22 July 2026 — Nicolas Daüy
Blocked nozzle, unbalanced half-boom: spotting them during spraying
You have been spraying for an hour and everything looks normal: the engine is humming, the boom is out, the pressure gauge reads its usual figure. Except a nozzle blocked on the third pass. You did not see it — from the cab, 4 metres from the boom tip, under a cloud of fine droplets, nobody sees it. You will find out three weeks later, when a one-metre strip grows back faster than the rest of the plot. Too late to re-treat it cleanly.
The blocked nozzle is the most common and the most discreet spraying fault. This guide explains why it goes unnoticed, what it really costs, and the different ways to spot it — from the most hands-on to live flow measurement.
What a blocked nozzle actually does to your treatment
A partially blocked nozzle delivers less, or nothing at all. The strip of soil passing under that nozzle therefore receives an under-dose, sometimes a zero dose. The consequences are not cosmetic:
- In weed control, the under-dosed strip lets the weeds recover. On an already established resistance, a repeated under-dose is exactly what selects for it and makes it worse.
- In fungicide, the poorly protected zone becomes the entry point for the disease, which then spreads to the rest of the plot. You paid for the product, spent the time, and the outbreak starts anyway.
- Conversely, if you compensate “just in case” by raising the overall dose, you over-dose everywhere else: wasted product, and a risk of exceeding the approved doses, hence non-compliance at inspection.
The problem is not that a nozzle blocks — it happens to everyone. The problem is not knowing about it before the damage is done.
Why you cannot see it from the cab
Three reasons combine.
Distance and angle. The tip of a 24- or 36-metre boom is more than 15 metres from the driver, often behind them. You are driving, watching the guidance line and the obstacles — not 48 nozzles one by one.
The spray cloud. Modern nozzles produce fine droplets that form a continuous cloud. A nozzle switched off in the middle of a spraying boom does not create an obvious “gap”: the cloud from the neighbouring nozzles hides the missing one.
The pressure gauge barely moves. This is the most treacherous trap. The pressure shown in the cab is the overall circuit pressure. If one nozzle in thirty blocks, the total flow drops by around 3 % and the pressure barely rises — invisible to the eye on a gauge. A whole half-boom would have to drop out for the general pressure to react clearly. In other words: the gauge reassures you while a section is under-dosing.
Methods to spot an imbalance, honestly compared
| Method | What it sees | Its limits |
|---|---|---|
| Visual check + water-sensitive paper | One nozzle at a time, when stopped | One-off: says nothing about what happens during the job |
| Bucket flow test, nozzle by nozzle | Each nozzle’s flow deviation | Slow, when stopped, a snapshot at one moment |
| Mandatory sprayer inspection | The general condition every 3 (then 5) years | A periodic inspection, not job monitoring |
| Live flow per section | The flow drop of a half-boom during spraying | Measures at section level, not nozzle by nozzle |
The first two methods are useful and worth keeping: water-sensitive paper remains the best tool for checking a specific nozzle and the spray quality. But they are done when stopped, before or after. None of them warns you if a nozzle blocks in the middle of a six-hour job.
Flow per section, live
To catch the fault while it is happening, you have to measure the flow where it leaves: on the boom, continuously. That is what a flow meter fitted on each section does.
The principle is simple. A boom is fed by half-booms (one left, one right, sometimes more). A flow meter on each feed measures the flow actually delivered by that section. When a nozzle blocks, the flow of its half-boom drops — and you see it straight away on the screen, not back at the farm. Combined with pressure and forward speed (via GPS), the flow also tells you whether the volume per hectare applied matches the intended dose.
That is the role of the Débitdouille, a unit born at the AgroTIC Mobilab with the Hérault chamber of agriculture. It displays pressure, flow per section and speed live on your smartphone, with no subscription. Two things to know before you decide:
- It measures at section level, not nozzle by nozzle. Two flow meters track the left and right flow: an imbalance between the two halves of the boom, or a clear drop on one side, shows up immediately. To then identify which nozzle is at fault, you go back to water-sensitive paper — but you already know which side to look at, and above all you have stopped spraying before wasting ten rows.
- The price follows the configuration. The version that tracks only pressure and speed starts at 220 € excl. VAT. Measuring the left/right flow adds the two SIKA flow meters and comes to 900 € excl. VAT. Beyond that (a boom with more than two sections) is on request. The details are on the sprayer flow monitoring page.
In short
- A blocked nozzle under-doses a strip of soil: weed recovery, a disease outbreak, or general over-dosing if you compensate blindly.
- From the cab the fault is invisible: distance, spray cloud, and an overall gauge that only reacts to a big drop.
- Water-sensitive paper and the bucket flow test remain essential — but when stopped, never during the job.
- Live flow per section is the only method that warns you during the treatment, at half-boom level.
To understand the full picture, see the sprayer flow monitoring page; for the specifications and configurations, the pre-assembled Débitdouille page. Does your boom have a particular configuration (more than two sections, vineyard, orchard)? Describe it to us — we give you a straight answer, including if it is not a good fit.
Frequently asked questions
Does the Débitdouille detect one specific blocked nozzle?
It detects the flow drop of the affected half-boom, live. It does not point out the exact nozzle among the thirty: for that, water-sensitive paper remains the tool. But it alerts you during spraying, tells you which side to check, and stops you finishing the job under-dosing without knowing it.
How is it better than my cab pressure gauge?
The gauge shows the overall circuit pressure. A single blocked nozzle barely moves it. Flow measured per section, on the other hand, reacts to the local drop: that is information the general pressure never gives you.
Do I need technical skills to install it?
No. The pre-assembled version arrives fitted and configured; FURGO installs the whole system on your sprayer, calibrates the sensors and supports you for the first use. The same system also exists as a self-build kit within the Hérault chamber of agriculture’s training course, for those who want to understand every component.
Does it replace the mandatory sprayer inspection?
No, and that is not its purpose. The periodic inspection remains mandatory. Live flow is a day-to-day operating tool: it shows you the real state of your spraying on every pass, between two inspections.
Going further: Sprayer flow-rate monitoring — the full page with hardware, prices and real cases.