Overview

Besides the modular PDI systems and the PDI Flight Probes, Artium also offers several dedicated, turnkey PDI Probes for special applications.

The Demeter PDI Probe has been specifically designed for agricultural applications that require turnkey operation in the field under difficult operating conditions. It measures  the size and one component of velocity of individual spray droplets as they enter the measurement probe volume. Various spray statistics such as mean diameter, median diameter, and size-velocity correlation are computed and displayed in real-time. The probe can be used for characterizing spray nozzles, studying spray deposition, and measuring spray drift. The probe can also be used for wind tunnel testing and can be mounted in an aircraft for aerial spraying applications.

The probe includes an optical transmitter and optical receiver which are packaged together in an aerodynamic, water-tight enclosure. The system is aligned and calibrated in the factory. Routine alignment and calibration are not required.  The operating measurement size range can be manually changed by the user. The high powered DPSS  laser built into the probe provides stability, compactness, ruggedness, and high reliability; it eliminates the need for inefficient and unreliable fiber optics and bulky Ar-ion lasers.

The Artemis PDI Probe has been specifically designed for rugged, industrial and R&D applications that require turnkey operation. It measures size and one component of velocity of individual particles as they enter the measurement probe volume. Various spray statistics such as mean diameter, median diameter, and size-velocity correlation are computed and can be used to monitor and control the spray process in real-time.

The instrument incorporates an optical transmitter and optical receiver which are packaged in an environmental enclosure. The system is aligned and calibrated in the factory. Routine alignment and calibration is not required. The high powered DPSS lasers built into the transmitter provides stability, compactness, ruggedness, and high reliability; it eliminates the need for inefficient and unreliable fiber optics and bulky Ar-ion lasers.

Theron PDI Probe has been specifically designed for rugged, outdoor applications that require turnkey operation. It measures size and one component of velocity of individual particles as they enter the measurement probe volume. Various spray statistics such as mean diameter, median diameter, and size-velocity correlation are computed and can be used to monitor and control the spray process in real-time. Key features include:

  • Flow entrained through duct
  • Concealed laser beams
  • Bug spray application
  • Turn-key operation
  • NEMA enclosure
  • Inexpensive

The Turnkey PDI’s offer permanent alignment and are ready to acquire data right out of the box. The optical heads are sealed and rated IP66 for spray ingress protection. These instruments are designed to be placed directly into a spray with optional purge-air hoods to prevent window fouling and are lightweight enough to easily traverse.

Turnkey PDI instruments utilize the same second-generation Advanced Signal Analyzer (ASA2) processor and Automated Instrumentation Management System (AIMS) software as Artium’s research-grade Modular PDI systems. This allowsTurnkey users access to the powerful processor automation capabilities, data processing, and export capabilities of AIMS. The Turnkey system makes an ideal easy-to-use PDI but may also be used to enhance the measurement capabilities and flexibility of other Artium PDI systems.

The Turnkey line offers two body styles with differing working distances. Both body styles are available in three fixed measurement ranges. Multiple optical heads may be interchanged to optimize instrument sensitivity and diameter size range.

The PDI Flight Probes have been developed specifically for aircraft-based cloud studies that require the measurement of liquid droplet size distribution, velocity distribution, number density, and liquid water content (LWC). The probes incorporate the well-established phase Doppler technique for directly measuring the size and velocity of individual droplets in the cloud. The measurement method is sensitive only to spherical particles and therefore non-spherical ice crystals are rejected. Furthermore, the probe has the ability to differentiate between liquid droplets and droplets that are frozen. The PDI Flight Probe overcomes the inherent problems, such as depth-of-focus, measurement uncertainty, and coincidence errors in high number density environments, faced by older and obsolete measurement technologies which are based on forward light scattering.

The PDI Flight Probe offers turnkey operation with a fully automated setup feature. The flight probe system can be used for the real-time, non-intrusive measurement of individual droplet size and single velocity component in a variety of flight and wind-tunnel applications. The complete instrument includes the flight probe (including the optical transmitter and receiver), ASA signal processor, and the AIMS system software. The diode-pumped solid state (DPSS) laser used in the probe provides stability, compactness, ruggedness, and high reliability; it eliminates the need for inefficient and unreliable fiber optics. The probes have optional built-in heaters to prevent ice accretion. This allows the flight probe to be used under extreme icing environments without any signal loss.

The dual size range version essentially combines two PDI instruments in a compact, aerodynamic package. The output of the two PDI channels are combined to form a single histogram of the droplet size distribution. A canister version is also available.

Advantages of the PDI Flight Probes over Legacy Probes

Characterization of icing clouds and wind tunnel simulations require modern instruments for measuring droplet size distributions, droplet velocities, and liquid water content in realistic icing cloud and simulated icing cloud environments. Currently, instrumentation developed in the 1970’s by Particle Measuring Systems (PMS) have been widely used for this purpose. Liquid water content (LWC) measurements are generally acquired using hot wire devices based on the early Johnson-Williams approach (~1955). Although these instruments can provide accurate measurements of drop size distributions and LWC, they are also known to be limited in terms of measurement accuracy and reliability when subjected to challenging conditions. The key limitation of the PMS FSSP instrument is due to coincidence errors when measuring cloud droplet number densities greater than ~300/cc. Furthermore, in helicopter applications the uncertainty in the droplet incident angles of trajectory can produce unacceptable measurement uncertainty. The hotwire devices are challenged by low flow speeds and small droplets. There is also a concern that when measuring SLD conditions, the large droplets may shatter on the hotwire devices producing measurement uncertainty. The phase Doppler interferometry (PDI) method has been demonstrated to be capable of handling a very wide range of droplet size distributions and has the added advantage of measuring the droplet size and velocity simultaneously. As such, not only the MVD but the liquid water content and droplet number densities can be measured with this single instrument. The PDI method is capable of measuring droplet sizes that are as small as 0.5 µm in diameter to droplets as large as 5000 µm in diameter or larger, provided that the droplets remain quasi-spherical. Artium has integrated the phase Doppler method into a single compact instrument that can be carried on aircraft including helicopters. To cover the full size range of 0.5 to 1000 µm, two instruments with overlapping measurement ranges are incorporated into one single package. Measured results are combined automatically using fundamental principles to produce an unprecedented dynamic droplet measurement range. Legacy instruments used for cloud characterization include the FSSP and the OAP developed by PMS. Besides being an old technology, these methods have several sources of measurement uncertainty including:

  • Oscillations in the light scattering intensity as a function of particle size
  • High particle concentrations
  • Coincidence errors due to multiple drops being present in the measurement volume at the same time
  • Change in calibration due to aircraft vibrations and/or contamination of the optics by particles
  • Insufficient de-icing of the probe causing flow blockage
  • Depth-of-field related errors (for OAP)

Such instrument deficiencies have been by addressed by the PDI Flight Probes including:

  • Limitations and measurement error due to high droplet number densities, coincidence
  • Flow blockage by ice accretion in the flow straightener and proximity to the main instrument body
  • Large droplet shattering on the flow straightener, measured as small droplets
  • Limited dynamic range for droplet sizing Measurement uncertainty due to flow speed and angle of trajectory
  • Depth of field errors with the OAP
  • Insufficient heating to de-ice the probes
  • Requirements for frequent calibrations and uncertainties due to calibration issues
  • Uncertainties due to optical contamination and misalignment
  • Uncertainties due to merging measurements from two disparate methods, FSSP and OAP
  • Uncertainties due to mixed phase conditions and the inability of the FSSP to discriminate ice particles and droplets

Comparison of the PDI and intensity based light scattering particle sizing methods

Measurement characteristics

PDI

Light Scattering Intensity

Measured quantity Light wavelength in the phase shift between Doppler signals which is linearly related to the drop diameter Absolute light scattering intensity which is proportional to droplet diameter squared
Response Function Response is linear with droplet diameter, d Response proportional to d2 and has significant oscillation for small droplets and is approximately parabolic
Particle Detection Based on sinusoidal character of the signals, based on digital detection using signal level and SNR Based on simple signal peak threshold detection, may be triggered by noise
Sample Volume Off axis light scatter detection reduces to well-defined cylindrical sample volume truncated by a selectable aperture in the receiver, off axis light scatter detection of 30° Confocal light scatter detection from a focused laser beam, length of the probe volume determined by the depth of field of the receiver and is dependent on droplet size
Effects of Gaussian beam intensity distribution Sample volume is a function of the drop diameter and must be corrected (and is corrected) Second detectors required to measure only those drops passing the center of the beam, no apparent correction on sampling statistics for sample volume size changes with droplet diameter
Number density limitations Number densities to 100,000/cc may be measured without significant coincidence errors Number density greater than 300/cc is known to create sampling problems with coincidence
Droplet shape sensitivity Requires spherical or quasi-spherical spherical particles for accurate measurements Particle shape is not a severe restriction since the light scattering is primarily due to diffraction mechanism
Quantities measured Simultaneous measurements of drop size, velocity, and time of arrival leading to determination of drop number density and LWC Measures droplet size and time of arrival, true only for the Fast FSSP

PDI Flight Probe Applications

The PDI Flight Probe measures all the drops passing through a measurement volume to form statistical distributions of droplet size and velocity. Since every droplet is measured, the time history of the droplet size and velocity are provided. This allows the observation of temporal changes in the icing cloud and enables the user to sample over discrete periods of time to form the statistical information and perform higher-level analyses on the character of the simulated icing simulation clouds. A long record can be re-processed into suitable time intervals, if desired. The instrument provides real-time calculations of all the mean values including MVD, number density and cumulative volume distributions. Since the droplet size and velocity or measured for each droplet, the droplet dynamics information is available using the size velocity correlation plot. These results can be used to determine whether there is an aerodynamic bias to the sampling due to the smaller drops responding more readily to the airflow in the vicinity of the probe. Results have also demonstrated that in wind tunnels, the larger drops may not reach the test section flow velocity which will bias the sampling results obtained with such devices as the J-W probes.

Accurate measurements of cloud of water content requires that all droplets passing the sample volume be detected and measured accurately in terms of their size and velocity. Accurate characterization of the instrument measurement volume is also required. A key advantage of the PDI method is that the signals have a sinusoidal character which can be easily detected in the presence of noise as compared to, for example, methods that simply use the threshold level detection system. Digital signal detection invented by our team has been perfected and incorporated into our latest signal processing system. This method continuously samples the incoming voltage from the detectors and analyzes overlapping segments to detect coherent signals (sinusoidal voltage fluctuations). If the signal to noise ratio exceeds a specified level that indicates the presence of a sinusoidal Doppler signal which sets the gate signal alerting the signal processor to send that packet to the computer for high level analysis. These detection means are reliable even under low signal-to-noise ratio measurement conditions. For aircraft icing applications, the signals are generally well-defined due to the fact that the droplet number densities are not high as compared to other applications.

The PDI-FP and the PDI-FPDR have been tested under typical aircraft icing conditions in various aircraft icing facilities including the U.S. Air Force McKinley Climatic Laboratory, CIRA Italy, and the Boeing Research Aerodynamics Icing Tunnel (BRAIT). It has been flown on various fixed wing aircraft including a U.S. Navy Twin Otter, a C-130, and the UH 60 helicopter. These results have been compared to the PMS FSSP and OAP.