The measurement of spray systems can be challenging for optical techniques such as laser diffraction. Sprays are often difficult to control and can easily deposit on optical components, invalidating any measurements which have been made. As such, it is important that the measurement system optics are placed as far from the spray as possible. For laser diffraction systems, the working distance of the optics is the most important consideration, as this defines the maximum distance between the spray droplets and the optics which can be used if accurate results are to be obtained.
This application note describes the importance of the working distance for laser diffraction systems. The working range of the Malvern Spraytec system is also tested using two spray systems which produce different particle sizes.
The optical configuration of the Malvern Spraytec laser diffraction system is shown in figure 1. The laser light source is located in the transmitter module on the left hand side of the instrument. The laser beam is expanded to provide a beam diameter of 10mm and is then transmitted across the measurement zone where the spray is introduced. The laser light scattered by the spray particles within the measurement zone is collected by a lens and focused onto a series of light-sensitive detectors which measure the angular dependence of the scattered light intensity. Coarse particles tend to scatter light at small angles whereas fine particles scatter more light at wide angles. As such, by analyzing the changes in light scattering intensity as a function of angle it is possible to calculate the size distribution of the spray.
The lens used in the Spraytec system is a Fourier transform lens. The function of this lens is such that any light scattered to the same angle by the particles within the measurement zone is focused to the same point on the detector system (figure 2). This is achieved independent of the velocity or position of the spray particles within the measurement zone, such that at any instant the light scattering pattern measured by the detector system is indicative of the particle size distribution of all the particles present in the laser beam. It is important to ensure that the spray particles pass close enough to the lens to ensure that any light scattered at wide angles by fine particles within the spray can be collected and measured. If the particles are too far away, the light scattered at wide angles will not be measured correctly, impairing the system's ability to detect fine particles (figure 2).
The maximum allowable distance between the particles and the lens defines the Working Distance for the laser diffraction system. This is defined by considering the maximum scattering angle which must be measured (which in turn relates to limit of detection for fine particles) and the physical size of the lens (the working distance can be increased by increasing the diameter of the lens). In the case of the Spraytec's 300mm lens, the minimum median particle size (Dv50) which can be measured for a spray is 0.5μm. This sets the maximum working distance as 150mm if particles of this size are to be accurately measured. This is much larger than that achieved for traditional laser diffraction systems, where the working distance for this size is typically only 20mm. If the minimum size of the particles within the spray is larger than 0.5μm then the practical working range will increase, as larger particles will scatter light at a smaller angle. As such, particles of 5 microns in size can be measured 1m away from the Spraytec optics.
The working range of the Spraytec system can be experimentally verified for a specific spray by determining how the reported particle size changes as the spray is moved further away from the receiver optics. The measured particle size should be independent of spray position when measuring close to the lens. However, as the distance increases, a point will be reached when the detection of fine particles within the spray is no longer possible, and a result shift is observed. The distance at which this occurs will be smaller for fine droplet sizes compared to coarse droplet sizes.
Figure 3 shows the data obtained during the verification of the working range for the spray produced by a pharmaceutical nebuliser. Nebulisers produce very fine particles, so represent a good test for the working range of the optics. The quoted distance in each case is for the centre of the plume, with the plume width being approximately 20mm. As can be seen, the results are consistent until the nebuliser is placed 170mm away from the receiver optics, at which point a narrowing of the distribution is obtained. This becomes more pronounced when the nebuliser is moved to 220mm and 270mm away from the optics.
The reason for the change in results observed when the nebuliser is positioned greater than 150mm from the optics can be seen in figure 4, where the scattering data collected for the nebuliser at each distance is shown. Here, the low detector numbers relate to measurements at small angles (large particles) and the high detector numbers relate to measurements at large angles (small particles). Data is observed across all of the detectors when the measurement distance is less than 170mm, hence the complete size distribution is obtained. At 170mm, part of the spray plume is positioned beyond the working distance, leading to a loss of data at very wide angles (detector 36). This data loss becomes more pronounced when the spray is positioned at 220mm, such that the response on detectors 35 and 36 is no longer correctly measured. At 270mm data loss occurs on detector 34 and higher. It is this which leads to the narrowing of the reported size distribution.
The same experiment can be carried out for sprays containing coarser particles. Figure 5 shows the particle size reported as a function of distance for a spray system producing droplets with a median particle size in the range from 55μm to 60μm. The results obtained are independent of the measurement distance (the Dv50 varies by only 1.9% across all of the measurements, which is within the expected reproducibility of the device). Accurate measurements are able to be made even when the spray is positioned almost 0.5m away from the optics, ensuring that the risk of optical contamination is minimized. These results confirm the considerable increase in the effective working range of the Spraytec when large particles are measured.
The working range of a laser diffraction system is critical in the case of spray measurements, as it defines the maximum distance the spray can be positioned away from the optics if accurate measurements are to be made. This, in turn, relates to the likelihood that optical contamination will occur during measurements. The results presented here show that the working range of the Spraytec is large, allowing fine droplets to be accurately measured far from the optics.