Often the selection of a particle counter for use in a cleanroom is done based upon the specifications and cost of the instrument.
Before getting into the details of the specifications it is important to look at what the instrument will be used for, the environments it will be used in, and who will be using the instrument. Without this information taken into consideration, a less then optimal choice of particle counter for the application could be made. Here are some items to consider prior to selecting a particle counter:
What type of environment will the particle counter be used in? Will it be used in an ISO Class 3 Cleanroom for routine particle counting or will it be used for verifying a flow bench is operating prior to a critical process?
What type of data is the particle counter expected to collect? Will this information be recorded as simple pass/fail or will the information have to be logged into a spreadsheet or database?
Will the operator be carrying the particle counter around and placing it on a critical work surface or will it be cart mounted?
Will this particle counter be used to certify cleanrooms and travel from location to location?
Will the particle counter be used to monitor the cleanroom on a continuous basis? Is the particle counter intended to interface with a Facility Monitoring System (FMS)?
Though all manufacturers use the same principle, the details of the design are what set one manufacturer apart from the rest. Things like sample flow rate, sensitivity, size range and number of counting channels, durability of the laser or laser diode, lifetime of the light source, the ability to hold calibration all are important factors to consider.
Sensitivity: The smallest size particle that can be detected.
Zero Count Level or False Count Rate: The number of falsely reported particles using filtered air at the optimum flow rate for a given amount of time. The correct reporting of this is number of particles per 5 minutes. (Expected Zero Count rate should be less then 1 count per 5 minutes)
Counting Efficiency: The ratio of the measured particle concentration to the true particle concentration. The true particle concentration is measured with a more sensitive instrument that has a counting efficiency of 100% at the minimum particle size of the instrument under test. A properly designed instrument should have a 50% counting efficiency.
Channels: This is the number of “bins” the particles are placed in based upon the respective size of each particle counted. Channels are represented in microns. For example, you may have a particle counter with 4 channels. This means that the particles can be counted and binned in 4 different channels. Examples of channels are: 0.1 µm , 0.2 µm , 0.3 µm, 0.5 µm , 1.0 µm , 5.0 µm .
Flow Rate: This is the amount of air that passes through the particle counter. This is typically represented in cubic feet per minute. Common flow rates are 1.0 cfm and 0.1cfm. The greater the flow rate, the larger the pump to pull the air and the bigger the particle counter.
All too often minimum size is chosen over the other criteria. Though this is an important consideration, other parameters should also be considered.
Typically the more sensitive instrument, the higher the initial investment, and the higher the maintenance cost. If the instrument is used in environments with extremely high concentration of particles, it may require frequent cleanings by service technicians.
By understanding the intended use of the particle counter and the specifications, a more educated decision can be made when selecting a particle counter.