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Taking droplet size measurements to the field

| August 17, 2016

To most users of pesticides, the key information on the label is the dose of active ingredient or actual product to be applied per hectare. Little information has been given on the spray volume or spray quality required. Historically, many pesticides were applied in large volumes of water to wet all the plants surfaces. As much as 2000 litres per hectare (L/Ha) were often applied, especially on orchard crops, despite most of the liquid dripping off foliage and being wasted on the ground. However, with water not always readily available in many parts of the world, and also costly to take to fields, the majority of pesticides are now applied at what are generally referred to as low volume sprays (<200 L/Ha). In some cases, very low volumes (<50 L/Ha) of spray have proven to be equally effective. Effectiveness with lower volumes is partly due to choice of the correct droplet size, which will depend on the target pest.

Figure 1: The VisiSize Portable with the LED diffuser positioned at maximum distance from the camera.

Figure 1: The VisiSize Portable with the LED diffuser positioned at maximum distance from the camera.

Whereas a droplet size of 70-150µm may be appropriate for insect pests, larger droplets are needed in some situations, such as where spray drift needs to be minimal, so sprays with droplet sizes of >150µm are needed. Unfortunately, the hydraulic nozzles most commonly used to apply agricultural sprays, produce a range of droplet sizes. Advice on droplet size has depended on using a spray quality classification initially introduced by the BCPC in 1985, which compares the spectrum of droplets produced by different nozzles, with selected standards that differentiate between very fine, fine, medium, coarse and very coarse sprays. More recent versions of the classification have additional criteria and include extra coarse sprays.

Measurement of the droplet spectra, produced by nozzles, was a tedious and lengthy process, until the development of instruments using lasers and computer technology. There are now several systems commercially available but these have generally been limited and confined to special laboratories. The spectrum will vary between different instruments, depending on how the droplets are detected and the position of the laser beam relative to the spray. Reference nozzles are used to relate data to the spray quality criteria. Recently there has been interest in scaling down the equipment so that a portable version can be used to measure the spray droplet spectra in the field.

Figure 2: (A) Close up view of the -8002E nozzle with water in front of the camera of the VisiSize Portable and (B) An image of the droplets gained from the VisiSize Portable.

Figure 2: (A) Close up view of the -8002E nozzle with water in front of the camera of the VisiSize Portable and (B) An image of the droplets gained from the VisiSize Portable.

The new VisiSize Portable, P15 (Fig 1) from Oxford Lasers Ltd measures spray droplets using shadowgraphy and is designed to operate with a laptop. Spray from a nozzle is back lit with an LED diffuse light source. The image of the spray from the -8002E nozzle, shown in Figure 2A, highlights the typical blurred image of droplets taken without a short pulsed light source. The LED’s short pulse length freezes the motion of the particles, allowing blur-free visualization of drop size and shape, as can be seen in Figure 2B.

Figure 3: The diameter number frequency.

Figure 3: The diameter number frequency.

The high resolution camera captures up to 15,000 droplets per second in real time mode. Images from the digital camera are transferred to a laptop and highspeed real-time particle sizing software analyses the images obtained, in order to build up the distribution of different sizes of individual droplets in flight within a spray (Fig 3 and 4). Software also allows for the velocity to be determined, hence the direction is provided. The high-magnification image-based measurement permits measurement of droplets without any motion-blur even close to a nozzle.

Figure 4: The diameter frequency by volume.

Figure 4: The diameter frequency by volume.

To demonstrate the new VisiSize Portable, water was sprayed from flat fan nozzle (8002E), operated at 1.5 bar pressure – a setting used for indoor residual spraying to control mosquitoes – was directed between the camera lens arrangement and the LED diffuse light source. A sample time of approximately 3 minutes allowed characterization of 10,000 droplets. This showed that the Volume Median Diameter (VMD) of the spray in the middle of the fan was 160.2 µm ± 1.3 µm (n=2) and the average velocity was 3.96 m/s at 20 cm from the nozzle (Table 1). The measurement was also completed with the Malvern Laser Diffraction particle sizer, providing a VMD of < 10% difference of 148 µm.

Table 1_001

With such a lightweight portable instrument it is now far easier to check the spray quality of nozzles to ensure that the correct droplet spectrum is being produced when fitted on spray equipment.

Published in International Pest Control – July/August 2016 issue

Author: Graham Matthews*
*Emeritus Professor, IPARC, Imperial College, London

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Category: Agriculture