Optical spectrometer FM-120

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Summary
FM-120 is a ground-based forward‑scattering optical spectrometer that continuously samples droplet‑laden ambient air and provides droplet size distributions from 2 µm to 50 µm for number concentration, liquid water content (LWC) and extinction/visibility calculations.

Key product overview
The instrument accepts and sizes only particles that transit a region of the laser beam with uniform power. Forward‑scattered light is collected, split and measured by photodetectors to produce particle size distributions (PSD) and derived metrics.

Features and benefits

• All‑weather ground measurement — Continuous PSD measurements for droplets 2–50 µm, suitable for fog and visibility analysis.
• Particle‑by‑particle (PBP) option — Optional high‑resolution PBP data for improved size discrimination and reduced Mie ambiguity.
• Instrument diagnostics — Access to raw single‑particle records for troubleshooting and instrument health checks.
• Dynamic thresholding — Automatic correction for polarization signal drifts to maintain measurement stability.
• Lifecycle Care Program — Manufacturer service program available to maintain performance over the instrument lifecycle.

Technology

• Particle light‑scattering — Forward‑scattering optical spectrometer that sizes particles passing through a uniform laser region.
• Collected forward‑scattered light is directed through a 50/50 beam splitter and measured by photodetectors to determine particle size and depth of field.

Main applications

• Weather modification research
• Visibility and fog studies

PADS — Particle Analysis and Display System
PADS provides a virtual instrument panel for control, real‑time display and logging. Typical capabilities include:

• Sample and record data
• View particle volume and number concentrations, Median Volume Diameter (MVD) and Effective Diameter (ED)
• View LWC as measured by the FM-120
• Monitor laser health and instrument temperatures
• Play back data for post‑processing
• Reprocess data with new parameters for further analysis

Accessories
The FM-120 can be supplied with a swivel‑head mount to keep the instrument inlet facing into the wind.

Technical specifications

• Measured parameters: Single‑particle forward light scattering
• Auxiliary parameters: Temperature, Pressure
• Derived parameters: Particle diameter; Particle number concentration; Liquid water content (LWC); Effective diameter (ED); Median volume diameter (MVD)
• Particle size range: Droplets 2–50 µm diameter
• Number of size bins: 30
• Typical sample area: 0.24 mm²
• Sample flow volume: 1 m3/min
• Sampling frequency: Standard data rate selectable 0.04 s–20 s; PBP data rate: no limit until particle coincidence
• Refractive index (assumed): Non‑absorbing, 1.33 (water)
• Light collection angles: ~3.5°–12°
• Data interface: RS-422 serial up to 460 kB
• Operating conditions: Temperature -40 to 40 °C; Relative humidity 0–100% non‑condensing; Altitude 0–4,000 m
• Weight: 18.5 kg (instrument); 13.0 kg (pump & hose)
• Probe dimensions: 23 cm H × 28 cm W × 37 cm L
• Power: Analyzer head universal input 50–60 Hz; pump 115–120 V or 230 V (factory/user‑configurable); consumption 200 W (instrument), 400 W (pump)