FILTER LEAK TESTING WITH PARTICLE COUNTERS
by Sheesh Gulati, MeasureTest Corporation, Bombay
Problems Testing HEPA Filter Systems with a Particle Counter

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 400,000 particles per
cubic foot per minute (or 40,000 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 centimetre which requires diluters to sample the
upstream challenge concentration. This equates to 2.83x1010 to 2.83x1011 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 2.83x1010 particles/cu. ft.= 2.83x1010/100 = 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 litre and are sensitive
enough to measure accurately and instantaneously concentrations down to 0.00001
micrograms per litre.

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.