Application Note

6 Fluke Corporation Indoor air quality monitoring made easy with the Fluke 985 Particle Counter
Making sense of particle counter features
Using a particle counter is relatively simple; however, understanding the features
that distinguish counters can sometimes be a challenge. The following terms are
commonly used to describe the accuracy, efficiency, and other attributes of an
airborne particle counter.
Count Mode: The count mode defines how the
particle counter displays data to the user. Concen-
tration and Raw Count are two typical sampling
modes, and the Fluke 985 adds an Audio mode.
Concentration mode samples a small volume of air
then calculates the value based upon the volume
setting (cm3, ft3, or litre) in the counter while
raw count mode allows the user to collect actual
particle counts as they accumulate per sample time.
It can be set to cumulative (the total number of
particles > each channel size) or differential (the
number of particles fall between the channel sizes).
Audio mode is useful when searching for areas that
exceed predefined particle levels. Once exceeded,
the counter notifies the user in audio alarm. The
count mode defines how the particle counter dis-
plays data to the user.
Zero Count: Zero count is a measure of the
particle counter’s accuracy, and should be taken
prior to use and periodically thereafter, or when
sampling error is suspected. The zero count filter
is attached to the particle counter per the manu-
facturer’s instructions, then the counter is run for
15 minutes. The counter should not have detected
more than one particle greater than 0.3 μm in a
five-minute period.
Coincidence Loss: Coincidence loss occurs
when two particles cross the counter’s light beam
simultaneously, creating a single pulse and result-
ing in a single particle count. This type of error
occurs more frequently as the concentration of
particles increases within the sample. Per 21501-4,
coincidence loss must be ≤ 10 % by the maximum
particle number concentration. Fluke 985 is 10 %
at 4,000,000 particles per ft
3
.
Counting Efficiency: The probability that the
counter will sense and count a particle passing
through the sample volume. Counting efficiency
is a function of size up to a minimum sensitivity
threshold, above which all particles are sensed and
counted. A counting efficiency of 50 % at the most
sensitive threshold is typically considered optimal,
and facilitates consistent comparisons between
counts from OPCs and those of higher-resolution
instruments.
Sensitivity: A device’s ability to detect small
particle sizes at a certain counting efficiency.
The Fluke 985 detects 0.3 μm at 50 % counting
efficiency.
Calibration: A set of operations or actions taken
to establish the relationship between the measured
values obtained through a device and the values
of the corresponding parameters as defined in a
standard. The Fluke 985 is calibrated using PSL
(polystyrene latex) spheres, widely used due to
their uniform size and light-refraction properties
and meets ISO 21501-4: Light scattering airborne
particle counter for clean spaces.
NIST traceable: Traceability is a characteristic
of a measurement or standard and its relationship
to stated references, which are often national or
international standards. The PSL spheres used in
the Fluke 985 calibration process can be traced to
NIST (National Institute of Standards and Technol-
ogy) standards.
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
Fluke. Keeping your world
up and running.
®
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©2012 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 3/2012 4153340A A-EN-N
Modification of this document is not
permitted without written permission
from Fluke Corporation.