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Cloud, Aerosol and Precipitation Spectrometer (CAPS)
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This multipurpose particle spectrometer includes three DMT instruments plus temperature and relative humidity sensors that are packaged into a single, integrated measurement system. The CAPS provides the following data:
Aerosol particle and cloud hydrometeor size distributions from 0.51 to 50 µm
Precipitation size distributions from 25 µm to 1550 µm, or 15-930 um with optional 15-micron resolution
Particle optical properties (refractive index)
Particle shape assessments (discrimination between water and ice for probes with depolarization feature)
Liquid water content from 0.01 to 3 g/m3
Aircraft velocity
Atmospheric temperature and pressure
This instrument replaces the older PMS/PMI FSSP-100, FSSP-300, 2D-C, 2D-P and KLWC and can be used in many applications, including weather modification, aircraft icing, hurricane and storm research, and agricultural and industrial spray characterization.
Depolarization Feature
The CAPS with depolarization (CAPS-DPOL) can differentiate between water and ice particles for particles in the 0.5 - 50 µm range. A standard CAPS measures the forward and back-scattered light from these particles, while the CAPS-DPOL measures forward-scattered light and the S-state and P-state polarizations with two backscatter detectors. For spherical particles, typically droplets, the polarization of the incident light will be retained and the crossed polarization in the back-scatter will not generate any signal. Depending on the asphericity of the particles, there will be increased signal in the backscatter detector with the crossed polarizer.
Photo at right: Depolarization ratio measured by CAS-DPOL in AIDA cloud chamber showing freezing of water droplets (dark to light color changes).
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Advantages
Combines multiple instruments in one flight canister, covering a sizing range from 0.51 µm to 1550 µm (or 0.51 µm to 930 µm for a 15-µm resolution CIP), plus Liquid Water Content, Temperature and RH sensing
Generates 2-dimensional images of particles from 25 - 1550 µm
Offers optional depolarization feature that provides information on particle shape
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Applications
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CAPS Probe mounted on a Twin Otter; Photo Courtesy of the British Antarctic Survey
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Principle of Operation
The three DMT instruments included in the CAPS are the Cloud Imaging Probe (CIP), the Cloud and Aerosol Spectrometer (CAS), and the Hotwire Liquid Water Content Sensor (Hotwire LWC).
The CIP, which measures larger particles, operates as follows. Shadow images of particles passing through a collimated laser beam are projected onto a linear array of 64 photodetectors. The presence of a particle is registered by a change in the light level on each diode. The registered changes in the photodetectors are stored at a rate consistent with probe velocity and the instrument’s size resolution. Particle images are reconstructed from individual “slices,” where a slice is the state of the 64-element linear array at a given moment in time. A slice must be stored each time interval that the particle advances through the beam a distance equal to the resolution of the probe. Optional grayscale imaging gives three levels of shadow recording on each photodetector, allowing more detailed information on the particles.
The CAS, which measures smaller particles, relies on light-scattering rather than imaging techniques. Particles scatter light from an incident laser, and collecting optics guide the light scattered in the 4° to 12° range into a forward-sizing photodetector. This light is measured and used to infer particle size. Backscatter optics also measure light in the 168° to 176° range, which allows determination of the real component of a particle’s refractive index for spherical particles.
The Hotwire LWC instrument estimates liquid water content using a heated sensing coil. The system maintains the coil at a constant temperature, usually 125 °C, and measures the power necessary to maintain this temperature. More power is needed to maintain the temperature as droplets evaporate on the coil surface and cool the surface and surrounding air. Hence, this power reading can be used to estimate LWC. Both the LWC design and the optional PADS software contain features to ensure the LWC reading is not affected by conductive heat loss.
Software
The Particle Analysis and Display System (PADS) is optional software that provides a user-friendly virtual instrument panel for the control, data display, logging, and playback of the CAPS instrument. For instance, the program enables the user to do the following tasks:
Start data recording and sampling
View particle image data acquired with the CIP
View particle volume and number concentrations, as well as Median Volume Diameter (MVD) and Effective Diameter (ED)
View LWC as measured or calculated by the hotwire sensor, CIP, and CAS
Monitor instrument parameters like CIP laser current and various electronics voltages
Play back data for post-flight viewing
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Particle Analysis and Display System (PADS) Software
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Specifications:
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| CIP (for 25-µm resolution CIP) |
CAS* |
Hotwire LWC |
| Technique: |
Optical Array Probe with 64 elements: 62 sizing elements, end diodes reject
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Forward and Back Scatter Light Sensors |
Temperature-Controlled Hotwire Sensor |
| Measured Particle Size Range: |
12.5 µm – 1.55 mm (standard) |
0.51 µm to 50 µm |
N/A; measured LWC range is 0 - 3 g/m3 |
| Sample Area: |
10 cm x 1.55 mm |
11.1 mm x 120 µm |
N/A |
| Upper Concentration Range: |
Depends on particle size, but up to 500 particles/ cm3 for a CIP with standard tips and arm width |
Greater than 1,000 particles/cm3 after corrections for coincidence that are about 25% at 800 and 30% at 1,000 particles/cm3 |
3 g/m3 |
| Air Speed Range: |
10 – 300 m/sec |
10 – 200 m/sec |
10 – 200 m/sec |
| Number of Size Bins: |
62 |
Selectable, 10, 20, 30, or 40 |
N/A |
Sampling Frequency: |
1D histogram data: 0.05 to 40 Hz
2D image data: variable interval, when buffer fills
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Selectable, 0.05 to 40 Hz |
Laser: |
658 nm, 30mW |
658 nm, ~50 mW |
Calibration Verification: |
Spinning glass disk with opaque dots of known size |
Precision glass beads and latex spheres for sub-micron range |
Not required |
Other: |
N/A |
Non-absorbing refractive index, 1.3 – 1.7
Light collection angles: 4° - 12°, 168° - 176°
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N/A |
Auxiliary Parameters: |
Ambient Temperature, Relative Humidity, Static Pressure, Dynamic Pressure (CIP) |
Data System Interface: |
2D CIP data: RS-422, High Speed, 4 Mb/sec Baud Rate
RS-232 or RS-422, 56.6 kb/sec Baud Rate for system data
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Software: |
Particle Analysis and Display System (PADS) (Optional) |
Weight: |
45 lbs./20.4 kg |
Power
Requirements: |
28VDC: 10A for probe system, and 45A for anti-ice heaters, optional AC voltages for anti-ice heaters |
Environmental Operating Conditions: |
Temperature: 0 – 40°C (32 – 104 °F)
RH: 0 – 100%, non-condensing
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Routine Maintenance: |
DMT recommends conducting basic instrument performance checks and inspecting the CIP optical windows before a flight. A weekly calibration check of the CAS and CIP is also recommended. |
Specifications are subject to change without notice. The CAPS is a Class IIIb Laser Product.
* CAS specifications apply to both standard CAS and CAS-DPOL.
Calibration
CIP Calibration is verified with a spinning disk (see picture under “Included Items”), while CAS calibration is checked with precision glass beads and latex spheres for sub-micron range. The Hotwire LWC does not require calibration.
Options
The CAS is offered with or without the depolarization feature.
The CIP is offered in a 15 um resolution in addition to the standard 25 µm resolution. In addition, it is offered with standard tips or Korolev anti-shatter tips (below left), and with monoscale or grayscale imaging (comparison shown below right).
Korolev anti-shatter tips, pictured above, significantly reduce the incidence of particle artifacts in the sample area.
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Grayscale imaging (above left) provides detail about particle composition that is missed with monoscale imaging (right).
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Items Included in the Purchase Price
Instrument
Shipping case
Operator manual
Zero-count filter
Glass beads and dispenser for CAS calibration check
Spinning disk for CIP calibration check
One day of training at DMT’s facility
One-year warranty
Email and telephone technical support
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Spinning Disk to verify CIP Calibration
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Accessories (Purchased Separately)
PADS software
Data acquisition system
Selected Bibliography
Baumgardner, D., H. Jonsson, W. Dawson, D. O’Connor and R. Newton. “The cloud, aerosol and precipitation spectrometer (CAPS): A new instrument for cloud investigations,” Atmos. Res. 2001: 59-60, 251-264. Link to abstract
Baumgardner, D., H. Chepfer, G.B. Raga, G.L. Kok. “The Shapes of Very Small Cirrus Particles Derived from In Situ Measurements,” Geophys. Res. Lett., 2005: 32, L01806, doi:10.1029/2004GL021300, 2005. PDF
Chiriaco, M., H. Chepfer, P. Minnis, M. Haeffelin, S. Platnick, D.Baumgardner, P. Dubuisson, M. McGill, V. Noël, J. Pelon, D. Spangenberg, S. Sun-Mack, G. Wind. “Comparison of CALIPSO-like, LARC and MODIS Retrievals of Ice Cloud Properties over SIRTA in France and Florida during CRYSTAL-FACE,” J. Appl. Meteor., 2007: 46, 249-272. PDF
Gao, R.S., D. W. Fahey, P. J. Popp, T. P. Marcy, R. L. Herman, E. M. Weinstock, J. B. Smith, D. S. Sayres, J. V. Pittman, K. H. Rosenlof, T. L. Thompson, P. T. Bui, D. G. Baumgardner, B. E. Anderson, G. Kok, A. J. Weinheimer. “Measurements of relative humidity in a persistent contrail,” Atmos. Env. 2006: 40, 1590-1600. Link to abstract
Garrett, T.J., H. Gerber, D. Baumgardner, C. H. Twohy, E.H. Weinstock. “Small, Highly Reflective Ice Crystals in CRYSTAL-FACE Anvil Cirrus,” Geophys. Res. Lettrs. 2003: 30, NO. 21, 2132, doi:10.1029/2003GL018153 Link to abstract
Heymsfield, A.J., C.G. Schmitt, A. Bansemer, D. Baumgardner, E.M. Weinstock, J.T. Smith and D. Sayres, “Effective Ice Particle Densities for Cold Anvil Cirrus.” Geophys. Res. Lettrs. 2004: 31, L02101, doi:10.1029/2003GL018311. Link to abstract
Heymsfield, A.J., C. Schmitt, A. Bansemer, G-J van Zadelho, M. J. McGill, C. Twohy, D. Baumgardner. “Effective Radius of Ice Cloud Particle Populations Derived from Aircraft Probes,” J. Atmos. Oceanic Tech. 2006: 23, No. 3, pp. 361–380. PDF
How to Order
For more information or to obtain a sales quote, contact DMT at +1.303.440.5576,
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