Spray to protect your crops

Part 4: The knapsack sprayer

Compiled by J Fuls (Pr Eng)

The knapsack sprayer works like a charm on a small farm or when those pesky weeds surrounding your vegetable garden are sticking out their heads. This month we shall look at the finer workings of this incredible tool. We thank the ARC Institute for Agricultural Engineering in South Africa, who made this article available to the readers of ProAgri Zambia.

The knapsack sprayer can be used to:

Components of the knapsack sprayer:

How does the knapsack sprayer work?

The pump inside the tank looks quite like a bicycle pump. Instead of moving the push rod, the pump cylinder is now moved up and down. The rod is fixed to the bottom of the tank.

The hand pump directly pumps liquid out of the tank for spraying. The tank is not placed under pressure. The sprayer is carried on the back, hanging on both shoulders.

Note the following differences between a bicycle pump and a knapsack pump:

  1. The bicycle wheel has a valve to stop air from returning to the pump, but the spray nozzle does not have a valve. Now an outlet valve is installed in the pump, to prevent liquid from returning to the pump.
  1. The washer of the bicycle pump is soft and allows air to flow around it to the pumping side only, as the pump rod is retracted. The seals of  the piston pump cannot do the same, and now an inlet valve is needed to allow the chemicals to flow from the tank to the pump, but not back again.
  1. On the sprayer pump the chemical enters the pump at the bottom end of the push rod.
  2. The sprayer pump has a plastic tube that connects the outlet hose to the bottom of the cylinder. This tube is very important. Its function will be discussed below.

If one understands how the pump works, it will be easier to find faults and to repair it.

Move the cylinder upwards:

Now move the cylinder downwards:

The plastic tube and the air pocket:
Let us look at the bicycle pump again. Fill it with water and then start pushing the handle. If we stop pushing the handle, it will immediately stop delivering water.

If the knapsack sprayer does the same, it will immediately stop delivering the chemical to the nozzle, the moment we pull the handle up for the next pumping stroke. Even spraying will then be very difficult.

Now this is how the problem is overcome:

This property of air is used on a spray pump:

There is also another alternative to the inside of the pump:

The main difference is that with the pump discussed so far, the push rod is fixed to the bottom of the tank and the cylinder is moved up and down for pumping. With the alternative pump, the cylinder is fixed to the bottom of the tankand the push rod is moved up and down for pumping. The rest of the functioning remains the same.

Next month we shall take the knapsack sprayer apart to have an even closer look. Published with the acknowledgement to the ARC Institute for Agricultural Engineering for the use of their manuals. Visit for more information.



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