DJI Agras Spray Rate Calibration: A Field-Proven Bench Test (L/ha You Can Trust)

Field Forge 3D · Field guide

If your prescription says 30 L/ha and your Agras is actually putting down 24, you're under-applying every acre — and you won't see it in the app. The flight controller reports the rate it commands, not the rate your nozzles physically deliver after a season of wear, a viscosity change, or a clogged screen. The only way to know is to measure the water that comes out. Here's the bench procedure we run before any contract job, and the simple tool that makes it repeatable.

Why the on-screen rate lies to you

DJI Agras spray systems (T20, T25, T30, T40, T50 and the older T10/T16) meter flow with a pump and flowmeter, then trust that the nozzles pass what the flowmeter measured. That assumption drifts:

• Nozzle wear. XR/ceramic tips open up over hundreds of tank-loads, raising real output above the commanded rate.
• Partial clogs. A screen or tip half-blocked by tank residue drops output below target — often on just one or two nozzles, so coverage goes uneven.
• Liquid viscosity. A thick suspension concentrate or UAN blend does not flow like clean water; the flowmeter calibration done on water no longer holds.
• Pump/diaphragm aging. Pulsation and pressure sag at high duty cycles shift the delivered volume.

None of that shows up on the remote. A 60-second catch test does.

What you need

• A measuring vessel with clear graduations in mL (a printed calibration cup with a 10 mL scale beats a kitchen jug — you're reading small volumes).
• A stopwatch (your phone is fine).
• Clean water in the tank for the dry run.
• A notepad or the notes app to log each nozzle.

The procedure: a 60-second catch test

1. Fill with clean water. Calibrate and verify on water first. Never bench-test with live chemical.
2. Set a known rate and speed in the app. Pick the L/ha and forward speed from your actual job so the system commands the same pump duty it will in the field.
3. Trigger spraying on the bench (props off or motors disabled per your model's maintenance mode). Let pressure stabilize for a few seconds.
4. Catch one nozzle for a timed interval. Hold the cup under a single nozzle for exactly 30 or 60 seconds and capture everything it delivers. Record the mL.
5. Repeat for every nozzle. Two-pump systems and four-/six-nozzle booms can have one lagging tip. You want each nozzle's individual output, not just the average.
6. Convert to flow rate. mL caught ÷ catch time × 60 = mL/min per nozzle. Multiply by your nozzle count for total boom output in mL/min, then ÷ 1000 for L/min.

Turning mL/min into L/ha (the number that matters)

Your application rate ties output to ground speed and swath:

L/ha = (Total boom output in L/min × 600) ÷ (Speed in km/h × Swath in m)

Example: 4 nozzles each catching 250 mL in 30 s = 500 mL/min each = 2.0 L/min total. At 6 km/h with a 4 m swath:

(2.0 × 600) ÷ (6 × 4) = 1200 ÷ 24 = 50 L/ha

If your prescription was 30 L/ha, you're 67% hot — either slow the commanded rate, speed up, or check why the nozzles over-deliver. Run the math the other direction to find the speed or rate that lands you on target.

Pass/fail and what to do with a bad result

• All nozzles high or low together → recalibrate the flowmeter in the app (DJI's flow calibration routine), or your viscosity has shifted enough to need a fresh calibration on the actual mix.
• One nozzle off → pull and clean that tip and its screen; if it's worn, replace the set. Tips are cheap; a misapplied field is not.
• Spread between nozzles → uneven coverage. Replace the full nozzle set so they wear together.

Document the cup reading per nozzle and the date. A logged catch test is also your defense if a customer ever questions coverage — you can show the boom was verified the morning of the job.

Make it repeatable

The reason most operators skip this is fumbling with a random container and doing mental math at the truck. A purpose-made Spray Calibration Cup with a printed mL scale, a stable base, and a wide catch mouth turns the whole thing into a two-minute ritual you'll actually do before every job. It's printed in chemical-tolerant material, packs in the case, and is parametric — set the scale and capacity to your nozzles and rates.

Get the Spray Calibration Cup → — part of the Drone & Ag line at Field Forge 3D, designed around real spray-drone fieldwork.

Always verify calibration on clean water, follow DJI's maintenance-mode procedure for your model, and wear appropriate PPE when handling spray chemicals.