A sprayer is calibrated to apply 30 gallons per acre at 40 psi. Approximately what pressure is needed to decrease the output to 15 gallons per acre without changing speed or nozzle size?

• A. 10 psi
• B. 20 psi
• C. 80 psi
• D. 320 psi

Answer: (B)

When you keep the spray speed and the nozzle size the same, the amount of liquid applied per acre depends on how much liquid is flowing through the nozzle, which is controlled by the pressure at the nozzle. For many common nozzles used in pesticide application, the discharge rate changes roughly in proportion to the pressure in the typical operating range. So, to cut the output per acre by half, you adjust the pressure to about half of what it was while keeping speed and nozzle constant. That’s why the option representing about half the original pressure is the best choice. Keep in mind that, in exact physics, flow is related to the square root of pressure, but for practical calibration in this context the near-linear relationship is used, making a halved pressure produce roughly half the output.