Apparatus and Method for Measuring Liquid Water Content of a Cloud or Fog

Various means have been used in the prior art to determine the accurate measurement of total water content of an optical path through clouds or fog. The prior art meteorological devices frequently used rotating cylinders to sample air volumes, hot wires, collector heads, paper-tape impactors, and instrumentation measuring dew point and dielectric constants of the medium being sampled. The problem with some of the aforementioned apparatus was that it did not permit rapid and continuous monitoring of the liquid content of water in a cloud in a precise or reliable manner. In addition the prior art devices failed to operate where the cloud being sampled varied in concentration over a range of as much as 120 to 1 of both super cooled and normal droplets. The prior art methods and apparatus failed to give rapid response, caused disturbance of the sampled medium, would not lend themselves to easy calibration, were not rugged and reliable for aircraft use and did not give a continuous easily-interpreted output reading.

It is generally well known in the art that electromagnetic radiations may be attenuated by aerosole or droplets of fine particles and that extinction can be used to monitor aerosol concentration when particle sizes are known along with the complex indices of refraction of the medium. In the past measurements made at visible or near visible wavelengths have depended upon optical scattering arising from the "real" (refractive) part of the complex index. Because extinction with wavelength in the prior art devices was extremely dependent upon particle size and size distribution it has been necessary to measure these parameters and to correlate extinction readings with aerosol concentrations. The problem with these devices and methods was that errors were frequently large. At infrared wavelengths greater than 2.5 um, the "imaginary" (absorption) component of the complex index of refraction, for example of water, becomes an important factor. When the radiating wavelength is extended to the 8-13 um region, the region where normal atomospheric vapors do not absorb, approximately equal parts of total optical extinction are contributed by the "real" and "imaginary" components of water droplets. Thus, droplet size and size distribution must be first known before total optical extinction at these wavelengths can be used as a monitor of total aerosol concentration.

The present invention relates to an apparatus and method for rapidly and continuously measuring the liquid water content of clouds or fogs. The present invention utilizes a valved environmentally controlled chamber disposed intermediate a spectrally scanning radiometer with liquid nitrogen cooled detector, and a temperature controlled radiation infrared energy source.

An object of the present invention is to provide an apparatus and method for rapidly and continuously measuring the liquid content of a cloud or fog.

Another object of the present invention is to provide an apparatus and method for rapidly and continuously measuring the liquid content of a cloud or fog by use of a single-wavelength transmissometer operating in a specific critical wavelength in an atmospheric "window" region of the infrared radiation spectrum.

Another object of the present invention is to provide an apparatus and method for rapidly and continuously measuring the liquid water content of a cloud or fog at a critical wavelength which makes the operation of the apparatus substantially independent of droplet size distribution of the cloud.

Another object of the present invention is to provide an apparatus and method for continuously measuring the liquid content of a cloud while insuring little or no disturbance of the sampled medium.

Another object of the present invention is to provide an apparatus and method for continuously measuring the liquid water content of a cloud over a 100:1 concentration range of both supercooled and normal droplets.

A further object of the present invention is to provide an apparatus for continuously measuring the liquid water content of a cloud which lends itself to calibration and easily-interpreted output readings in gm/m.sup.3.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following descriptions taken in connection with the accompanying drawings.

FIG. 1 is a partial cross-sectional schematic view of an apparatus for measuring the liquid water content of a cloud or fog created in an environmental test chamber.

FIG. 2 is a plot of the total optical extinction versus droplet diameter for water for a variety of visible and infrared wave lengths.

Referring now to FIG. 1 a tubularly shaped environmental chamber 10 has a fifteen centimeter diameter first optically transparent window 12 positioned in one side of chamber 10 and a second optically transparent window 14 diametrically disposed in the opposite side of chamber 10 having an optical path length of 3.05 meters. The material of windows 10 and 12, chosen to be optically transparent at a selected critical wavelength of 11.5 um, is made of 0.0015 inch thick polyetheylene film. The window could be made of other materials such as Kodak "Irtran" series, KRS-5, fused salts or any other material or means such as circularly variable filter wheel as manufactured by Optical Cooling Laboratories, Inc., Santa Rosa, Calif., having optical transparency at a critically chosen wavelength which would allow transmission of an optical bandwith of approximately 2% of the center critical wavelength. Two electric fans 16 and 18 are operatively positioned on both ends of chamber 10 to maintain homogeneous aerosol concentrations during the testing procedure. Steam or the cloud or fog to be tested is introduced into the chamber 10 at top side via valved steam pipeline 20 and condensate may be exhausted therefrom at the end by valved pipeline drain 26. An exhaust pump or system not shown may be used to reduce chamber pressure through valve 22. Smoke or other type atmosphere may be introduced into the bottom end of chamber 10 by valved pipeline 24. Condensed liquids are drained from the bottom end of chamber 10 by drain line 26. A spectrally scanning radiometer 28, such as Exotech Model 10, infrared radiometer manufactured by Exotech, Inc., Gathersburg, Md., with its infrared detector modified to be cooled by a liquid nitrogen trap 30 is axially aligned and operatively disposed opposite to the first optically transparent window 12. A circular variable filter (CVF) 32 as aforesdescribed is positioned intermediate the scanning radiometer 28 and the first optically transparent window 12. A temperature controlled infrared energy source 34, such as model MS-153 black body manufactured by Electro-Optical Industries of Santa Barbara, Calif., is capable of generating radiations in the 3-5 um and 8-13 um wavelength region. Infrared energy source 34 is proximately disposed in alignment with the first and second optically transparent windows 12 and 14 respectively and in axial alignment with the wavelength selection filter 32 and scanning radiometer 28. The humidity and temperature of the chamber is determined and controlled by a hygrometer and a temperature controller 36 and 38 respectively each having its sensing elements operatively protruding through the chamber wall into the chamber 10.

Referring now to FIG. 2, plots of extinction coefficient in square meters per gram versus particle diameter in microns for various water droplets measured at various wavelength varying from the visible spectra to the infrared radiator on the apparatus as illustrated in FIG. 1. The various wavelengths of illumination are labeled upon the individual curves. It can be seen for the 0.53 um curve in the visible spectrum that the extinction coefficient has an extreme dependency upon particle diameter. At 0.53 um wavelength total extinction is virtually entirely due to "real index" scattering of the water droplets. As the wavelengths of illumination is increased, as shown on curves marked 1.05 um, 4.0 um, 8.5 um etc. the curves begin to flatten, with the peak or "hump" portions thereof flattening out toward the shorter droplet diameters. At the higher wavelengths of illumination reel index scattering, refraction, becomes unimportant when the droplet sizes fall well below illumination wavelengths. The curves approach a level determined, for smaller droplet sizes, by their "imaginary index" droplet absorption alone. It is also noticed that when the absorbing drops are small compared to the illuminating wavelength they behave almost as a gas, with no real-index scattering, with the important exception that the "gas" behaves according to the spectral characteristics of the aerosol material. The overall curve shapes at various wavelengths of illumination are thus seen to be dependent upon the shaping effects of the "real" (refractive) and "immaginary" (absorption) components of the complex index of refraction. It has been empirically determined using the apparatus of FIG. 1, that at or near a critical wavelength of 11.5 um illumination, because of the delicate balance between the refractive index contributions to curve shape at this precise wavelength, the instrumentation is virtually independent of droplet size number distribution over a very wide range of droplet sizes for water. From FIG. 2 we see that over the entire droplet diameter range of stable fogs and clouds of water, from droplets of less than 0.1 microns to droplets of more than 15 microns there is a variation of only approximately plus or minus 10% from a total extinction coefficient of 0.15 meters sq/gram for water clouds at the critical wavelength of 11.5 um.

In operation, calibration of the apparatus is accomplished by introducing a cloud of the type to be measured through either pipelines 20 or 24 into preheated chamber 10. Fans 16 and 18 are initiated to obtain a homogeneous mixture in chamber 18. The temperature controller 38 is adjusted to sustain saturation humidity. When the desired humidity is obtained on hygrometer 36 a homogeneous sample is drawn through exhaust pipeline 20 into a hygroscopic, preweighed sample tube or "gravimetric train," not shown, for determination of cloud concentration, typically in grams per cubic meter. At the same time, a signal generated by scanning radiometer 28 that was received from infrared source 34 through cloud 40 and circular variable filter 32 is monitored and compared to that obtained in the absence of any cloud. Knowing the ratio of the signals, the path length and droplet concentration at the critical infrared wavelength, the radiometer 28 is adjusted to give a scale setting on radiometer output meter 42 corresponding to the concentration of liquid of the cloud 40 in the sample volume using wellknown laws of optical extinction. As the cloud concentration varies, the radiometer output meter, calibrated in grams per cubic meter, will vary accurately indicating liquid water content. The critical wavelength of illumination for water was chosen at 11.5 um as shown in FIG. 2 for the reasons aforementioned.

The aforementioned method and apparatus is an example given for measuring the liquid content of a cloud for water, however the technique and apparatus are applicable to a wide variety of airborne droplet and aerosol materials provided that a suitable wavelength of illumination is selected or can be found to make operation of the instrument independent of droplet size and particle distribution.

The foregoing disclosure and drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense. I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described for obvious modifications will occur to a person skilled in the art.