Fog Monitor

This optical spectrometer continuously samples droplet-laden ambient air and produces size distributions in the range from 2 µm to 50 µm.


Rely on the only all-weather ground-based instrument capable of measuring particle size distributions (PSDs) in the 2 µm to 50 µm size range – critical for understanding the evolution of fog, and useful for calculating number concentration, liquid water content, and extinction/visibility.

Get high-resolution information on particle light-scattering intensity with the PBP optional feature. This allows much more precise and detailed size distributions, as well as minimization of the well-known Mie scattering ambiguities.

Access raw data for one particle in each sampling instance. Use this information as an important instrument diagnostic.

Depend on automatic correction for drifts in polarization signals through the dynamic thresholding feature.

Particle light-scattering

  • The FM-120 is a forward-scattering optical spectrometer that accepts and sizes only particles that pass through a region of the laser beam with uniform power.
  • Forward-scattered light is collected, directed through a 50/50 optical beam splitter, then measured by photodetectors to determine size and depth of field.
Parameter Specification
Measured Parameters

Single-particle forward light scattering

Auxiliary Parameters


Derived Parameters

Particle diameter
Particle number concentration
Liquid water content (LWC)
Effective diameter (ED)
Median volume diameter (MVD)

Particle Size Range

Droplets with 2-50 µm diameter

Number of Size Bins


Typical Sample Area

0.24 mm²

Sample Flow Volume

1 m3/min

Sampling Frequency

Standard Data Rate: Selectable, 0.04 sec to 20 sec
PBP Data Rate: No limit until particles are coincident

Refractive Index

Non-absorbing, 1.33 (the industry standard for water)

Light Collection Angles

~3.5º – 12º

Data System Interface

RS-232 or RS-422 serial interface
Up to 460 kB

Environmental Operating Conditions

Temperature: 0 to 40 °C
Relative Humidity: 0 – 100%, non-condensing
Altitude: 0 – 4,000 meters


10.0 kg (instrument), 9.5 kg (pump)

Probe Dimensions

23 cm H x 28 cm W x 37 cm L

Power Requirements

Universal input for analyzer head, 50-60 Hz
115-120V or 230V for pump (set at factory, but user-configurable)
200W (instrument), 400W (pump)

The FM-120 can be purchased with a swivel-head mount. The swivel head ensures the instrument inlet is always facing into the wind.

Droplet offers a three-year service plan to ensure that your FM-120 continues to deliver high-quality data. This plan locks costs upfront at the time of funding application and covers annual calibration, alignment, cleaning, wear-item replacement, calibration documentation, and shipping. Specific services provided for the FM-120 include:

  • Shipping to and from Droplet facility for service
  • Incoming instrument evaluation and health check
  • Pre-calibration data recording and testing
  • Pump power cycle testing through PADS
  • Optics cleaning
  • O-ring replacement
  • Heater functionality tests
  • Align laser, qualifier, and sizer
  • Pressure transducer
  • Calibrate air-speed correction factor
  • Calibration of instrument using spinning pinhole calibrator
  • Calibrate sizing with Soda Lime Glass Microspheres
  • Beam-mapping of instrument
  • Extreme cold testing of instrument
  • Software, firmware, and antivirus update
  • 100% unit functionality test
  • Certificate of Conformance, Calibration Certificate, Pressure Calibration Worksheet, ASCF Calibration Worksheet, Pre-Calibration and Post Calibration sizing histograms and housekeeping data provided

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