mobile feed weather

Spray Management Valves for knapsack sprayers: achieving the correct flow of spray solution.

The most important factor that ensures that a nozzle emits the correct flow of spray solution is to have a constant spray pressure at the nozzle itself. Spray Management Valves such as the 'Constant Flow', CFValve, make this easier to achieve.

Why bother?

Yellow nozzle (32 Kb JPEG)

The flow of spray solution that is sprayed by the nozzle on the knapsack must be consistent with the spraying activity that you plan to do so that you: 

  • apply the pesticide at the rate that you intend to use;
  • evenly on every surface sprayed; 
  • using the required drop coverage and non varying water volume;
  • reduce problems such as unexpected drift; and – most importantly 
  • reduce your exposure to spray.

How do problems occur?

Side lever Knapsack Sprayers:

The ideal is to have a consistent nozzle output despite changes in spraying pressure.  In practice, problems with the spray output from the nozzle occur when the pumping speed changes. A  regular pumping speed is not easily maintained when spraying, perhaps because of operator tiredness or operator skill or because the terrain is difficult. A variable pumping speed means that the pressure at the nozzle on side lever knapsack sprayers, changes as the piston or diaphragm pump is being worked.  This means that the amount of spray applied to the crop varies which means that the spray and, hence the pesticide/drop coverage applied, will not be evenly distributed on the target surface.

Compression Sprayers

The challenge of achieving a constant flow from the nozzle is greater when using a compression sprayer. A compression sprayer works by pressurising the air (which is in the same sealed tank as the spray solution) by hand, usually using built in pumps. The pressure created forces the spray solution through the lance and nozzle. The spray solution is emitted at maximum pressure and volume when the operator starts spraying but as the liquid is used in the tank, the air expands to fill the space vacated by the spray solution and the pressure lowers. This means that the pressure and the volume of the spray being emitted from the nozzle also reduces, causing the spray or drop pattern to change, until eventually the pressure is so reduced that no liquid comes out of the nozzle.  (See section on nozzle selection for more information. )

How do CF VAalves solve the problem – how do these valves work?

(25Kb JPEG)

The pressurised spray liquid forces the (yellow) diaphragm valve to open and allows the liquid through into the (green) chamber and then through to the nozzle orifice [opening] itself. If the pressure is too low then the diaphragm will not open and no liquid will be sprayed. If the pressure is too high the throttle pin will close to restrict and control the flow. This means that any liquid that is sprayed will be emitted at the correct pressure. There are four different colour coded valves which will enable you to spray at either 1, 1.5, 2 or 3 bars pressure.

Does it deliver on its promise?

According to one expert - yes; the nozzle does not emit spray until the correct pressure is reached and stops emitting spray when the pressure falls below the correct pressure.

Red constant flow valve (33 Kb JPEG)
Red constant flow valve

Studies by Professor Graham Matthews of IPARC in the UK (e.g. Matthews, Thornhil & Dobson, 2008) have shown with the red CFValve, that input pressure must reach its rated minimal pressure of 1.5 bars [21 psi] before the nozzle sprays. The CFValve then continues to hold that required pressure despite an initial input pressure of  more than 6 bars; a pressure that would otherwise have increased the amount of spray solution emitted (and therefore the pesticide rate).  As input pressure lowers, so the red CFValve maintains the 1.5 bars [21 psi] - until this minimal pressure requirement can no longer be achieved when the nozzle stops spraying.

The red CFValve makes the rate of nozzle spray solution output consistent so that it does not change from start to finish making it more likely that the pesticide will be correctly applied in an even spray pattern 


Matthews, G.A., Thornhill, E.W. and Dobson, H. (2008) The compression sprayer and its use in vector control. Paper presented at Aspects of Applied Biology; International Advances in Pesticide Application 84, 337 - 342