Hach Ultra MET ONE 227, User Manual

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9
General Information
2.2 General product information
The MET ONE 227 particle counter is a battery operated, laser based particle counter
that is used in a walk-around sampling routine. The MET ONE 227 particle counter stores
up to 200 records with a different location label for each sample. The data records may
later be printed or downloaded to a computer for analysis. The MET ONE 227 particle
counter is used in environments where the particulate contamination does not exceed two
million particles per cubic foot of air, such as clean-rooms, medical instrument
assemblies, computer rooms and downstream of air filter installations in HVAC systems.
The Concentration mode in the MET ONE 227 particle counter is used to take a brief
sample and estimate the probable cleanliness in areas with unknown particulate levels.
This is based on built-in calculations performed in the microprocessor of the MET ONE
227 particle counter. Long term use of the MET ONE 227 particle counter in uncontrolled
environments such as open air office spaces or outdoor air will require frequent sensor
maintenance.
The MET ONE 227 particle counter operates at a flow rate of 0.1 cubic foot per
minute (cfm). The flow rate is required to set the sampling parameters. For instance, the
MET ONE 227 particle counter takes 10 minutes to sample 1 cubic foot of air. To obtain
the counts per cubic foot, multiply the results of a 1-minute sample by ten. All counts are
reported as cumulative counts; that is, all the reported 0.3 micron particles are 0.3 micron
and larger in size.
2.3 Theory of operation
The MET ONE 227A/B particle counter is a combination of a sensor, pump and
electronics (refer to
Figure 2 on page 10 ) that includes full particle counter features. Both
variants use the same sensor, pump, CPU and display electronics. The MET ONE 227A
particle counter is calibrated with channel 1 sensitivity of 0.5 micron and the MET ONE
227B particle counter is calibrated with a channel 1 sensitivity of 0.3 micron. Both operate
at a flow rate of 0.1 cfm.
The sample passes through a laser beam in the sensor, where the particles scatter the
laser light. The sample then passes through a pump and a filter to trap particles in the
counter rather than return them to the environment. The flashes of scattered light are
converted to electronic pulses directly proportional to the amount of scatter, which
creates a correlation to the particle size. The electronic pulses are counted and stored in
channel bins in a data record. A data record also includes the date and time from the
CPU clock and snapshots of the environmental conditions, if the Relative
Humidity/Temperature (RH/T) probe is attached.