Tank size decides how often you refill, and pump type decides how much of the day goes into pumping rather than spraying. Settle those two questions first and the choice comes down to which chemicals you are actually applying.

Tank size and how often you refill

All four knapsack sprayers in the range sit between 15 and 20 litres, and the difference mostly comes down to refill frequency against weight on your back. The Cooper Pegler CP15 Classic holds 15 litres (17 actual), the Berthoud Vermorel 1800 holds 16, the Berthoud Cosmos 18 Pro holds 18, and the Cooper Pegler CP3 Classic holds 20 litres (25 actual). On a large site or a boom-fed job, the CP3’s extra capacity over the CP15 means genuinely fewer walks back to the vehicle; on a smaller site, that extra capacity is mostly just extra weight for no benefit.

How much pumping you are prepared to do

Tank size is not the only thing that decides how hard the job feels by the end of the day. The Vermorel 1800 needs only 8 to 10 pump strokes a minute to hold its working pressure, against 30 to 40 on a conventional knapsack, and its curved back frame spreads the weight across the shoulders rather than pressing a flat panel into the spine, both of which matter over a full spraying day. The CP15 and CP3 Classic use a diaphragm pump with no moving parts in direct contact, which does less for arm fatigue but a great deal for longevity: Spindrift, the UK distributor, still services Cooper Pegler sprayers over 50 years old. The Cosmos 18 Pro takes the opposite approach and pushes for pressure, 6 bar against 5 bar on the Vermorel and 3 bar on the Cooper Pegler pair, which helps with fine atomisation and canopy penetration rather than an easy pumping action.

Chemicals that need mixing, not just spraying

Wettable powders and suspension concentrates settle out in a standard tank over the course of a treatment, so the mix reaching the nozzle at the end is not the mix you started with. The Cosmos 18 Pro is the only sprayer in the range with a built-in agitator mixer, integrated into the pump itself, so it keeps stirring as you pump. If you are mostly spraying straightforward liquid formulations such as glyphosate, that feature will not matter to you; if wettable powders are a regular part of the job, it is the reason to choose the Cosmos over the other three.

All four sprayers use Viton seals and either brass or GRP components in the lance and pump, which is what lets them stand up to herbicides, fungicides and insecticides over years of use rather than degrading after a season or two. The Cooper Pegler pair also ship with two nozzles as standard, a green anvil nozzle for herbicide work and a yellow hollow cone for fungicides and insecticides, alongside a dual pressure valve to switch between them in the field, so you are not buying separate nozzles just to change the job.

Which sprayer for which job

For daily professional use where longevity and low maintenance matter most, the CP15 Classic on smaller sites or the CP3 Classic on larger sites and boom work are the safer long-term buy. For operators spraying for extended periods, where fatigue is the real cost, the Vermorel 1800’s low-stroke pump and ergonomic frame are worth the difference. Where the job regularly involves wettable powders or suspension concentrates, the Cosmos 18 Pro’s built-in mixer and higher pressure earn their place over the other three.

Whichever sprayer you choose, operating one professionally in the UK requires a PA1/PA6 certificate of competence or equivalent, and it is the chemical you put in it, not the sprayer, that carries the label instructions and any qualified-user restriction. Read the label for the product you are applying before you calibrate the sprayer, and get in touch with the Phoenix Amenity team if you want help matching a sprayer to a specific spraying programme.

Frequently asked questions

Can I use the same knapsack sprayer for herbicides, fungicides and liquid fertiliser?

Yes, all four sprayers here take the full range of liquid plant protection products and feeds, though you should rinse the tank, lance and nozzles thoroughly between different chemicals to avoid cross-contamination, and check the label of whatever you are applying for any product-specific cleaning instructions.

Do these sprayers come with different nozzles for different jobs?

Yes. The Cooper Pegler CP15 and CP3 Classic both include a green anvil nozzle for herbicide work and a yellow hollow cone nozzle for fungicides and insecticides, plus a dual pressure valve (1 bar and 3 bar) to match. The Berthoud sprayers accept flat fan, hollow cone, evenspray and deflector nozzles as accessories.

What is the difference between rated and actual tank capacity?

The rated capacity, 15 or 20 litres on the Cooper Pegler models, is the working fill level; the actual tank volume is slightly larger (17 and 25 litres respectively) to leave headroom so the tank does not need filling right to the brim. Berthoud specifies the Cosmos 18 Pro and Vermorel 1800 at their working capacity of 18 and 16 litres.