How to calibrate a sprayer: pump-up, backpack and hose-end

A label might tell you to apply a product in a set volume of water over a set area. Whether you actually do that depends on three things: your nozzle, your pump pressure and how fast you walk. Two people with the same sprayer can put out very different amounts, and neither will know unless they measure.

Calibrating is how you measure it. You spray clean water over a marked area, see how much you used, and work out your output for a given area. Once you know that, you can match the label, mix the right amount for your lawn, and get even results without guesswork. It takes about fifteen minutes and only needs doing again when something changes.

Why it matters

Getting the output wrong causes problems either way.

• Too much puts more product on each square metre than the label intends. With a herbicide, that can damage the lawn; with a fertiliser, it can burn it. It also wastes product and increases what can wash into drains.
• Too little leaves weeds that survive, or a lawn that barely responds. People then spray again, or make the next mix stronger, which is where the real damage happens.
• Uneven output gives stripes and patches. A nozzle that’s partly blocked or worn, or a pace that slows down at the end of each strip, shows up in the lawn a few weeks later.

The Victorian Department of Primary Industries makes the point that pump-up and backpack sprayers can be highly accurate, provided they are serviced, maintained and calibrated correctly. The accuracy comes from the calibration, not the sprayer.

What the label tells you

Lawn product labels give their directions in a few different ways. Read yours closely before you start, because it decides what you need to measure.

• A product rate and a water volume for an area. The label says how much product to use on a given area and how much water to apply it in. This is where calibration matters most: you need to know your sprayer puts out that water volume over that area.
• A dilution and a coverage. The label says how much product to mix with each litre of water, and how far that mix should go. Calibration tells you whether you’re spreading the mix over the area the label means.
• A dilution for spot spraying. The label says how much product to mix per litre and to wet the weed’s leaves. Calibration matters less here, but the nozzle and pressure still need to be right.
• Ready to use, or hose-end. The product is already diluted or the hose-end bottle doses it for you. You can’t change the mix, but you can check you’re covering the area the label intends.

Never invent a rate or round one off. If you can’t find the information on the label, check the product’s label on the APVMA’s PubCRIS database, or ask the supplier.

Before you start

• Use clean water only. Calibration is never done with the product in the tank.
• Use the sprayer and nozzle you’ll use for the job. Calibration only applies to the exact set-up you tested.
• Service the sprayer first. It should be clean, with no leaks, cracked hoses or worn seals, and working as the manufacturer intends.
• Check the label allows this method. Some products may only be applied in certain ways. Check before you buy or mix.
• Pick a calm day. Wind changes the spray pattern and makes it hard to see what you’re doing.
• Calibrate on a similar surface to the one you’ll spray. Lawn is best; a driveway lets you see the wet strip more easily.

A sprayer with a pressure gauge or a pressure-regulating valve makes calibration easier, because it helps you keep the pressure the same on the day as it was during the test. Without one, pump at a steady rate you find comfortable and use the same rate every time.

Step by step: pump-up and backpack sprayers

1. Mark out a test area. Use pegs at the corners. It can be any shape, as long as you know its area: 5 m × 2 m gives 10 m², and 5 m × 5 m gives 25 m². Bigger is more accurate.
2. Mark a fill line. Put the sprayer on level ground, fill it partly with clean water, and mark the level on the outside of the tank with a permanent marker. Many tanks have printed marks you can use instead.
3. Pump to your normal pressure.
4. Spray the test area at the pace and pressure you’ll use on the day, holding the nozzle at the height you’ll use. Spray it just as you would the lawn: parallel strips with a slight overlap, no gaps, no double passes.
5. Measure what you used. Put the sprayer back on level ground and refill it to the line from a measuring jug. The water you add is what you sprayed.
6. Work out your output. Enter the area and the water used in the Sprayer Calibration Calculator. It gives your output for every 100 m² and compares it with the label’s water volume.
7. Adjust and repeat until your output is close to what the label asks for.
8. Write it down. Note the nozzle, the pressure or pumping rhythm, and your pace. Next time you’ll start from there.

Working it out by hand

To get your output for every 100 m², divide 100 by your test area and multiply by the water you used.

• On a 25 m² test area, multiply the litres used by four.
• On a 10 m² test area, multiply the litres used by ten.

These are water volumes from a calibration run, not product rates. The amount of product always comes from the label.

Matching the label

Once you know your output, compare it with the water volume on the label.

• Putting out too much? Walk a little faster, lower the pressure slightly, or fit a nozzle with a lower output.
• Putting out too little? Slow down, keep the pressure up by pumping more often, or fit a nozzle with a higher output.

Change one thing at a time and test again. Walking pace is usually the easiest to adjust, but it’s also the hardest to keep steady, so many people prefer to settle on a comfortable pace and change the nozzle instead.

If the label gives a range of water volumes, anywhere within that range is fine. What matters is that the amount of product on each square metre stays at the label rate. The Product Application Calculator uses your calibrated output and your lawn area to work out how much mix you need and how much product goes in it.

A worked example

Here’s how calibration feeds into a real job. The numbers are water volumes and areas only; the product amount always comes from your label.

• The job: a front lawn of 120 m², measured with the Lawn Area Calculator.
• The test: you spray a 25 m² test area and need 0.6 litres to refill the tank to the line.
• The output: 0.6 × 4 = 2.4 litres for every 100 m².
• The mix for the job: 120 m² is 1.2 lots of 100 m², so you need about 2.9 litres of mix. Round up slightly for the hose and the bottom of the tank, which never empties completely.
• The product: you add the amount of product the label gives for 120 m² to that water, following the label’s mixing directions.

If the label had asked for a lower water volume than your 2.4 litres, you’d adjust your pace or nozzle first, rather than spreading the same product in more water than the label allows.

Spray width and overlap

Your output depends on the width of the strip you spray as well as your pace. Before you calibrate, hold the nozzle at your normal height over a dry driveway and spray a short strip. Measure how wide the wet band is.

• Keep the nozzle height constant. Lifting it widens the strip and thins the spray; lowering it narrows the strip and concentrates it.
• Overlap slightly. Fan nozzles usually give a band that’s lighter at the edges, so a small overlap evens it out.
• Mark your lanes on a large lawn with string or a couple of pegs at each end, so each pass sits beside the last.
• Start at the far end so you aren’t walking through wet spray.

Keeping a steady pace

Pace is where most calibration goes wrong. People walk faster when the tank is heavy and slower when they’re concentrating on an edge.

• Count your steps during the test run, and match that count for the same distance on the day.
• Time a strip. Note how long it takes to walk one length of the test area, and check yourself against it on the lawn.
• Keep pumping at the same rhythm. Pressure drops as you spray, and lower pressure means less output.
• Don’t rush the end of each strip. Turn with the nozzle pointing away from the lawn, or release the trigger as you turn.

Checking a nozzle

A quick flow check shows whether a nozzle is worn, blocked or different from the others.

1. Pump the sprayer to your normal pressure.
2. Spray into a measuring jug for 30 seconds.
3. Note the volume. Repeat a couple of times and take the average.

Do the same with a new nozzle of the same type. A worn nozzle usually puts out more than a new one and gives a less even pattern, which is why replacing a cheap nozzle is often the easiest fix for patchy results. A partly blocked nozzle puts out less and sprays a lopsided pattern. Clean it with water and a soft brush while wearing gloves; never blow through a nozzle by mouth.

Hose-end sprayers

Hose-end sprayers work differently. The bottle mixes the product into the water flowing from the hose, so the mix rate is set by the bottle, the dial or the product itself, not by you. You can’t calibrate the mix, but you can check the coverage.

1. Fill the bottle with clean water to a marked level and set the dial as you would for the product.
2. Spray a measured area at your normal pace, with the tap open to the same point you’ll use.
3. Note how far the bottle level dropped and how long the area took.
4. Compare that with what the label says one bottle, or one dial setting, should cover.

If you covered much less area than the label suggests, you’re walking too slowly; much more, and you’re moving too fast. The hose-end sprayer guide goes into more detail.

Spot spraying

For spot spraying, you’re wetting the leaves of individual weeds, not covering an area, so output per area matters less. Still check:

• the nozzle gives an even cone or fan, not a jet or a drip;
• the pressure is steady, so you aren’t misting the spray on some weeds and drenching others;
• you’re wetting the leaves as the label directs, not spraying until the mix runs off, unless the label says to.

The spot-spraying guide covers technique.

Droplet size and drift

Pressure changes droplet size as well as output. Higher pressure makes finer droplets, which stay in the air longer and drift further. The APVMA’s guidance on spraying recommends ways to reduce drift, such as using equipment that produces coarser droplets. When you’re calibrating, use the lowest pressure that still gives an even pattern, and keep the nozzle close to the lawn.

Looking after the sprayer

The Victorian DPI’s guidance gives a few simple rules that keep a sprayer accurate:

• Clean it thoroughly inside and out after every job, wearing gloves.
• Check the seals and O-rings for wear, and keep a few spares.
• Store it out of the sun. Ultraviolet light breaks down plastic sprayers and shortens their life.
• Keep one sprayer for herbicides and label it, so traces of weed killer never reach plants you want to keep.

When to recalibrate

• Before using a new sprayer or nozzle.
• At the start of each season.
• When you change to a product with a different water volume.
• When results look patchy or striped.
• When someone else is doing the spraying. Their pace won’t be the same as yours.

Common mistakes

• Calibrating with product in the tank. Use clean water only.
• Testing one nozzle and spraying with another.
• Walking more slowly on the day than during the test, often because the tank is full.
• Letting the pressure drop halfway through a strip.
• Making the mix stronger to make up for a sprayer that puts out too little.
• Not writing it down, so next season starts from scratch.

Log it

Record your calibration in your Lawn Journal: the sprayer, the nozzle, the pressure or pumping rhythm, your pace and your output. When you log an application, note the area and the amount of mix you used. Over a season, you’ll see whether your sprayer’s output is drifting, which usually means a worn nozzle or seal.