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You are here: Agriculture » Farming and Management » Chemical Use » Agricultural Chemical Use » Spraying, Spray Drift and Off-target Damage » Using Knapsack Sprayers and Compression Sprayers

Using Knapsack Sprayers and Compression Sprayers

Note Number: AG1223
Published: November 2005
Updated: December 2010

 

Introduction

Many people who have a small spray job to perform are often concerned that a knapsack or compression sprayer is not capable of doing an accurate job, and that they are only useful for spot spraying where the water application rate is not as critical as in other spraying jobs.

This is far from the truth, and in fact both knapsack and compression sprayers can be highly accurate provided they are serviced, maintained, and calibrated correctly.

This Agriculture Note sets out the steps involved in ensuring that a knapsack or compression sprayer operates with a high level of efficiency, and is capable of producing a quality, accurate job.

Chemical Restrictions

Back-mounted knapsack application of certain chemicals is prohibited due to the Occupational Health and Safety (OHS) risks posed to the chemical user.  It is illegal to apply the following chemicals using back-mounted spraying equipment (i.e. knapsack or hand-held spraying equipment that is operated manually):

• mevinphos
• parathion
• parathion methyl
• paraquat
• methamidophos
• chlorthiophos.

These prohibitions are not specified on the product label, but regardless users must comply with these requirements.

Before applying any other chemical product using back-mounted or hand held spraying equipment, users should check the product label to confirm that the application is permitted. 

Equipment

Most modern knapsack and compression sprays are manufactured from materials that are both robust and easily maintained. Injection moulded plastics, stainless steel and brass are the materials commonly used by manufacturers, and produce an item of equipment, which should have a long and useful life.

Most manufacturers have also opted for a piston pump, either single or double acting for their sprayers for simplicity and longevity. Many of these pumps are field maintainable by hand and have few moving parts.

The range of attachments for these sprayers has also become much broader in recent years. It is now possible to buy a knapsack or compression sprayer fitted with high efficiency filters, pressure gauge, non drip nozzles, lance extensions and in some cases, even a small spray boom.

Service and Maintenance

Modern knapsacks and compression sprayers are designed for ease of service and maintenance. Only a few simple rules are needed to keep the equipment in top condition. These include:

• reading the service manual to check for any specific service requirement of your piece of equipment
• at the end of each spraying job, thoroughly clean the inside and outside of the sprayer, disassemble it, and clean each part meticulously
• check for wear and correct fitting of “O” rings (it may even be advisable to have a few spare “O” rings on hand)
• complete the service by reassembling the unit and returning it to storage. Plastic sprayers should not be left out in the sun, as the ultra-violet radiation harms the plastic and shortens its useful life.

Calibration

Calibrating a knapsack or compression sprayer is a simple task which takes little time. A sprayer with a pressure gauge is the best one to use because the pressure gauge assists in ensuring an even output is maintained.

There are a few points to remember before calibrating the sprayer:

• it must be clean, serviced, and operating according to the manufacturer's instructions
• only use water to calibrate the sprayer - calibration is never carried out using chemicals
• the accuracy of the calibration depends on whether you are able to use the pump to produce a constant pressure, and walk at a constant rate. This is why a pressure gauge is so valuable. If your sprayer is not fitted with a pressure gauge, you will need to operate the pump at a constant rate - so choose a rate you find comfortable.

Calibration method

1. Carry out a pre-operation service and prepare the sprayer for the job.
2. Mark out a 25m2 calibration plot in the area to be sprayed with a peg in each corner (the 25m2 can be any configuration – e.g. 5m x 5m, 12.5m x 2m, 25m x 1m or 16.6m x 1.5m, or any other combination to give 25m2).
3. Fill the sprayer to a known level, or use a permanent marker to mark the tank and fill it to that line.
4. Spray the calibration plot at the pressure and speed you will use when you do the job. Be sure to avoid over sprays, spills, missed areas, and any other spray faults.
5. When you have finished spraying the calibration plot, carefully take the sprayer off, take it back to a level area and measure the amount of water it takes to the fill the sprayer back to the original mark (from Step 3). The amount of water taken to refill the tank to the mark must be in measured in litres.
6. The amount of water applied to 1000 square metres`, or per hectare can be read directly off the table below or by using the following formulae:

Water rate/1000m 2 = Litres used on calibration plot x 40

Water rate/hectare = Litres used on calibration plot x 400

Litres used on 25 m 2 calibration plot	Litres/1000m 2	Litres/hectare
0.25	10	100
0.30	12	120
0.35	14	140
0.40	16	160
0.45	18	180
0.50	20	200
0.55	22	220
0.60	24	240
0.65	26	260
0.70	28	280
0.75	30	300
0.80	32	320
0.85	34	340
0.90	36	360
0.95	38	380
1.00	40	400

Mixing the spray (a worked example)

Let’s say we want to use a 15L knapsack to spray annual weeds in an area 50m long and 30m wide (1500m2) between two sheds, using a glyphosate herbicide product (360g/L) at 3L/ha.

We marked out a 25m2 calibration plot and sprayed it using water only and following the chemical label instructions. The amount of water used was measured as 0.4L. Using the table, this means that the water rate per hectare will be 160L/ha (refer to the yellow highlighted row).

To work out the number of tanks of water it will take to spray a hectare, use the formula:

Therefore, 10.6 tanks full of water are required to spray a hectare using 160L of water/hectare.

Next, calculate the quantity of herbicide to be added to each knapsack tank full of water using the formula:

This means that 0.283L of the glyphosate product must be added to each knapsack tank full of water to ensure an application rate of 3L of glyphosate product per hectare in 160L of water per hectare. (Note: 0.283L is 283mL)

If we want to go one step further and calculate many knapsack tanks full of water we will use in total to complete our job, the formula below can be used.

Remember, the total area we have to spray is 50m x 30m, or 1500 m2 (this is 0.15 hectare, as there are 10,000m2 in a hectare).

It will take nearly 1.6 knapsack tanks of water to apply the herbicide at 3L/ha to our 1500m2 of weeds in the area between the sheds, and we will need a total of 453mL of glyphosate product to complete the job (1 tankful with 283mL of glyphosate and 0.6 of a tankful with 170mL of glyphosate).

Completing the spraying

Some points to note before spraying include:

• Be absolutely sure that you understand all the information contained in the chemical label, Material Safety Data Sheet (MSDS) and sprayer manual.
• It is always good practice to inform your neighbours of any spraying you will be doing.
• Select and use attachments appropriate to the job (e.g. plant guard, mini boom, or lance extensions).
• Walk and operate the pump at the same rate you used when calibrating.
• If a pressure gauge is fitted, keep the pressure in the range of 2-3.5 Bar if using flat fan jets, and around 5 Bar if using cone jets.
• Do not spray if conditions are unsuitable.
• Specified records must be made within 48 hours of using an agricultural chemical product, and kept for a period of two years. This applies to all agricultural chemicals used, including poison baits used for pest animal control. This requirement came into effect on 24 July 2007 and excludes the use of household or home garden products.

Further References

Chemical manufacturers

Chemical resellers and agronomists

DPI Chemical Standards website - www.dpi.vic.gov.au/chemicalstandards

Contact/Services Available from DPI

DPI Chemical Standards Officers

North West	North East
Jo Robinson (03) 5355 0522	Jane Rhodes (03) 5833 5234
Alex Fahy (03) 5430 4591
South West	Gippsland
Neil Harrison (03) 5336 6616	Michael Laity (03) 9785 0191

Acknowledgements

This Agnote was developed by Alan Roberts, November 2005.

It was reviewed by:

Alan Roberts, Farm Services Victoria, December 2010.