Dr Nicholas Marsden

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Instrument Support Level 4

Instrument Support Level

Instrument Support Level 4

Manufacturer & Model

Droplet Measurement Technology. CAPS probe with depolarisation.

CEDA Archive Name


Data Products


Insurance Value


Physical Dimensions

1 m x 50 cm.: ~12 kg


See page for details

Daily Facility Charge



Cloud-Aerosol & Precipitation Spectrometer with Depolarisation

Cloud-Aerosol & Precipitation Spectrometer (CAPS) with Depolarisation is designed to measure size distributions of airborne particles.

It does this in the diameter range 0.65 < Dp < 50 μm using measurements of the Mie scattering of laser light (CAS), in the range 15 < Dp < 960 μm using a 1-dimensional shadow detector (CAS), which also produces 2-bit resolution particle images. Additional measurements include airspeed using a differential pressure measurement through a pitot tube, temperature, and relative humidity.

It is primarily an aircraft-mounted instrument, although CAS has been used in the past during ground-based deployments.

The CAPS probe is a combination cloud microphysics probe designed primarily for aircraft use. The four parts of the probe are:


CIP (Cloud Imaging Probe) is a 2D shadow probe that operates by measuring the level of shadowing of an incident laser beam onto a 64 element photo-detector. One dimensional slices of particle images are built up into 2D images by rapidly scanning the photodetector array voltages. The image resolution is 15 μm, giving a particle size range of 15 – 960 μm. This portion of the probe consists of two “horns” protruding from the front of the instrument PMS pod housing, – one to transmit and one to receive radiation from a continuous wave diode laser (680 nm; 50 mW) across an open path length of 7.5 cm. The tips of these horns are pointed and are moderately sharp to prevent the breakup of particles into the sample volume.


CAS is a Mie scattering probe which calculates particle diameter by measuring the intensity of light scattered in the forward direction by particles passing through a sample volume inside a sample tube. Illumination is provided by a continuous wave diode laser (680 nm; 50 mW). The intensity of backscattered light and depolarised backscattered light is also measured to provide insight into particle morphology. CAS has a particle size range of 0.6 – 50 μm for liquid water particles. This portion of CAPS is housed in a wing shaped section of the probe head beneath the CIP.


The hotwire probe calculates total liquid water content by measuring the electrical power required to keep a fine wire coil (2 cm x 0.18 cm) at a constant temperature of 125 oC. It is predominantly sensitive to droplets in the size range 5 / 10 – 30 / 40 μm. The hotwire protrudes from the leading edge of the probe between the CIP horns. Its power supply must be interlocked for no low-speed operation to prevent coil burnout /burn injuries on the ground.

Pitot tube

The pitot tube, located beside the CIP horns, measures static and dynamic pressure and uses these to calculate true airspeed. Data from a Honeywell Humicap temperature and humidity sensor are used in this calculation.

CAPS CIP sizing accuracy is checked for consistency against other cloud probes such as 2DS and CPI. Periodic calibrations take place in the laboratory in Manchester using a spinning disc assembly, but there is no requirement for field calibration. The sizing performance of the probe is very stable.

CAPS CAS is a Mie scattering probe, and as such should be checked regularly using calibrated glass beads. These can be provided along with the bead dispersion system, but not the costs associated with this.

There are no servicing requirements apart from the cleaning of the optical windows at regular intervals. Training can be provided for the rare occasions on which the probe is deployed without the IS.

  • For cleaning of optical windows, only cotton swabs and ethanol are required. These will be supplied by the IS operating the probe.
  • Glass calibration beads are required to check the sizing performance of the CAS-DPOL portion of the probe. The number of sizes and the quantity of beads required will depend on the duration and nature of the detachment. Glass beads normally last around one month of continuous operation and several sizes are normally required, at a cost of £250 per size.
  • Please contact the IS about your requirements), but a representative figure is approximately £1,000 per month of operation.
  • Instrument Insurance
    • This system must be insured by the user for £50K and covers loss, theft or damage to the instrument: damage is that over and above general wear and tear. The system has been designed to be rugged and autonomous. Even so, the end-user must respect the fact that the system is a precision optical instrument that must be treated with great care.
    • The user is responsible for the instrument from the time it leaves the AMF to the time it is returned and signed off as in an acceptable operating condition by the IS: this will be done as soon as is possible on its return.
  • Public Liability Insurance
    • The AMOF is not liable for any damage or injury arising from the deployment or operation of this instrument when unattended by the IS.
  • Shipping Expenses
    • The user is liable for all costs arising from the shipping of the instrument both to and from a deployment.
  • IS T&S
    • The user is responsible for coving the travel and subsistence expenses of the IS while attending the instrument.

The system when packed ready for shipping consists of two boxes:

One flight case with wheels containing the probe, measuring 125 x 45 x 44 cm and weighing approximately 45 kg.

Typically one aluminium box containing the data acquisition system, monitor, keyboard, power transformer and spares. Size is 78 x 58 x 41 cm. This normally weighs around 50 kg.

For use on the FAAM aircraft, CAPS must normally be used in conjunction with the Manchester cloud physics rack. Please contact the IS for further information.


The design of ground-based cloud experiments is beyond the scope of this document. Please contact the IS for further information or to discuss your requirements.

Eye safety
  • CAPS is a class 3R laser device. Cleaning of the optical windows should not take place with the lasers powered on.
  • Only the IS is permitted to conduct maintenance work on CAPS.
  • Safety training will be provided, but users other than the IS should be able to provide evidence of prior laser safety training adequate for the use of class 3R lasers.
  • AMOF cannot accept responsibility for the laser safety of the class 3R laser in CIP without the presence of the IS or another suitably trained person. Risk Assesments and local rules for usage are available from the IS on request.
Manual handling
  • The probe mass is approximately 12 kg without mounting canister.The packed mass of 45 kg means the supplied wheels must be used to transport the shipping case.
Electrical safety
  • Under normal circumstances, no live electrical connections are exposed. High voltages exist inside the probe and data acquisition system, so the covers must never be removed except in the presence of the IS.
Attended operation
  • No intrinsic hazards during operation have been identified. Most hazards are a result of the operating environment.
Current Powwer
110 V @ 400 Hz* 8 A 880 W
28 V DC 10 A 280 W
TOTAL 1160 W
CAPS fits within a standard PMS canister.
*For aircraft use. Can be 50Hz.
The data acqisition system runs on 240V AC, using approximately 4A including monitor.

While raw images can be produced fairly quickly, full quality control and production of corrected time series data is computationally expensive and time-consuming and hence are only offered as VAD data. Uncorrected products are made available in the field. Details of the products available in the field and as archived VAD products are given below.

Field Data

CAPS data are in the form of 1 Hz comma delimited text files containing bulk averaged quantities for each scalar instrument, binary time-stamped compressed image files from the cloud imaging probe (CIP-15), and time-stamped single particle scattering data from the cloud aerosol spectrometer (CAS-DPOL). The former are provided in compressed image files and image index files and the latter in comma-delimited text files:

  • Imagefile_1CIP Grayscale_YYYYMMDDHHMMSS.png
  • 00CIP_GrayscaleYYYYMMDDHHMMSS.csv
  • The user can download (but not delete) this data from the instrument but it should be noted that this data will not have been quality controlled.
Archive Data