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Calibration: the key to accurate application

Advisory Blog
27.05.2021

For sprayers, calibration is all about making sure you’re applying a known spray volume over a given area, so you can apply the correct dose of the products you’re using.

The principle is the same whatever the type of sprayer: you’re measuring the amount of spray over a set time and combining that with a calculation of the time taken to cover a known area.

 

Hand-held sprayers. With hand-held equipment, the speed you walk during spraying is a key factor – so when you’re calibrating, it’s important to choose an appropriate pace for the conditions you’ll be working in. Remember, you’ll need to maintain that pace consistently while working, including while wearing PPE. For most people that’s going to be around 3km an hour, the equivalent of 50m in a minute – but will be affected by the layout of the crop area and any obstructions such as greenhouse stanchions.

Nozzles and operating pressure should be those you’ll be using on the crop. Your nozzle choice may depend on the product and on the crop being sprayed so you’ll need to calibrate, and make a record for, each nozzle type used. Set pressure between 2 and 3 bar for the optimum balance between coarse droplets, which reach well into the canopy but are prone to run-off, and fine droplets that give good coverage but are susceptible to drift.

Unfortunately many compromises are made when spray pistols are used to apply products across a nursery bed or bench. That can lead to a range of inaccuracies including:

Poor coverage

Higher pressures being used in an attempt to ‘throw’ the spray, resulting in too-fine droplets that evaporate before they hit the crop and pose a potential risk to the operator

Uneven distribution – run-off on the nearest plants and poor coverage at distance.

Horticulture spray application

To calibrate a hand-held sprayer:

1. Walk at your operating pace for 100m and time that.

2. Spray water into a container (ideally a measuring cylinder) for that length of time, and record the volume.

3. Spray water to a path or similar area from an appropriate operating height and measure the width of the spray pattern.

 

From these three measurements you can calculate application volume per square metre.

 

For example (based on recommendation by the British Crop Protection Council):

Average time you take to ‘spray’ over a distance of 100m = 110 seconds

Amount of water you sprayed into a container during 110 seconds = 3,900ml

Width of your spray pattern = 1.6m

Spray volume per sq m = 3,900 ¸ 1.6 ¸ 100 = 24.4ml per sq m or 244 litres per ha.

 

That’s the figure you can now use to calculate the amount of product required in your tank. For example, if the required dose rate (from the product label) is 5 litres per ha and you require 20 litres of spray liquid in the tank:

 

Amount of product = 5 x 20 ¸ 244 = 0.41 litres of product and 19.59 litres of water.

 

The principle is the same if you’re using a hand-held lance or boom with more than one nozzle – you just need to collect the spray output from each nozzle for your calculation and measure the full width of the spray pattern (see section on boom sprayers below). For a wheeled sprayer with a boom and no power, measure the time taken to push it for 100m, as when calibrating a hand-held sprayer.

 

Spray pistols. Rather than interchangeable nozzles, most spray pistols have a rotating adjuster to vary the spray characteristics. Ensure this is set to deliver the correct characteristics (unless specified on the crop protection product label, you should aim for a medium to fine spray) before calibration. It may be useful to make a permanent mark on the adjuster. Smaller nozzle apertures help reduce flow rates and hence application volumes while pressure also has a considerable influence on spray pattern.

 

Calibration is very tricky with these: any adjustment of the nozzle will alter the output and therefore the application rate. Calibration should be undertaken using a fixed nozzle position. If for any reason it is subsequently adjusted, you’ll need to recalibrate again.

 

However carefully you calibrate this type of equipment, your spray technique, particularly your wrist movements, is crucial in delivering the correct application rate across the crop. Rather than relying on good calibration, delivery quality is very much in the hands of the operator. In many cases a long lance with a single nozzle, multi-nozzle or ‘mini-boom’ head will give a more consistent application.

 

Self-propelled boom sprayers. As with hand-held and pedestrian-operated sprayers you need an accurate measure of forward speed, spray output and area covered in order to calculate the volume of product to put in the tank to achieve the required dose.

 

1. Measure in seconds the time taken to operate the sprayer over a marked 100m on a similar surface to where you’ll be spraying, at the speed you plan to use when making applications.

2. The true speed in km/h is this time, divided by 360.

3. Measure output from each nozzle for 30 seconds, noting each one separately. Add the total output and divide by the number of nozzles to arrive at an average – if any individual nozzle differs by more than about 5% from the average it is worn and needs replacing. Double the volumes to get output in litres per minute.

4. Calculate spray volume applied in litres per ha:

Volume = nozzle output (l/min) x 600 ¸ speed (km/h) ¸ nozzle spacing (m)

 

So for example with a flow rate per nozzle of 1.36 litres per minute and nozzles at 0.5m spacing, the water volume application rate when spraying at 3.8km/h would be:

1.36 x 600 = 816 ÷ 3.8 = 215 ÷ 0.5 = 430 l/ha

Product rates can then be calculated.

Note that, unlike with hand-held equipment, you don’t need to measure the width sprayed. By using the nozzle spacing, the amount delivered to an area is calculated rather than the amount per boom so the calibration is correct no matter which boom is on or off.

 

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