Droplet size beats water rate: getting real coverage on the target

One of the most common mistakes we see in aerial spraying is asking for more water and assuming it means better coverage. It doesn't. What actually puts chemical on the target is droplet size — and understanding that can dramatically improve your result while cutting your cost.

Why more water isn't more coverage

Here's the rule worth remembering: every time you halve the droplet size, you increase the number of droplets by up to eight. Coverage is about how many droplets land on the target, not how many litres you pump out.

Say a farmer asks for 100 L/ha because that's what his mate said was the go. At a given speed, the drone might only run a coarse 440-micron droplet at that rate — and at the target you land around 8 droplets per square centimetre. Sounds like plenty of water. But drop that droplet to 220 microns and you multiply the droplets by eight: the water rate can fall to about 50 L/ha — effectively halving the cost of the application — and you're now at around 64 droplets per square centimetre. The chance of contact or ingestion goes up dramatically.

Put it another way: 100 L/ha is only 10 ml of liquid per square metre. 50 L/ha is 5 ml. Five millilitres goes a very long way when it's broken into fine droplets.

The fine print on fine droplets

Smaller droplets come with responsibilities. You should always follow the label — including recommended surfactants — and comply with maximum winds, buffer zones and minimum droplet sizes. Flying at a slower speed also helps: it gives better circulation through the crop and more even coverage. And spray systems should be tested regularly to confirm droplet-size accuracy, the percentage of fines and the true effective swath.

What this means for you

When you're weighing up quotes, don't fixate on the headline water rate. Ask the operator how they manage droplet size and coverage, and whether they pattern-test. A smaller, correctly-managed droplet can beat a big water rate on both result and cost — as long as it's used within the label's limits.

What the trials show

Independent field trials back this up. Testing reported by Sprayers101 found the actual effective swath from ag drones came out noticeably wider than programmed — roughly 30% wider for a T50 and around 40% for a T100 — and that finer droplets spread wider (better coverage, but more drift to manage). A good reminder that the numbers you set aren't always the numbers you get: you have to test on the day.

Further reading: Sprayers101 — RPAS herbicide application.

All Flight pattern-tests every job and dials in droplet size for real coverage. Call 0437 761 273.