Back-Scatter Cloud Probe

The BCP is a compact probe that measures 5 µm to 75 µm particles with no contamination from ice crystal shattering and no airflow distortion.


Minimize airflow disturbance and enable undisturbed particle measurement with the BCP’s aerodynamic design. Mount the BCP easily on aircraft, even when space is at a premium.

Monitor multiple output variables like total particles, average transit time, over-range particles, and various probe health indicators.

Depend on automatic adjustments to both sizer and qualifier signals to account for temperature drifts and ensure accurate particle sizing.

Program the instrument's sample rate and size bin thresholds. Plus, get size distributions accumulated in the probe, with serial transmission to a computer via any standard RS-232 or RS-422 communications port.

Particle light-scattering

  • Particles passing through the laser beam scatter light in all directions.
  • Light transmitted within a cone with subtending angles between 143° and 169° (156° ± 13°) is directed onto a photodetector that converts photon pulses into electrical pulses, which are then transmitted to a signal processor that amplifies and digitizes them.
  • Mie theory is used to determine the particle’s size from the peak amplitude of the scattering signal, then used to determine particle number concentration.
Parameter Specification
Measured Parameters

Single-particle 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

5 – 75 µm

Number of Size Bins
  • 10, 20, 30, or 40 (configured at time of purchase)
  • User-Selected boundaries
Number of Concentration Range

0 – 1,000 particles/cm3

Air Speed Range

10 – 250 m/sec

Sampling Frequency

Selectable, .04 sec to 20 sec

Light Collection Angles

Center-line: 156°, +/- 13° (see “How it Works” diagram)


658 nm

Laser Power

50 mW or less

Data System Interface

RS-422 serial interface

Environmental Operating Conditions
  • Temperature: -40° to 40°C
  • Relative Humidity: 0 – 100%, non-condensing
  • Altitude: 0 – 50,000 feet (0 – 15,000 meters)

1.5 kg

Probe Dimensions

11.7 cm x 10.7 cm x 4.5 cm, with 5.9 cm diameter mounting flange

Electronic Box Dimensions

21.6 cm x 12 cm x 5.7 cm

Power Requirements

28 VDC, 5 A for system and heaters

Droplet offers a three-year service plan to ensure that your BCP 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 BCP include:

  • Freight to and from Droplet facility for service
  • Incoming instrument evaluation and health check
  • Optics cleaning and alignment
  • Clean and replace heater heatsink compound
  • Laser evaluation and adjustments
  • Laser splitter block alignment
  • Offset voltage verification
  • Baseline amplitude verification
  • Baseline voltage evaluation on DSP board and PADS
  • O-ring replacement
  • Hardware replacement
  • Reseal windows
  • Extreme cold testing of probe
  • Software, firmware, and antivirus update
  • 100% unit functionality tests
  • Calibration data, Calibration Report, Pitot Calibration Report, and Certificate of Calibration provided

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