March 18, 2019

Droplet size: what to understand about the measuring methods

droplet-size-measurement

Droplet size: how do we measure the diameter of a water droplet?

As nozzle manufacturer, we often use the term spray droplet diameter, mean droplet diameter, water droplet size, droplet spectrum, spray drop size, etc. But how do we measure the diameter? Is there an international standard to align all companies around the same measurement? How important is the size of a water droplet?

Let’s have a closer look to droplets size!

Classification of spray droplet diameter

When talking about water droplets, there is no general classification. As “The Fog Engineers”, we classified them based on spray droplet diameter measured by the immersion sampling method. The size of a water droplet differ depending on the measuring method.

Classification-of-Spray-Droplet-Size

How important is the size of a water droplet?

Having a water droplet of 10 μm (fine fog) or 1 mm (light rain) has a huge difference on the impact and the wetness. Depending on the application, different size of water droplets is required. For example if you want an effective washing application, you will not use a fine fog that evaporate in the air before reaching the surface you want to clean. And on the contrary if you need to cool down a room without wetting, you will not spray a light rain droplet diameter as you will wet the entire room creating water puddles on the floor.

Spray droplet measuring methods

There are several methods used to measure droplet diameter.

  1. Immersion sampling method
  2. Laser analyzer: Fraunhofer diffraction method ; Laser Doppler method

The immersion sampling method and the laser analyzer are used as industrial methods of measuring spray droplet sizes.

Immersion sampling method

Droplets are collected on a glass plate coated with silicone oil and they are immediately photographed at high magnification for subsequent scanning. In this method, the collected droplets quickly settle in the silicone oil and do not evaporate even underneath the strong light while being photographed. Stuck in the silicone oil, they are measured as perfect spheres.

immersion sampling method

Immersion sampling method illustration

However, ultra-fine droplets, too small to break the surface tension of the oil and stick, will evaporate. Thus, the droplet sizes of the fine and ultra-fine fog determined by the immersion sampling method are larger than the actual values.

Laser analyzer: Fraunhofer diffraction method

Fraunhofer diffraction method

Fraunhofer diffraction method

In this method the principle of diffraction by laser beam is used. When the droplets get in the way of the laser beam, they scatter the direction of the laser rays creating a diffraction pattern (Fraunhofer diffraction).

The diffraction pattern depends on the size of the droplets and their distribution.

This method can simultaneously measure all the droplets that interfere with the laser beam.

But if the concentration of droplets in the spray is too high, a phenomenon called multiple dispersion can be caused in which, a ray diffracted by a drop can be diffracted again from another drop. This alters the measurement by making the measurement smaller than the actual droplet size.

Laser analyzer: Laser Doppler method

This method is based on the creation of a beam where two laser beams interfere.

When a droplet passes through this beam, two or more sensors located at a certain distance detect the phase difference of the scattered light by determining the size of the droplet.

The advantage of this method is that it does not affect the spray concentration range of the spray because it measures the size of the droplets one by one and also measures the speed of the droplets at the same time. However, measurement is made only at a single point of the spray.

Laser doppler method

Laser doppler method

Comparison between the 3 methods

conversion factor for mean droplet diameter

Correlation of Sauter mean droplet diameters among three measuring method

Depending on the measuring method used, the size of the same water droplet will differ. Assuming the mean droplet diameter measured by the immersion sampling method is equal to 1 (as a relative coefficient number), the value will be different when measured with the Fraunhofer diffraction method and the Laser Doppler method.

We use the immersion sampling method in our hydraulic catalog to give an indication about the spray droplet diameters of each nozzle series. And for our pneumatic catalog, we always specify which method we used to measure the diameter.

Choose the correct spray droplet diameter for the application you are looking for and ask our project engineer to help you choose the optimum nozzle.