Knapsack and backpack sprayers: choose equipment for professional use

The product label sets the crop or situation, dose, application limits, operator protection and environmental requirements for a backpack weed sprayer. The best weedkiller for a backpack sprayer is therefore a label question, not a sprayer question: "Roundup" alone does not identify a label, so take the label of the exact product. The UK product-register guide leads to the label.

For a knapsack job in Great Britain, Article 55(2) includes compliance with the authorised conditions specified on the labelling within proper use.

Best knapsack sprayer, or knap sprayer: hand pumping or a battery operated backpack sprayer

A battery operated backpack sprayer, also sold as a cordless, rechargeable or electric knapsack sprayer, replaces hand pumping with a battery-driven pump. HARDI's BPE is the electric model and SOLO's professional unit is battery powered; a manual knapsack sprayer is a hand-operated piston-pump unit such as the HARDI BP.

HARDI BP piston-pump backpacks use trigger-valve control and an unpressurised tank. SOLO's battery backpack has adjustable pump output and a charge-status display.

HARDI manual backpacks have top-positioned tank openings, shoulder pads and a waist belt. SOLO's battery model has a padded adjustable carrying system. Compare these features if comfort over a long working day matters most.

Tank capacity and filling

HARDI lists two figures for its backpacks: a rated fill of 15, 18 or 20 litres and a total fill of 16 or 22 litres. Compare the rated fill when you judge how much liquid you will carry: a 15, 18 or 20 litre backpack sprayer is named by its rated fill, while 16 and 22 litres are total fills. Prevent contamination of the outside of the backpack during filling.

Weed killer backpack sprayer: a backpack sprayer for weed killer, herbicide and knapsack weed sprayer

A weedkiller backpack sprayer, weed killer backpack sprayer, herbicide backpack sprayer or knapsack weed sprayer is an ordinary knapsack used with a herbicide, and a builders' knapsack sprayer for weed control on a site is the same machine, so two choices stay separate: the machine and the product. The sprayer decides how evenly and how safely the liquid is delivered. The label of the herbicide decides the crop or situation, the dose and the protection.

If one machine does both jobs, clean it between them as the washings guide describes. For spot treatment, work with the nozzle cover and pressure control valve described below, so that the intended application rate is reached and spray does not drift off target.

Types of knapsack sprayer: lever-operated, compression, battery, petrol and solar

Knapsack sprayers are named by their drive, not by the job:

• Lever-operated, manual or hand-pumped (also called a conventional knapsack sprayer): a lever works a pump. HARDI's BP backpacks use a hand-operated piston pump.
• Compression, pneumatic or pressurised: air is pumped into the tank to push the liquid out. The HARDI BP tank, by contrast, is not pressurised.
• Battery, electric, cordless or lithium (an electric backpack sprayer, electric knapsack sprayer or battery pack backpack sprayer): a battery-driven pump, as in the HARDI BPE and the SOLO 442.
• Petrol, gas, gasoline, engine, motorised or powered (a gas powered backpack sprayer or motorized backpack sprayer): an engine drives the pump or a fan. The fan version is the mist blower described in the application methods guide.
• Solar (a solar operated sprayer for farming): a battery sprayer that a solar panel charges; compare runtime and charging time as for any battery model.

A knapsack power sprayer, a 12 volt (12v) knapsack sprayer, a 12v spot sprayer, a 12v chemical sprayer and an automatic backpack sprayer are powered models: the voltage names the supply of the electric pump and does not say anything about the spray quality, so compare the pump output and the nozzle. Stainless steel, brass and plastic knapsack sprayers differ in the materials that touch the liquid; check them against the product, as in the section on chemical compatibility below. A backpack sprayer boom or spray boom for a backpack sprayer is an accessory that holds nozzles in a row for even coverage of low weeds; its nozzles are set and calibrated like any other, see the nozzle chart.

Backpack sprayer sizes: 10, 12, 15, 16 and 20 litres, and gallons

For a 10, 12, 15, 16 or 20 litre backpack sprayer (10l, 12l, 15l, 16l or 20l, as sellers write them), ask which figure the seller gives: HARDI lists rated fills of 15, 18 and 20 litres and SOLO's 442 has a 16 litre container. A 16 ltr knapsack sprayer (also written 16ltr, 16 litre or 16 liter), a 12ltr backpack pressure sprayer and a 20 liter sprayer are all judged the same way. Gallon sizes need conversion, because a US gallon is 3.785 litres and an imperial (UK) gallon is 4.546 litres.

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A full tank adds weight in proportion: the spray liquid alone weighs roughly as many kilograms as the tank holds litres, so a large, heavy duty or commercial backpack sprayer is judged by harness, hip belt and balance as much as by volume. Ratings such as "highest rated", "top 10" or "most comfortable backpack sprayer" do not replace this comparison, and neither does the price: put pump type, runtime, nozzle fittings and service support side by side.

Backpack insecticide sprayer, bug sprayer and fertiliser sprayer: one machine, different products

A backpack insecticide sprayer, backpack bug sprayer, portable insecticide sprayer or backpack poison sprayer is the same knapsack used with an insecticide. The label of the product in the tank sets the crop or situation, the dose and the operator protection; the machine does not. A backpack fertiliser sprayer or farm fertiliser sprayer carried on the back needs the same component check as any chemical knapsack: seals, hose, trigger, pump and nozzle must suit the liquid. Rinse the machine between a plant-protection product and any other liquid as the washings guide describes.

Model names and rankings: Cooper Pegler CP15, Stihl, spot sprayers and the most powerful backpack sprayer

A Cooper Pegler CP15 manual sprayer, a Stihl manual backpack sprayer, a Spot Shot or deluxe spot sprayer, a pro series or pro backpack sprayer and a quick-fit nozzle such as Quick TeeJet are compared by the points above: pump type, rated fill, harness, nozzle fitting and service support. The most powerful backpack sprayer and the best motorized backpack sprayer are the ones whose pump output, runtime and nozzle suit the job; a model name or a pressure figure alone does not show that.

Battery powered backpack sprayer: electric backpack sprayer runtime and charging

The SOLO 442 runs for up to 5 hours on its standard 2.5 Ah battery, and the supplied charger restores a full charge in 1.1 hours. The optional 5.0 Ah battery gives up to 12.6 hours and 516 litres at 1 bar with a flat spray nozzle, so compare runtime figures only at the same pressure and nozzle.

Check chemical compatibility by component

A chemical knapsack sprayer has to be compatible in every component that touches the liquid: seals, hose, trigger, pump and nozzle as well as the tank. HARDI manual backpacks use polypropylene (plastic) tanks and Viton seals.

HARDI manual backpacks allow maintenance without special tools.

Set up a knapsack sprayer for orchard and protected crops: a backpack sprayer for fruit trees

Set the knapsack up for the crop situation on the label, and fix the application rate before you load product. The label names the crop or situation that may be treated, so an orchard block or a protected crop needs that use listed for the product in the tank.

1. Fit a pressure control valve and a nozzle cover. With a knapsack you must take care to achieve the intended application rate and to prevent spray drifting off target, and a suitable pressure control valve and nozzle cover may help.
2. Calibrate with clean water at your own working pace. Measure the band the nozzle sprays at working height and time a walked strip, as the knapsack calibration guide shows.
3. Check the water volume against the label. Do not use reduced-volume spraying where the label bans it (for example, with a minimum application volume), says PPE must be worn at the ready-for-use dilution, or carries 'corrosive', 'very toxic', 'toxic' or 'risk of serious damage to eyes'; there you may cut the volume only if you cut the dose in the same proportion. In other cases the reduced-volume spray may be at most 10 times the label's maximum concentration, with a spray quality no finer than medium for hand-controlled equipment.
4. Fill without wetting the outside of the backpack. Do not overfill, replace the filler cap firmly, and fill inside a portable drip tray or a bunded area to prevent spillage.
5. Plan the row order before you start. Try not to walk back through the crop you have just treated, especially a high crop, and avoid working into the wind.

Decide whether a backpack fits the job

A misting backpack sprayer is a different delivery method: see the application methods guide for fogs and mists. For sustained field-scale work, compare the agricultural sprayer selection guide. For an existing machine whose output needs setting, use the nozzle and calibration guide.

You may apply a product by a method other than the one the label recommends only if the equipment suits that method, your COSHH assessment shows no increased risk to health, your assessment shows no increased risk to wildlife or the environment, and control measures are in place. This does not apply where the label legally requires a specific type of equipment.

For a tank on other equipment instead of a carried unit, see the tank selection guide.

Next step

See the product documents, nozzle output and the washings guide.