Calibration is the job of making sure the rate printed on the chemical label is what actually lands in the field. When the two drift apart, you lose money in both directions: over-application wastes product and raises the risk of crop injury; under-application makes the treatment ineffective and sends you back into the same field — fuel, time and a second tank of chemical.
The good news: calibration is not complicated. It takes a measuring jug, a stopwatch and 20 minutes. Here are the 7 steps, simplified for field use.
When should you calibrate?
- At the start of every season, before the first application
- Whenever the nozzle set changes, or individual nozzles are replaced
- When switching application types (e.g. herbicide to fungicide, low volume to high volume)
- When the tank runs out noticeably earlier or later than the math says — that is the system asking for calibration
Step 1 — Start with a clean system
Calibrating a dirty system means fine-tuning a wrong measurement. Clean the suction filter, the line filters and the nozzle filters, in that order; fill the tank with clean water. Even a single clogged nozzle filter drops that nozzle's output and skews the whole test.
Step 2 — Measure your real speed
Tractor speedometers lie — tire wear and slip see to that — and your rate drifts along with the error. Pick the gear and rpm you will actually spray at, then measure true ground speed. Your phone is enough: our GPS Speed Meter shows real speed and carries the result straight into the nozzle calculation.
Step 3 — Turn the target into a number
The required flow per nozzle comes from a simple formula:
Nozzle flow (l/min) = Target rate (l/ha) × Speed (km/h) × Nozzle spacing (m) ÷ 600
Example: a 200 l/ha target at 8 km/h with 50 cm spacing → 200 × 8 × 0.5 ÷ 600 = 1.33 l/min. That is what every nozzle should deliver per minute.
If you would rather skip the arithmetic, the Nozzle Selection calculator does the same math and then recommends the right nozzle and pressure by ISO colour code.
Step 4 — Set the pressure, verify the nozzle
There are two roads to a target flow: the right nozzle, or forced pressure. The first is the correct one. Chasing flow by cranking pressure shrinks droplets and increases drift; chemical landing on the neighbour's parcel is both a loss and a liability. Compare your nozzle's ISO colour code against the target flow — if they disagree, change the nozzle, not the pressure.
Step 5 — The bucket test
This is the heart of calibration, and no electronics replace it:
- Run the system at the real working pressure
- Hold a measuring jug under one nozzle for exactly 1 minute
- Compare the litres collected against the target from Step 3
Don't stop at one nozzle; test at least 3-5 across the left, centre and right of the boom. If a single nozzle deviates, the culprit is usually its filter or the nozzle itself; if they all deviate together, look at pressure, the flowmeter or the speed signal. More than ±5% off target means adjustment.
Step 6 — Catch the wear: the 10% rule
Nozzles wear silently: the orifice grows, flow rises, the spray pattern collapses — and you over-apply on every hectare without noticing. The simple check: test a new nozzle of the same type at the same pressure. If a used nozzle delivers more than 10% above the new one, it is time to replace the set. A nozzle set is the cheapest part on a machine that carries a tank full of chemical.
Step 7 — Record it, repeat it
Write down the date, nozzle type, pressure, measured speed and the bucket result. Next calibration you will have a baseline, and you will see how fast drift builds up. Even a 5-minute single-nozzle spot check mid-season catches surprises early.
From once-a-day calibration to continuous
Manual calibration is a snapshot: correct on that day, at that speed, at that pressure. In the field, speed changes — and on a conventional system the rate drifts with it. Automatic rate control closes exactly that gap: a rate controller (often called a spraying computer) like the Nova 100 measures flow continuously, adjusts the valve as speed changes and holds litres per hectare constant. It retrofits onto your existing sprayer; the bucket test remains your seasonal verification, but day-to-day rate keeping moves to the device.
Frequently asked questions
At what pressure should I run the bucket test?
Always at the real working pressure you spray at. A low-pressure test says nothing about high-pressure reality.
Do I have to test every single nozzle?
For routine checks, 3-5 nozzles across different boom sections are enough. If you find a deviating nozzle, widen the check around it.
The tank still runs out early and my calibration looks right — why?
Overlap, leaks or a bad speed signal are the usual suspects. The Nova Support wizard walks you through eliminating them step by step.
Instead of doing the math by hand, use the Nozzle Selection and GPS Speed Meter tools in the field, on your phone — free, no sign-up.
