Here at Weed Free we use pesticides almost every working day, normally having a break only when it is blowing a bit of a storm or torrential rain.

The key aspect of pesticide application is having the correct methodology to make the work effective but also as easy as possible for the Team.

You could have the most expensive knapsack in the world but unless it is calibrated correctly, you may end up doing the task twice in order to get the desired results.

Recently, I was talking to a client who told me that he applies a Glyphosate based weed killer on his farm, but he commented that the Farmer owning the adjacent land, always achieved better results than him even though both were applying the total weedkiller with the same active ingredient.

When I asked about the output of the sprayer and amount of product that went into one full tank, the client looked at me as if I was talking double-dutch.

We went through the calibration process with his knapsack and established that he should have been putting 1000 ml or 1 Litre of Glyphosate in a full tank in order to comply with the label and get the desired results.

Our pesticide training services are always interactive and very hands on, offering advice and additional information that is not in the NPTC pesticide schedules but taken from our own experience of application of pesticides and turf care products in many varying situations where our work dictates.

We 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 chemicals 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 turf care products get a bit confused with the product label - 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 it is advised 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 specific 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 2x100 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 receptacle 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 each other 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 Ltrs 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 Ltrs of Spray Volume per Ha.

If the Spray Volume is too low as defined by the label (which may state apply 5 Ltrs per Ha in a minimum of 200 Ltrs of water per Ha) then you will need to change the nozzle, or walk a little slower to achieve 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.

Hi there,

Really useful article on how to calibrate- thank you.

I don't know if you can help, but I am struggling on how best to use a specific liquid fertiliser in terms of Nozzle selection or what would be the best practice to achieve the desired Spray Volume, as the water volume is quite high.

The label states 10-20l/ha in a minimum 600-800l or by knapsack 10-20ml per 10l.

I've passed the pa1&6 but do not really have any experience. I understand the calibration process but obviously do not want to mess up the rates by not achieving them.

Could you give me an example of how you would go about calibrating and what Nozzle you would use.

Also, if you use a boom attachment holding 2 Nozzles, does that double the application rate or just increase the Swath?

Many thanks

Reply: We have emailed you for some more information and we can help you via email.

Posted by: Jonny | 16 June 2017 at 21:32