TSI Alnor 9306-V2 User Manual
THE PURPOSE
AND APPLICATION
OF ISO 21501-4
APPLICATION NOTE CC-117 (US)
Introduction
ISO 21501 is titled “Determination of particle size distribution – Single particle light interaction
methods” and consists of 4 parts. Part 4: Light scattering airborne particle counter for clean spaces
applies to handheld, portable and remote airborne particle counters used in cleanrooms. This
standard was released in 2007, and, together with ISO 21501-2 and 21501-3, replaces ISO 133231:2000. The purpose of ISO 21501-4 is “to provide a calibration procedure and verification method
for particle counters, so as to minimize the inaccuracy in the measurement result by a counter, as
well as the differences in the results measured by different instruments.” ISO 21501-4 sets
minimum performance standards for airborne particle counters and defined methods for
calibration and verification of performance. While standards existed previously that covered these
topics, none were as comprehensive as ISO 21501-4.
Scope of ISO 21501-4
Table 1 shows the parameters that are listed within ISO 21501-4 and the accompanying
requirements. In addition, ISO 21501-4 requires that the calibration particles have a standard
uncertainty of the mean particle size ≤ ±2.5%, defines the minimum information to be included in a
test report, and the minimum parameters that should be calibrated at the calibration interval. The
minimum information for each test report is listed in Table 2, and the minimum calibration
parameters are listed in Table 3.
Table 1. Scope of ISO 21501-4
Parameter Requirement
Size calibration
Verification of size setting ≤ ±10%
Counting efficiency (50 ±20)% close to the minimum detectable particle size
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Calibrated using median voltage (or internal PHA channel)
for each calibration particle with the peak of the distribution
at least 2X the minimum noise level at the valley of the
distribution
(100 ±10)% for particles 1.5 to 2 times larger than the
minimum detectable size
Parameter Requirement
Size resolution ≤15% for particles at a size specified by the manufacturer
False count rate To be determined in units of counts/m3 at the minimum
reported size range (note: there is no specification in the
standard)
Maximum particle number
To be specified by manufacturer at a coincidence rate ≤ 10%
concentration
Sampling flow rate Standard uncertainty of volumetric flow rate ≤ ± 5%
Sampling time Standard uncertainty of sampling time duration ≤ ± 1%
Response rate Response rate of the particle counter shall be ≤ 0.5%
Calibration interval Recommended calibration interval to be one year or less
Table 2. Minimum Required Test Report Information
Date of calibration
Calibration particle sizes
Flow rate
Size resolution (with particle size)
Counting efficiency
False count rate
Voltage limit or channel of internal pulse height analyzer
Table 3. Minimum Calibration Parameters
Size calibration
Size resolution
Counting efficiency
Sampling volume uncertainty (dependent on sample flow
rate and sampling time uncertainty)
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