What Is a Truck-Mounted Herbicide Sprayer? (PSI & GPM Calibration)
A truck-mounted herbicide sprayer is a tank, pump, plumbing system, and spray outlet carried by a truck. Correct calibration matches nozzle flow, pressure, and travel speed to the product label’s application rate. PSI measures pressure; GPM measures flow. Neither number alone confirms the correct dose, so calibrate with clean water before mixing herbicide.
A basic setup may use a tank and pump, while a larger or more complex system may add a boom, hose reel, spray gun, agitation, and pressure regulator. If you are weighing budget options, compare the full working system, not just the tank or pump: it must deliver the label-approved method and rate. A lower-cost setup can be suitable if its parts are compatible and you can measure and control its output. Do not use a cheaper component to bypass label limits.
What a truck-mounted herbicide sprayer does
A truck-mounted herbicide sprayer carries liquid in a tank and uses a pump to move it through hoses to a boom, wand, or spray gun. The pump creates flow and pressure; nozzles shape and distribute the liquid. Calibration checks whether that system applies the label-required amount over the intended area.
A boom sprayer has several nozzles along a frame and is often used for broadcast coverage. A hose-and-gun sprayer sends liquid through a hand-held wand for spot work or targeted treatment. The method matters: the same pump can behave differently with different nozzles, hose lengths, and settings.
The pump may be powered by an engine, a vehicle power take-off (PTO), or a hydraulic system. Check that the drive and pump can provide the required flow at the working pressure. Also confirm that the tank, seals, hoses, valves, filters, and nozzles are suitable for the herbicide and use conditions. The product label and equipment manuals are the final references.
PSI, GPM, and application rate
PSI is pressure, measured in pounds per square inch. GPM is liquid flow, measured in gallons per minute. Application rate is the amount of spray mixture applied to an area, often stated as gallons per acre (GPA). To calculate a broadcast rate, you also need travel speed and nozzle spacing.
A pressure gauge shows pressure at its location, not the amount applied per acre. A pressure reading can look normal even when a nozzle is partly blocked or worn. For that reason, check nozzle output as well as pressure.
For a broadcast boom with one nozzle per spacing, use:
GPA = 5,940 × nozzle GPM ÷ (speed in mph × nozzle spacing in inches)
For example, a nozzle delivering 0.50 GPM at 5 mph, with 20 inches between nozzles, gives:
5,940 × 0.50 ÷ (5 × 20) = 29.7 GPA
This is an example, not a universal target. The label may call for a different application rate or method, and the actual sprayer must be measured.
| Calibration input | Example only | What to measure |
|---|---|---|
| Pressure | 40 PSI | Pressure while the intended nozzle is operating |
| Flow per nozzle | 0.50 GPM | Liquid collected over a timed interval |
| Travel speed | 5 mph | Actual speed over a measured course |
| Nozzle spacing | 20 inches | Distance between adjacent nozzles |
| Calculated broadcast rate | 29.7 GPA | Compare with label and intended use |
The pressure-flow relationship is useful for understanding changes, but not as a substitute for measuring output. With the same nozzle, flow changes approximately with the square root of the pressure change:
New GPM = old GPM × √(new PSI ÷ old PSI)
For instance, increasing pressure does not double flow. More importantly, raising pressure to compensate for the wrong nozzle or speed can create poor spray patterns or exceed equipment and label limits. Choose a suitable nozzle and verify its output.
Prepare and calibrate before spraying
Calibration means measuring the sprayer’s output and matching it to the planned application. Start with clean water, not herbicide. This lets you find leaks, uneven nozzle flow, or pressure problems before chemical is in the system.
Before filling, read the product label and safety data sheet (SDS). Confirm the approved site, target, application method, rate, weather limits, required personal protective equipment (PPE), and re-entry instructions. Check the tank, strainer, agitation return, hoses, clamps, gauge, regulator, and nozzle. Keep people, pets, and unprotected workers away, and take steps required by the label to limit drift and protect drains, water, and desirable plants.
- Park and inspect. Secure the truck and inspect the treatment area. Do not spray if plumbing leaks, a hose is damaged, or the nozzle is clogged or mismatched.
- Use clean water to test. Fit the intended nozzle and set pressure within the nozzle, equipment, and label requirements. Check pressure at the operating nozzle or manifold, not only at an idle pump.
- Measure each nozzle. Catch each nozzle’s output for the same timed interval. Compare results across the boom and with the nozzle’s expected output. Replace worn or mismatched nozzles rather than trying to correct them with excessive pressure.
- Confirm travel speed. Drive a measured distance at the speed you will use while spraying. Record the time and calculate speed, or use a reliable speed display and verify it against the measured course.
- Calculate the rate. Record nozzle flow, speed, and spacing, then use the GPA formula. Adjust nozzle choice, speed, spacing, or pressure only within the label and equipment limits. Recheck after any change.
For a hose-and-gun system, do not infer coverage from pump PSI. Collect the wand’s output for a timed period, and determine the area treated using the approved method and rate. Aim for consistent coverage while following the label; a hand-held spray pattern can vary with the operator.
Apply, monitor, and troubleshoot
Once calibrated, maintain the same pressure, speed, nozzle height, and overlap used in the check. Changes to any of these can change the amount or distribution of spray. Watch for leaks, shifting pressure, clogged filters, poor spray patterns, and conditions that increase drift risk.
| What you notice | Possible cause | What to do |
|---|---|---|
| Pressure looks normal but coverage changes | Blocked or worn nozzle; inaccurate gauge | Stop, inspect nozzle flow, and verify the gauge |
| One nozzle delivers less than others | Clog, damaged nozzle, or restriction | Stop and clean or replace using approved procedures; retest |
| Pressure varies during spraying | Regulator, pump, filter, or plumbing issue | Stop and inspect before resuming |
| Spray drifts or weather exceeds label limits | Conditions are unsuitable | Stop and follow the label’s weather restrictions |
| Flow or speed changes after adjustment | Rate is no longer verified | Recalibrate before applying herbicide |
A blocked or worn nozzle, faulty regulator, or inaccurate gauge can cause under- or over-application even when the displayed PSI seems normal. That can lead to poor control, damage to desirable plants, or an application that does not comply with the label. If a reading or spray pattern changes, stop and verify the system before continuing. Never spray with a leak, damaged hose, clogged or mismatched nozzle, or unverified calibration.
Cleaning, records, and seasonal storage
Maintenance keeps measured output more dependable. After use, clean filters, strainers, nozzles, and lines according to the label and equipment instructions. Collect rinse water and handle it as the label requires; do not discharge it into drains or waterways. Never clear a nozzle by mouth or by blowing through it.
Inspect pump seals, regulator, gauge, agitation return, hoses, and fittings. Replace damaged hoses and check the gauge against a known-good instrument when accuracy is in doubt. Record the product, dilution, nozzle, measured PSI and GPM, speed, spacing, calculated rate, date, and operator. Recalibrate when parts or settings change and at the interval required by the operation or label.
For storage, follow label directions for rinsing, decontamination, and disposal. Keep chemicals in their original labeled containers, secured and apart from incompatible materials. Drain or winterize the sprayer as the pump maker directs, protect it from freezing, and store clean hoses and nozzles where they will not be damaged.
Illustrative setup scenarios
These examples show how to think through common configurations. They are not reports of specific installations or prescriptions for a particular herbicide. In each case, the label and the sprayer’s actual measurements determine whether the setup is suitable.
Boom on a truck: Suppose the planned setup uses 20-inch nozzle spacing and a measured speed of 5 mph. If each nozzle measures 0.50 GPM at the selected pressure, the calculated rate is 29.7 GPA. Compare that result with the label and intended treatment. If it does not match, choose an appropriate adjustment and measure again.
Hose and spray gun: A pump gauge reads 40 PSI, but that figure does not tell you the area the wand can cover at the label rate. Measure the wand’s output with clean water and assess the treated area using a label-compliant method. Keep delivery consistent, then verify the result before mixing product.
Normal gauge, uneven spray: A system may show steady pressure while one nozzle has reduced flow. Stop, inspect the nozzle and filter, and measure output again. Do not raise pressure to hide a blockage; correct the cause and recalibrate the full system.
Frequently asked questions
What does PSI mean on a herbicide sprayer?
PSI means pounds per square inch, a measure of pressure. It helps describe the force moving liquid through the system, but it does not show how many gallons reach an area. Measure nozzle flow and calculate application rate as well.
What does GPM mean?
GPM means gallons per minute. It describes the volume flowing through a nozzle or hose over time. For boom calibration, measure each nozzle’s output under the intended operating conditions.
Is higher PSI better for spraying?
No. Use pressure within the label, nozzle, and equipment limits. Higher pressure is not a reliable way to correct the wrong nozzle, travel speed, or application rate.
How do I calculate GPA for a boom?
For one nozzle per spacing, use GPA = 5,940 × GPM ÷ (mph × spacing in inches). Measure all inputs on the actual sprayer, then compare the result with label directions.
Should I calibrate with herbicide?
Calibrate with clean water first. This helps identify leaks and uneven output without using chemical. Then mix and apply only according to the label.
How often should I check nozzle flow?
Check during calibration, after nozzle or plumbing changes, and whenever the spray pattern, pressure, or flow changes. Follow any additional schedule required by your operation or label.
Why is the pressure gauge normal if one nozzle is blocked?
A gauge reports pressure at its location, not the flow from every nozzle. A blockage can reduce one nozzle’s output while the gauge still shows a normal reading.
Can I use a hose-and-gun sprayer without calculating GPA?
You still need to apply the label rate and use an approved method. Measure the wand’s output or treated area as appropriate; do not assume PSI alone proves the correct coverage.
What should I do if calibration does not match the label?
Do not spray the herbicide yet. Check measurements, nozzle condition, speed, spacing, and equipment settings. Make only permitted adjustments, then retest.
Can I spray if a hose is leaking?
No. Stop and repair or replace the damaged part before spraying. A leak risks exposure and changes system output.
(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)
