and the counter counts them as only one larger particle. As a result, the true concentration of the challenge aerosol is always greater than that reported by a counter exhibiting coincidence counting.

Filter integrity testing requires aerosol challenge concentrations from two million (2 x106) to thirty million (3x107) Particles per cubic foot. This is 20 to 3,000 times the upper limit of the aerosol particle counter with a 1.0 cfm sample rate. The challenge concentration must be known to determine the maximum scan speed and to quantify the size of a defect. Therefore, it is necessary to have a simple reliable means to accurately dilute the aerosol to a concentration that can be measured accurately with the particle counter.

The dilutor mixes a known quantity of clean air with the aerosol sample. By knowing this ratio of clean air, a “dilution factor” is calculated. The actual particle counter display is multiplied by this factor to determine the actual aerosol concentration.

Dilution devices with factors of 500:1 are typically used to allow the particle counter to accurately measure concentrations up to 50 million (5x107) Particles per cubic foot. (100,000 x 500 = 50,000,000).

Evolution of the Dilutor
Biocon, Inc. of Raleigh, NC began using simple dilutors in the late 1980’s. Revisions of the early dilutors lead to the basis of the current capillary tube technology. The capillary is simply placed in the sample line between the particle counter and the aerosol to be measured. ULPA filtered air is used as a carrier for the particles passing through the capillary. These units have proven to be very durable and rugged for field use. The dilutors do not require compressed air or electrical power.

Aerosol diluter extends use of particle counters beyond concentration limitsPrinciple of Operation
The volumetric flow through a capillary tube is a function of the pressure differential across the tube. Thus any changes in pressure will result in a change in the aerosol sample volume passing through the capillary. The plenum pressure has a direct effect on the capillary pressure and thus aerosol flow and thus the dilution factor. The pressure across the capillary is measured via a Dwyer Magnehelic gauge. The resulting dilution factor for the corresponding capillary pressure for each specific application is referenced on a dilution factor chart.

Particle Counter Airflow
It is not necessary for the particle counter to sample exactly 1.0 cfm of air through the dilutor. The counter is only counting particles that have passed through the capillary or about 0.002 cfm. The remainder of the sample air is clean filtered air and has no effect on determining the final concentration. Thus, the particle sample flow rate can be adjusted to obtain a wide range of pressure across the capillary and thus a variable dilution factor range.

However for maximum particle counter accuracy, you should adjust the flow on the particle counter to 1.0 cfm with the dilutor attached to then sample tubing. The counter was calibrated at this flow. If adjusted for the resistance of the dilutor, one must reset the counter to 1.0 cfm sample flow rate once the dilutor is removed from the inlet sample line of the counter.

Diffusion Loss
It should be noted that particles in the size range of 0.1 um to 2 um are easily sampled with virtually no sample line losses. Smaller particles will be lost to diffusion and larger particles will be lost to sedimentation in the sample tubing. It is always best to keep the inlet sample line to the dilutor (1/4” OD tube) to the same length it was at the time of calibration of the dilutor. The tubing should be as short as possible with a small internal diameter to minimize losses. Large diameter and / or longer inlet tubes would result in significant particle loss.

Very little particle losses occur in the 0.1 um to 2 um particles once the sample is diluted with clean air. The 3/8” OD sample tube from the dilutor to the particle counter can be up to 30 meters long, provided it is made of a conductive polymer such as Tygothane.

Density Correction
As with any laminar flow device, airflow is affected by the density and viscosity of the air. The dilutors are calibrated to sccm flow (29.92 “ Hg and 70 degrees F). To correct the dilution factor for your local conditions, use the following formula:

As a general rule, there is an estimated 3% correction for every 1,000 ft above sea level, and a 1% correction for every 5.3 degrees F from 70 degrees.

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