Understanding Zero Count Testing and False Count Rate

By Randy Grater, Technical Services Manager
November 1, 2011

Zero count testing has become a fairly standard SOP that companies follow before making daily rounds of particle count checks.  This is accomplished by installing the zero count filter (also known as a 'purge filter') over the inlet to the particle counter.

The purpose of the zero count test

The zero count test started out as a manufacturer's verification that the sensor was not leaking, nor had excessive contamination.  Early in the history of particle counting it was used as a troubleshooting aid to isolate counting problems as internal or external to the particle counter.  It then became an SOP for many companies to verify that the particle counter was working properly by obtaining zero counts before starting the morning rounds of sampling.

Unfortunately, the term zero count was not a good choice of names for this test, as it implies that the expectation was "no counts whatsoever - zero." 

Particle counters produce counts by collecting light energy scattered by particles as they pass through a laser beam.  The semiconductor photodetectors used to detect light scatter, however, also respond to high energy solar and stellar radiation that producing small pulses indistinguishable from pulses produced by particles.  The current standards refer to this as a false count rate.

Test duration

When performing a zero count test before making the daily round of particle counts, the test should only verify there is nothing catastrophically wrong with the instrument. 

A one-minute test will tell you as much as you will learn by taking a full cubic meter sample.  Therefore, there is no real benefit in performing a longer test.  In fact, significantly longer testing over a long period of time may have a detrimental affect, albeit perhaps nominal, on the product life cycle.

If there is something wrong with the instrument that it produces tens or hundreds, or more counts per minute, this will be quickly obvious.  If there are a couple of counts per minute with a zero count filter, then one can surmise that nothing has been found that will materially alter the evaluation of the room or location.  Once again, there is no added benefit to a longer test.

The zero count test should be viewed simply as a false count rate, not an absolute.  The testing should be designed to satisfy the context of the test:  Is there something fundamentally wrong with the particle counter?  - This is all that needs to be known before beginning the round of sampling. 

If there are few to no counts in one minute, there is no reason to expect the instrument to suddenly develop problems in 5 minutes.  However, if there are several counts during the first minute, then it would be advisable to see if the instrument cleans up during 5 additional one-minute periods.

In formulating an SOP on zero count testing, the test should reflect the purpose of the test, and there should be a definable added benefit for additional time spent on the test




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