PMS/CSIRO Hot Wire Liquid Water
Probe
1. Introduction
The PMS/CSIRO is an instrument developed
by Warren King (CSIRO) and marketed by Particle Measuring Systems
(PMS Inc., Boulder, Co) for the measurement of cloud liquid water
content. This sensor, commonly referred to as the "King"
probe, is used primarily for the study of cloud microphysical
processes and in icing studies.
2. Operating Principles
The King probe operates under the principle
that liquid water can be calculated from measurements of the amount
of heat released when vaporized. As shown in the drawing in Fig.
1 and photograph in Fig 2, a heated cylinder is exposed to the
airstream and intercepts on coming droplets. The electronics maintain
this sensor at a constant temperature (approximately 130o
C) and monitor the power required to regulate the temperature
as droplets vaporize. This power is directly related to the amount
of heat taken away by convection plus the heat of vaporization.
The convective heat losses are known empirically and vary with
airspeed, temperature and pressure. The liquid water content is
calculated from power loss found from the difference between total
and convective power losses.
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: None required
Range: .05 - 3.0 g m-3
Accuracy: ±15%
3b. Primary Output
RAF Parameter Name Plain Language Name Description
PLWC Power This is the power consumed in watts to maintain the hot wire sensor at a constant temperature.
3c. Derived Output
RAF Parameter Name Plain Language Name Description
PLWCC Liquid Water Content Cloud Droplet
water mass - grams per cubic meter
where l is the sensor length, d is its width,
v is the air velocity, Lv is the latent heat of vaporization,
c is the specific heat of water, Tb is the boiling
point of water, Ta is the air temperature, Ts
is the sensor temperature, k is the thermal conductivity of the
air, Re is the Reynol's number, Pr is the Prandtl number and A0,
x and y are constants for a heated cylinder at high Reynold's
number.
4. Data Interpretation
The King probe sensor is limited by collection
efficiency considerations on the small droplet end of the spectrum
and by vaporization time on the large end. The sensor has a diameter
of approximately 2 mm and small water droplets, less than 10 mm
will not impact with 100% efficiency as they follow the airflow
around the sensor. These losses are typically about 5% for 10
mm droplets
but increase to greater than 20% for diameters less than 5 mm.
This is normally not a major problem since the largest fraction
of the water mass is typically carried in droplets greater than
10 mm.
In developing clouds, however, near cloud base where droplets
are still quite small, or in cloud edges where entrainment and
evaporation is occurring, the underestimation of liquid water
content can be significant.
On the large droplet side, the King probe begins to underestimate the liquid water contained in drops larger than 30-40 mm as a result of incomplete evaporation as these larger droplets impact and are carried away by the airstream before sufficient heat has been transferred to vaporize them.
