Chapter 4 : Connections
4-12 Lasair® Pro Aerosol Particle Counter Operations Manual
• Min: Sets the minimum x-axis value of the log graphical display.
• CHANNEL: Press the dropdown arrow to select the particle size to be used for
analog output generation. Channel units are set in the CHANNELS tab of the
SYSTEM screen.
• MAX: Press inside the field to define the maximum value for the particle size
channel pertaining to 20 mA. The minimum number of counts (pertaining to the 4
mA signal) is assumed to be 0.
Lasair Pro Analog 4-20 mA Output Log Mode
The Lasair Pro allows scaling of 4-20 mA output data in a log format rather than a linear
format. This is useful for encoding values with high dynamic ranges of particle counts as it
provides an equal number of discrete values per decade. This improves resolution at lower
count values and especially helps maximum output values of greater than 50,000. The
resolution of the 16-bit converter is unable to discriminate between 0 and 1 count above
this value. If overall system accuracy is also taken into account, a lower level than 50,000 is
the more practical limit for linear scaling.
The log setting uses a global minimum particle count value. Particle counts below the
minimum value will be clipped. The limits of this parameter are 0.001 to 100.0.
NOTE: Set the minimum value to less than 1 (e.g., 0.1) in order to dierentiate
between 0 and 1 particles. For example, when sending raw particle counts, if
the minimum is set to 1, then if 0 particle counts are received, the system will
output 1 count. This is an inherent limitation of logarithmic conversions.
Each channel has a maximum value output. The global minimum and channel-based
maximum are used to calculate the output. The two equations in this section show the
conversion used from mA to particle counts and back.
Particle counts are scaled to a normalized value, if that output option is selected, before
the conversion.
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