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Accurate Calibration of a Knapsack or Spraying Equipment

SprayiingWe are often asked by pesticide users, amateur and professionals alike about the process of calibrating application equipment such as Knapsacks. Calibration of the applicator will ensure that the correct amount of chemical active ingredient(s) is/are mixed with the correct amount of water and then applied to the appropriate number of square metres of lawn or grass or non crop area to guarantee the results.

Users of such products get a bit confused with the product label. After all, this is the first place to look for statutory, advisory and instructions for use information. The work has already been done for you by the Manufacturer, so why not take a few minutes to read the product label fully ahead of use?

Most liquid turf care products whether weed killers or fertilisers etc are quoted on the product label as needing to be applied at a permissable dose rate (amount of chemical concentrate to apply) per Hectare, expressed as one Ha. For example, you may see 'Apply at 5 Litres per Ha'. One Hectare is 10,000 square metres (sqm). There are 2.4 Acres per one Ha so it is easy to calculate that one Acre is approx 4040 sqm in area or 0.4040 Ha.

I like metric as it is all divisible by 10, unlike imperial which relates to pounds and ounces and gallons which is not divisible by ten and I get a bit lost with it!

There are only three measurements to collect when calibrating a new/old knapsack -

1. The walking speed of the operator expressed in kilometres per hour (Kph)
2. The output per minute of the sprayer expressed in Litres or a apart thereof
3. The width of each pass of the sprayer, commonly known as the Swath Width expressed in metres or part thereof

So armed with your new knapsack, put together correctly and half filled with water only, you can continue to calibrate it. Put you protective clothing on first!

The methodology is thus:

1. Assess walking speed - Measure a distance of 100 metres. Put the knapsack on your back and start pumping, walk at a steady walking pace, spraying with the nozzle at knee height and recite the word 'one thousand' over and over again making one pump stroke per 'one thousand'. Time yourself in seconds, and take an average from two 100 metre passes. Let us say that the total number of seconds is 75 seconds. (The 360 figure below is a 'constant' that never changes so do not worry about this, it was invented by a clever Chap who was good at numbers...)

The formula is 360 divided by time to travel 100 metres equals x Kph
360 divided by 75 seconds equals 4.8 Kph

2. Measure the output of the sprayer - When the knapsack is up to pressure, release the trigger and hold the nozzle right into a metric calibrated vessel like a jug and time for 1 minute, only pumping and reciting the 'one thousand' over and over again, making sure that you hand pump once only per 'one thousand' bit. After a minute, assess how much water has been sprayed out and express this as a litre or part thereof. So for example let us say that the single nozzle sprayed out 1.2 Litres per minute. Make sure that the receptable is big enough to collect the expected amount of water to be sprayed out in a single minute or time for 30 seconds and double the actual amount in the jug. If you have more than one nozzle such as on a trolley sprayer, multiply the single nozzle output by the number of nozzles and use this figure in the calculation. For example if each nozzle puts out 1.2 Litres per minute so the total output of the spraying unit per minute is therefore 3.6 Litres.

3. The swath width - walking along a dry piece of tarmac or driveway, once the sprayer is up to pressure and walking at the previously discussed speed of one stride per recital of 'one thousand' holding the nozzle at knee height, spray continuously for 10 strides then stop and quickly measure with a tape measure the wet footprint width of the spray before it dries. Let us say 0.8 metres or 80 centimetres average width. If you have more than one nozzle fitted such as on a trolley sprayer, measure the overall swath width of the sprayer / wet footprint, not the actual physical boom width. The extra nozzles and overall width will almost cancel eachother out in practice. All it means is that you will cover the area in fewer passes but the calibration will still be the same on spray volume per Hectare - see later.

The formula for assessing the overall output of the sprayer per Hectare is thus -

600 multiply by the output per single nozzle per minute divided by the operator speed divided by the swath width equals the total output in Litres of the operator and knapsack per Hectare. (The '600' is another one of those constant figures like the '360' in the speed formula).

600 multiply by 1.2 Litres (Output) divided by 4.8 KPH (Speed) divided by 0.8 metres (Swath width) = 187 Lts per Ha Spray Volume.

The Spray Volume is actually the amount of chemical and water per Ha. So 5 Lts of chemical per Ha in a minimum of 200 Lts of water per Ha will actually be 5 Lts of chemical and 195 Lts of water to make up the overall 200 Lts of Spray Volume per Ha.

If the Spray Volume is too low as defined by the label (which may state apply 5 Lts per Ha in a minimum of 200 LTs of water per Ha) then you will need to change the nozzle, or walk a little slower to acheive the desirable spray volume to keep within the label and the law!

Now you need to know how much chemical concentrate (at label Dose Rate per Ha) to put in a full knapsack -

Remember this useful formula once the Spray Volume (SV) is known -

Tank Capacity (TC) multiply by the Dose Rate (DR) divided by The Spray Volume (SV) = Amount of Concentrate (C) in Litres or part thereof to put in a full knapsack. Let us say that we have a 15 Litre knapsack and a chemical which is applied at 5 Litres per Ha and our calibrated Spray Volume per Ha is 200 Lts.

TC multiply by DR divided by the SV = C
Answer is 15 multiply by 5 divided by 200 = 0.357 Litres

How much will one tank cover at this spray volume per Ha.

Simply divide the Spray Volume (SV) by the Tank Capacity (TC) to get number of full tanks per Hectare thus -

TC divided by SV equals number of tanks per Hectare
15 divided by 200 = 13.33

How many square metres will one single full tank cover?

Divide the number of square metres in a full Hectare which is 10,000 by the number of tanks per Hectare to get the answer thus - 10,000 divided by 13.33 equals 750 sqm per each full tank.

Assessing beforehand the area to be sprayed is therefore important. you should not be out between table top calibration and actually performing the spray job by more than +/- 5%. If you are, re check your calibrations.

One useful formula is knowing how much water and/or chemical is required for a particular job where the area to be sprayed has been measured with a metric measuring wheel.

If the spray volume is 200 lts per Ha and the Dose Rate is 5 Lts per Ha -

200 divided by 10,000 multiplied by the quantity of sqm that needs treating will tell you how much chemical concentrate to use on the area

5 divided by 10,000 multiplied by the quantity of sqm that needs treating will tell you how much water is required for the job too.

So at the end of the job, if you have chemical or spray volume left over, your area is smaller than you originally measured or you have not done it correctly!! Equally, if you run out of spray solution before you have covered all the area, you may have applied too much by walking too slow or over lapping spray swaths.

What I love about this calibration process is that because it is decimals, you can quickly ascertain so many other figures from those that we are working with here and easily see the relationships between them all. Maths was not really my strong point but I can grasp the process of calibrating a knapsack in theory and practice which is so important.

Once you have calibrated your knapsack or sprayer, half fill the sprayer with water, add the required amount of chemical and then fill to the full mark. If your calibrations show that you only need half a knapsack for the job in hand then work on 50% of the figures and do not mix up more than you require for the job in hand.

If you need more information on this subject, feel free to post a comment!

We have added an Excel Spread Sheet entitled calibration_sheet_example.xls that can be downloaded which clearly shows the boxes that need to be filled in and finished up detailing exactly how much water and how much chemical concentrate is required for pre determined number of square metres of area that needs spraying. Hover the mouse over the red boxes for a pop up comments box to help you.


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great help, well laid out and saves a lot of stress!!


Question - The herbicide label says that to treat 100 sq m I'll need 20ml of product and 10 litres of water. However, when I plug in all my figures into the spreadsheet, apparently, I only need 2 litres of water to treat 100 sq metres. The concentrate amount is correct (20 ml) but the water dosage is very low - does it matter?

Answer - the product is applied at 2 Lts per Ha which is 10,000 sqm. So this equates to 20 ml per 100 sqm.

The Ha rate of water is 1,000 Lts.

You should apply 20ml of concentrate in 10 Lts of water per each 100 sqm.

The calibration sheet is assuming that the knapsack calibration is for a 15 lts sprayer and calibrated to apply a spray solutuon at around 200 lts per Ha (chemical and water).

It really depends upon the applicator as to how much chemical goes into a full tank as the Excel spreadsheet shows because ours shows a calibration process to only 200 Lts per Ha and your product is asking for 1,000 lts per Ha - quite a difference so we would need to travel slower, with a large nozzle apperture to get that amount of water down on a lawn.

The reason why the sheet does not work for you is that the only figure I changed was the chemical dose rate down from say 5 lts per Ha to your 2 Lts per Ha.

It is important to always follow the label but in practice, 20ml in 5 Lts of water per 100 sqm would still be okay.

Hope this helps you Richard.

Mike Seaton

Hi Steve

Thanks for the enquiry. I have doubled checked the text and it still makes sense to me. If you read this inconjunction with the excel spread sheet it will all become clear. The spray volume is chemical and water total so 195 lts of water and 5 lts of chemical to treat 10,000 sqm or 1 Hectare


At the end where you have 200 divided by 10000! X a given area You say this gives chemical rate!
Then you have the water rate that should be the chemical rate 5li,
Or have I got it wrong?


Thanks for this info.
I am going to use the exel sheet with my school students

Great job Thanks a million!!!

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