One Dimensional Optical Array Probe
260X OAP Cloud Probe
1. Introduction
The One dimensional optical array probe
(1D-OAP), model 260X, is an instrument developed by Particle
Measuring Systems (PMS Inc., Boulder, Co) for the measurement
of cloud droplet size distributions. The sensor is used primarily
for the study of cloud microphysical processes, particularly the
growth of cloud droplets through condensation and coalescence
into drizzle and precipitation drops.
2. Operating Principles
The 260X measures the size of hydrometeors
from the maximum width of their shadow as they pass through a
focussed He-Ne laser beam (Fig. 1). The shadow is cast onto a
linear diode array and the total number of occulted diodes during
the particle's passage represents its size. The diodes at each
end of the array act as a mechanism for rejecting those particles
that would be undersized when they do not pass entirely within
the bounds of the array.
The size is categorized into one of 60 channels
and this information is sent to the data system where the number
of particles in each channel is accumulated over a preselected
time period. Figure 2 shows a typical size distribution where
the concentration of droplets in each size category is shown,
normalized by the width of the size channel. Figure 3 is a photograph
of the 260X in the canister that is normally mounted on an aircraft
pylon.
3. Sensor Specifications
3a. General Information
Manufacturer: Particle Measuring Systems
Inc., Boulder, Co.
RAF Resident Expert: Darrel Baumgardner
(303) 497-1054
darrel@ncar.ucar.edu
Typical Mounting
Location: Pylons on fuselage or wings
Calibration Method: Monodispersed glass
beads
Range: 40 mm
- 600 mm
Accuracy: Diameter: Function of particle size, shape and orientation
Concentration: Function of particle size
3b. Primary Output
RAF Parameter Name Plain Language Name Description
AC601-15 Channels 1-60 - 60 channels
of accumulated counts
3c. Derived Output
RAF Parameter Name Plain Language Name Description
CONC6 Concentration # of particles per unit volume - number per cubic centimeter
PLWC6 Liquid Water Content Total droplet mass (assuming spherical water drops) - grams per cubic meter
DBAR6 Average Diameter Arithmetic average
of droplet size - micrometers
where ni is the number of droplets
detected in size channel i, di is the diameter represented
by channel i, and V is the sample volume measured in a given sample
period.
4. Data Interpretation
The electronic response time of the 260X
imposes some limitations on the minimum detectable size. A photodiode
is registered as shadowed when its output is sensed as changing
by at least 50% and at least one diode must change by 67%. The
edges of particles will oftentimes be missed and particles in
the lower end of the size range can pass undetected when the velocity
of a particle through the beam exceeds the response of the probe.
At 100 ms-1 this imposes a lower size threshold of
30-40mm
on the 260X.
The 260X is a particle sizing instrument,
not a liquid water content probe. The 260X detects any particles
that cause the diode array to be occulted, however, these probes
cannot differentiate shapes or particle orientation. If liquid
water content information is desired, some fairly loose assumptions
must be made with regard to the phase, habit, and density of the
particles. These assumptions may lead to significant errors in
derived liquid water content.
The sample volume of this instrument is relatively small, and varies with particle size. This imposes a limitation on the minimum sampling time if a statistically significant measurement is to be made.
