Nozzle Output at a New Pressure

Flow through an orifice follows the square root of the pressure driving it, which makes pressure a disappointing lever on application rate. To double a nozzle's output you need four times the pressure. To raise output by a modest 10% you need 21% more pressure. A grower who wants 20% more water on the crop and reaches for the pressure regulator will find the gauge at nearly one and a half times where it started, and the spray quality ruined.

Ruined is the right word, because pressure does something else at the same time. Higher pressure atomises the sheet of liquid leaving the nozzle into finer droplets, and fine droplets drift. The ASABE S572 spray quality categories — very coarse through very fine — shift by one or two whole classes across the pressure range a nozzle is rated for, which is why the same nozzle can be a drift-reducing choice at 200 kPa and a drift hazard at 500 kPa. Lower pressure has its own floor: below about 100 kPa a flat-fan nozzle stops forming a proper fan at all, the pattern narrows and the overlap between nozzles fails, leaving visible stripes in the crop.

The practical consequence is that pressure is for fine-tuning within a nozzle's rated band, and nozzle selection is for everything else. If the arithmetic says you need substantially more or less output, change the tip. Modern nozzles are colour-coded to ISO 10625 so the flow at a reference pressure is readable across every manufacturer — an ISO 03 tip passes 1.2 L/min at 300 kPa whoever made it — which turns nozzle selection into a lookup rather than a catalogue hunt.