Rosemount Icing Detector
Model 871
1. Introduction
The Model 871 icing detector is an instrument
that was developed by Rsoemount Engineering (??, Minnesota) for
the detection of supercooled liquid water content and the onset
of airframe icing. It has been used by the cloud physics community
to detect the presence of supercooled water in mixed phase clouds.
It has been particularly useful for detecting supercooled water
in cirrus clouds at very cold temperatures when the water levels
are below the detection limit of conventional hot wire devices.
2. Operating Principles
The Model 871 detector, shown schematically
in Fig. 1, measures the amount of ice mass accumulation on a metal
cylinder. Using a property known as magnetostriction, the sensing
cylinder is driven at a natural frequency of 40 KHz. As the ice
accretes on the cylinder, the frequency of the vibration decreases.
A phase-locked loop converts this frequency change to a proportional
voltage from which the ice mass may be calculated. Once a pre-set
amount of mass has been accumulated, the cylinder is heated to
melt the ice. Figure 3 shows a typical time history of the probe
output as the voltage increases with accumulating ice mass then
dropping to its threshold value as the heater is activated to
remove the ice.
3. Sensor Specifications
3a. General Information
Manufacturer: Rosemount Engineering, ??
RAF Resident Expert: Darrel Baumgardner
(303) 497-1054
darrel@ncar.ucar.edu
Typical Mounting
Location: Underside of leading edge of
the wing
Calibration Method: Single Drop Freezing
Range: .001 - 1.0 g m-3 (airspeed
and temperature dependent
Accuracy: ±20% - 50%
3b. Primary Output
RAF Parameter Name Plain Language Name Description
?? Detector Volts Out Raw output voltage
3c. Derived Output
RAF Parameter Name Plain Language Name Description
? Liquid Water Content Supercooled liquid
water content - grams per cubic meter
where l is the sensor length, d is its width,
v is the air velocity, Ec is the collection efficiency,
and G is the sensitivity coefficient that relates the rate of
mass change to voltage change from the sensor.
4. Data Interpretation
The ice detector is limited by collection
efficiency considerations on the small droplet end of the spectrum.
As ice accumulates and the diameter of the cylinder changes, the
collection efficiency decreases. The collection efficiencies are
not well characterized but can exceed 20% for droplets less than
10 mm.
An additional uncertainty arises due to the change of diameter
with mass accumulation. The mass does not accumulate evenly so
there is not a simple expression relating the mass to diameter
change. A maximum error of 30% is associated with this source
of uncertainty.
The sensitivity factor is a function of
where the mass accretes on the sensor. This factor can be determined
empirically through comparisons with other instruments and can
vary depending upon mounting location.
