serviced. King et al. (1978) reported a sensitivity of
water content that received much use was the icing-
0.02 g m3, a response time better than 0.05 s, and an
rate meter, a horizontal rod with holes facing into the
accuracy of 5% at 1.0 g m3. Baumgardner (1983) could
relative wind (Perkins 1952). Ice plugging the holes
draw no conclusions about the King probe, other than
would be sensed with a pressure transducer, triggering
that it was promising and deserved more study. Cober
a deicing cycle. Assumptions about ice density and the
et al. (1996b) use the King probe on the Canadian Con-
meter's collection efficiency allowed rough estimation
vair and claim accuracies of 0.02 g m3 for liquid-
of liquid-water content. Electronic devices have since
water contents of less than 0.2 g m3. In a range of
replaced multicylinders.
liquid-water contents between 0.1 and 1.25 g m3, and
Today, the primary electronic devices for measur-
with MVDs between 10 and 40 m, Ide (1996) found
ing liquid-water content are the Rosemount ice detector
accuracy for the King probe to be within 0.1 g m3 of
and the King and JohnsonWilliams hot wire probes
the calibrated NASA Glenn Research Center Icing
(FAA 1991, Knollenberg 1981, Glass and Grantham
Research Tunnel (IRT). In general, wind-tunnel testing
1981). The Rosemount ice detector is a standard instru-
has shown the King probe to be generally accurate to
ment on most icing research aircraft and on the ground,
5% at 1.0 g m3, and it is generally superior to but more
and it may also be used to compute liquid-water content
fragile than the JohnsonWilliams probe (FAA 1991).
by relating the deicing rate of the detector to relative
Another hot-wire-based instrument is the Nezorov
wind velocity (Brown 1981, FAA 1991, Claffey et al.
probe developed in Russia (Korolev et al. 1996). Cur-
1995). A 6-mm-diameter by 25-mm-long probe vibrates
rently being flown on the Canadian NRC Convair and
axially at its resonant frequency of 40,000 Hz. As ice
the NASA Glenn Research Center Twin Otter, it has
accretes on the probe, its frequency drops until, at a
the unique ability to quantify both the supercooled
preset frequency, a heater deices the probe. Liquid water
liquid-water content and the ice-water component of
may be computed if the mass of ice, the exposure time,
clouds (Miller et al. 1998). Similarly to the Johnson
the relative wind velocity, and the collection efficiency
Williams probe, a reference heater corrects for convec-
of the probe are known. The detector is reasonably accu-
tive heat losses. Though details are not available, it
rate, within the range of conditions found in most moun-
appears that liquid- and ice-water components are sepa-
tain and aircraft applications, at moderate liquid-water
rated by the lag caused by phase changes as water
contents (Claffey et al. 1995). The typical liquid-water
performance range is 0.05 to 3.0 g m3 (FAA 1991).
vaporizes within the instrument (but ice particles may
The JohnsonWilliams probe exposes a hot wire to
break away, causing negligible heat loss--and error). The
the droplet-laden air flow, and a second "compensat-
exact process is not clear. Comparisons with the King
ing" wire is protected from liquid water but exposed to
probe in CFDE flights show less than 10% disagree-
the air flow (Knollenberg 1981, FAA 1991). The second
ment, with the Nezorov showing better performance in
wire compensates for variations in air speed, altitude,
SLD environments. Wind-tunnel tests demonstrated
and air temperature. The resistance of the wires changes
better stability than the King probe at low temperatures
as they warm and cool, and the change of resistance is
and the ability to measure the frozen component of
mixed-phase clouds. Verification in snow has not been
measured through a Wheatstone Bridge circuit. The
possible because of a lack of standards.
instrument has an absolute liquid-water range from 0.0
to 1.5 g m3 (Jeck 1980) to 6.0 g m3 (FAA 1991). Per-
Liquid-water content may also be measured opti-
cally, typically utilizing the interaction of laser-based
sonne et al. (1982) found undermeasurement of liquid-
collimated light and droplets. Gerber (1991, 1996) has
water content in large-drop environments for that por-
developed an instrument, the particle volume monitor
tion of the liquid-water content in droplets larger than
30 mm in diameter. A 20% error limit is often assumed
(PVM), that measures cloud liquid-water content, inte-
for the probe, but it can be smaller with wind-tunnel
grated particle surface area, and effective cloud droplet
calibration (Baumgardner 1983, Sand et al. 1984).
radius. All measurements are made simultaneously in
a large, 1.25-cm3 sample volume. The instrument oper-
The CSIRO, or King, hot-wire probe measures liquid
ates by passing droplets through a laser beam, which
water by maintaining a copper wire coil nominally 1.5
then forward-scatter laser light through a lens and a
mm in diameter exposed to the air stream at a constant
variable transmission filter onto a detector. Output from
temperature (King et al. 1978, Knollenberg 1981, FAA
1991). The electrical energy necessary to maintain a
the detector is mathematically inverted to derive liquid-
constant temperature under the cooling influence of the
water content and effective drop radius. The instrument
air stream and impinging water droplets is related to
resembles a class of instruments called "laser-diffraction
liquid-water content after corrections are made for air
particle-sizing instruments" (Gerber 1996). Compari-
temperature and wind speed. The probe requires no
sons of the PVM with other instruments in environ-
wind-tunnel calibration, is rugged, and is easily field
mental chambers, on mountain tops, and on aircraft have
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