No. 15 - Leak Testing with Particle Counters

Particle counters count the number of particles in a cubic foot of air. To do so accurately, a sample must be taken at a specific location for 1 minute. If a sample is taken for only 6 seconds (one-tenth of a minute), a '0' must be added to the total count, introducing a possible 10 % error.
Particle counters will only give accurate counts up to approximately 300,000 particles per cubic foot per minute (or 30,000 particles per cubic foot in 6 seconds). Concentrations higher than this introduce large COINCIDENCE LOSS which leads to errors of 5-20%, depending on the aerosol concentration and the particle counter. It is a acknowledged fact that two identical particle counters manufactured and calibrated by the same company will not produce identical results when exposed to the same aerosol sample.
A good example of the accumulation of errors is the Palas' filtration system, which purports to use 106 to 107 particles per cubic centimeter which requires diluters to sample the upstream challenge concentration. This equates to 2.83x10E10 to 2.83x10E11 particles per cubic foot. The diluters are 10:1 units which have a 5-10% error margin. By placing 2 diluters in series, the error margin increases to 10-20%, and gives a dilution of 100:1. The lower concentration is 2.83x10E10 particles/cu. ft. = 2.83x10E10/100 = 2.83x108 particles/cu. ft. or 283 million particles per cubic foot. Particle Counters CANNOT count that many particles accurately. Therefore, any downstream sample reading, even for less than 1 minute, would not be reliable.
From a practical standpoint (forgetting diluter error, count error, etc.) and just calculating scanning time, the Palas filtration test system becomes unwieldy. Using an Isokinetic Probe, per IEST Standards, sampling at 1 cfm and with an opening of 3" x 0.5", it would take [384 readings (counts) at 6 seconds each] 38.4 hours to scan a 24" x 24" HEPA filter. Remember, in addition, a 10 - 20% error rate due to extrapolating what the particle count would be for a full minute by multiplying the count obtained in just 6 seconds by 10.

A particle counter counts the number of particles passing through its viewing area. Most particle counters don't pass the entire 1 cfm sample through the viewing area but only a portion which introduces more room for error. Knowing the sample flow rate (1 cfm) and the duration of sample time (1 minute), it can be determined how many particles were in that one cubic foot sampled for one minute. This is what they were designed to do, and as such, make an excellent monitor for clean or sterile areas.

An aerosol photometer is a true ratio detector since it measures particles "en masse" and all of the particles travel through its viewing or detection area. ATI photometers are capable of measuring concentrations of particles as high as 600 micrograms per liter and are sensitive enough to measure accurately and instantaneously concentrations down to 0.00001 micrograms per liter. Using an ATI photometer, the same 24" x 24" HEPA filter can be scanned in accordance with IEST and other Standards in 1.6 minutes. If there is any area that has leakage > .01%, it can be pinpointed in seconds.

An excellent article on this subject was printed in Performance Review, a peer reviewed technical journal published quarterly by the Controlled Environment Testing Association (CETA). The article titled HEPA & ULPA Filter Installation Leak Tests by Ulrich Dietrich appeared in the summer of 1995 edition. For a copy, contact CETA at (202) 737-0204.