Hello everyone my name is Leo Rivera. I’m the Director of Scientific Outreach here at METER Group. And in our previous videos, we’ve talked about some of the sedimentation approaches for measuring particle size analysis. In this lesson, we’re going to talk about our optical approaches for measuring particle size analysis of soils. The three most common methods for making these measurements are:

• X-ray attenuation
• Laser light scattering or laser diffraction
• Vis-NIR visible near infrared spectroscopy

In this lesson, we’re primarily just going to focus on the laser diffraction method, but X ray attenuation and Vis-NIR spectroscopy are also both very good tools for measuring soil particle size distribution optically. Laser diffraction method works based on the fact that particles that are different sizes diffract the light at a different angle relative to that size. We need to disperse and treat that soil just like we would for the sedimentation methods, but that light is passed through that suspension of soil. You can see an example of that sample cell, and the light being passed through the sample and the receiver, and the diffraction of that light is measured with those detectors, and then we use the Mie theory to relate that diffraction to the different particle sizes that are in suspension. The typical sizes that we can measure with the laser diffraction method are 0.04 to 2000 microns. This is not quite down to your clay sizes, but it gets pretty close. That is one of the limitations of the laser diffraction method. Another challenge with the laser diffraction method is that you typically are using smaller sample sizes. It can be less representative of the measurement that we’re trying to make. Ideally, more sample helps us characterize things a little bit better. Laser diffraction is also very expensive, so that can be a challenge. Most of these devices are orders of magnitude more expensive than than the sedimentation methods. However, this method has a much higher throughput, so you do not have to wait for multiple hours to make measurements. It is a very, very quick measurement. That is an advantage if you have the budget and you have the time to invest in a tool like this, because you can get through a lot more samples. One of the other challenges with the optical method—especially with the laser diffraction method—is that we’re measuring the diffraction based on the particle size. And clay particles, for example, are flat, and so depending on the orientation of that particle, you may actually wind up measuring a different particle size than the actual average size of that particle, and how it how it relates to everything else. So, particle orientation can result in error in your measurement. We’ve talked about quite a few different methods for measuring soil particle size distribution, and we’ve talked about their pros and cons behind each method. I personally have a lot of experience myself, of course, with the hydrometer method and now the ISP+ method, but it’s really important to weigh the pros and cons that we talked about with your measurement goals and your research goals when it comes to choosing the right tool for making your measurements. And in our next video, we’re going to talk about the ins and outs of the PARIO method and the ISP+ method and how to best utilize those tools for your research.