Figure 1. MetroLaser's PDT particle sizing system.

MetroLaser's Pulse Displacement Technique (PDT) System is a prototype instrument that simultaneously measures particle size, velocity, and concentration in a spray of droplets. It is especially suited for applications characterized by a large size range and high particle concentration, such as in fire sprinklers. The instrument is based on the Pulse Displacement Technique, which involves measuring scattered light at two angles from droplets passing through two laser sheets. Experiments with the prototype system in a fire sprinkler have demonstrated a higher level of accuracy and a broader size range than is possible with other currently available instruments. The technique bases its measurement on accurate time-domain algorithms that are immune to fluctuations in laser intensity. Droplet diameters in the size range of 10 microns to 4 mm can be measured in a downpour of 200 cm/hr. Potential users include industries and government agencies interested in spray atomization, fire sprinklers, and powder technology, such as food, pulverized coal combustion, and sand blasting.

The instrument is shown in Figure 1. It consists of a probe assembly, seen on the right side of the photo, and a computer control system, seen on the left side. The probe consists of two gray boxes attached to a stand. These are the transmitter and receiver boxes, from left to right in the photo. Two laser sheets pass from the transmitter to the receiver, each at adifferent wavelength. Particles passing through the laser sheets scatter light that is collected in two directions by optics located in the receiver. Signals from photodetectors in the receiver are sent to the computer processing system via a 50-foot waterproof cable, enabling the control system to be placed a safe distance from the probe. Statistical data such as average, standard deviation, and size distribution are computed and displayed graphically.

Figure 2 shows a layout of the optical components of the instrument. The two laser beams, one infrared and one visible, are formed into sheets using cylindrical lenses L1 and L2. The laser sheets are made parallel to each other, spaced by 500 micrometers apart. Particles passing through the laser sheets deflect light from the beams into collection lenses L3 and L4. The collected light is then separated by wavelength using dichroic beamsplitters and bandpass filters, and then focused onto photodiodes PD1 through PD4.

Figure 2. Optical layout of PDT system.

Signals from the photodiodes as a particle passes through the measurement zone are depicted in Figure 3. As the particle crosses the first sheet, a large refraction pulse occurs on the upper detector, followed by a small reflection pulse. The same pattern of pulses occurs when the particle passes through the second sheet. Similar pairs of pulses appear on the lower detector after a short delay. For a given detector, the measured time difference between a pulse from the visible and IR lasers yields the particle velocity. Once the velocity is obtained, the particle diameter can be computed from the time difference between pulses from the upper and lower detectors for a given laser sheet.

Figure 3. Depiction of signals from a particle passing through the laser sheets.

The accuracy of the measurements is determined by the accuracy to which the laser sheet separation can be known, which is on the order of a few percent. This enables measurements of particle sizes that are much more accurate than with existing methods, which typically rely on frequency domain measurements and are therefore susceptible to errors caused by low-pass filtering. Figure 4 shows a comparison of particle size and velocity measurements from the MetroLaser PDT system and a PDPA system in similar fire sprinklers. The plot in Figure 4(a) is from a paper by Widmann using the PDPA system, while the plot in Figure 4(b) is from the PDT system using the Rasco G sprinkler, the same type as in Widmann’s paper. The PDPA data shows significantly more scatter than the PDT data. Also, the PDPA data shows some stratification that is obviously an artifact that the PDT system does not suffer from. These plots suggest that the PDT system offers a significant improvement in resolution. While the PDPA system is limited to a maximum diameter of 950 mm, the PDT system can measure particles up to 5000 mm in diameter. Figure 4(b) suggests that the PDPA system is missing a significant number of particles that are larger than 950 mm, while the PDT system is capturing them.

Where to Buy

This product is built from scratch to meet your unique requirements. Because each product is unique, we are unable to provide you with a demonstration unit. For more information, a quote, and delivery schedule, please contact MetroLaser Sales at: Tel: (949) 553-0688 - Fax: (949) 553-0495
sales@metrolaserinc.com