How to Calibrate a Knapsack Sprayer

The first thing that should be said about the application of pesticides is that if you are paying for it to be done, the operative must have a spray licence. It is illegal for anyone to apply pesticides commercially without one.

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Having said that, you may want to use pesticides yourself, and have recently purchased a portable sprayer from a garden centre and be wondering how much concentrate to use. For the purposes of this blog, we will assume you want to kill the weeds on a large gravel drive.

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I should also add at this point that if you are going to use a sprayer you should be wearing protective clothing, gloves and rubber boots at all times.

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Working out how to deliver the correct amount of active ingredient is called ‘Calibrating’. And where this is concerned, IT IS THE PERSON THAT HAS TO BE CALIBRATED and not the sprayer. This is because we all walk at differing speeds on level ground, so there is a possible variation there between how fast you might walk compared with someone else. Secondly, you may be walking uphill, or on soft ground or there may be factors which reduce your walking speed that you hadn’t considered. So if the terrain is a little difficult it is always a good idea to walk it beforehand.

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So, back to our gravel drive. ….  and the first thing to do is to see how far you can walk at a comfortable and manageable speed with the sprayer partly filled with a known quantity of clean water, until it runs out. So maybe using somewhere where the ground is dry and you’ll be able to see where you’ve been, like your own driveway, patio or some other similar location. So pump up your sprayer to its working pressure and then walk till you run out. Then measure the width of the spray pattern and the length you travelled before you ran out. When multiplied together, these two figures will give you the area you would have covered at your comfortable pace using say,1 litre of water. Then it’s simply a question of multiplying that area by the number of litres that the spray tank can carry. You then know what area 1 tank full will cover at your comfortable pace of walking.

[If you were then going to kill off an area of grass on a slope, you would have to calibrate yourself again to allow for the difference in your comfortable walking speed, because of the effect of the slope. Never forget you could be carrying 15 litres of fluid on your back, and it makes a difference.]

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So let’s say that with your 15 litre spray tank  YOU  (not someone else) can cover 285 square metres with 15 litres. The next thing to know is the total area of where you want to treat with your spray. And once measured, let’s say it’s 1640 square metres. Divide one into the other – we’ve all got calculators on our phones these days – and you end up with 5.754385964. So we’ll call that 5.75 Spray Tanks full.

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That’s good – we’re getting somewhere. But we’re not done yet, because there’s one vital thing missing.

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We need to know how much of the active ingredient to include in the spray solution. To be able to do this, we need to consult the product information leaflet. This leaflet is a ‘legal’ document which tells you, amongst other things the application rates which you must adhere to. To apply the incorrect concentration, whether it is too great or too small is illegal, and can attract a fine, so we really have to get this right.

On this label will be given an application rate which is often in ‘Litres per Hectare’. 1 Litre is 1000 millilitres, and 1 Hectare is 10,000 Square Metres.

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So, if the application rate is quoted as being 4l/ha, it means no more than 4 litres of concentrated product per hectare.

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So to find out how much product we must include in our 15 litres tank, we must calculate how much product we need to treat 1 square metre. By dividing the number of square metres in 1 hectare (10,000M) into the number of millilitres recommended per hectare (4000ml), our calculator will give us the result of 0.4 ml.. If we then multiply that by 285, then our answer is 11.4ml of product to be added to the tank.

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 Once you’ve done that, you will have all the information that you need to spray your driveway and kill your weeds, legally and safely.