A sprayer is calibrated to apply 15 gallons per acre at 15 pounds per square inch (psi). Approximately what pressure is needed to increase the output to 30 gallons per acre without changing speed or nozzle size?

• A. 30 psi
• B. 60 psi
• C. 75 psi
• D. 90 psi

Answer: (A)

The key idea is how nozzle discharge relates to pressure when speed and nozzle size stay the same. For a given nozzle, the flow rate is roughly proportional to the square root of the pressure (Q ∝ sqrt(P)). That means to double the flow, you don’t double the pressure—you need four times as much pressure.

Starting from 15 psi, doubling the output (to 30 gallons per acre) requires P2 such that (P2/P1) = (Q2/Q1)^2 = 2^2 = 4. So P2 = 15 psi × 4 = 60 psi.

Therefore, about 60 psi is needed to reach 30 gallons per acre. A pressure of 30 psi would only increase output by about 41%, not double.