Managing pressure

Better knapsack sprayers have valves to help you manage pressure.

1. If there is not enough pressure, the nozzle valve will remain closed.
2. At the correct range of pressures, the nozzle valve will open.
3. If the pressure is too high, the pressure relief valve in the pump chamber will open. This lets the spray mix back into the tank and stops the spray going through the hose to the nozzle.

Pressure regulators control the pressure at the upper and lower limits.

1. At low pressure, the gap is closed and spray mix cannot reach the nozzle.
2. At the right range of pressures, the force pushes the pin down to allow liquid to pass through the gap and into the nozzle.
3. At high pressure the pin moves further down the valve: the wider end of the pin makes the gap smaller and lowers the flow of spray mix at the nozzle to the right level.

Choose a pressure regulator (e.g. Cooper Peglar's pressure regulator, Gate's CFValveand Jacto's EcoValve [289 kb pdf]) that sprays at the pressure you will need. Some spray at 1, 1.45, 2 or 3 bars. Lower pressures are useful with deflector nozzles and higher pressures for hollow cone nozzles.

Choose a gauge with a scale that it is marked at useful intervals and shows upper and lower limits. For example, this pressure gauge has a:

• green area to show the safe pressures at which to spray
• red area to show when the spray pressure is too high.

Most gauges show the pressure of the spray liquid in bars. 

Use the gauge to help you maintain the pressure advised by the nozzle maker and used to calibrate your knapsack sprayer. Take care that the pressure does not vary as you tire. Check your gauge often.

Checking your pressure gauge before spraying pesticides