SprayMaster The 1/128 acre sprayer calibration method, step by step
Drive a course that equals 1/128 of an acre for one nozzle, then catch that nozzle for the same number of seconds: the fluid ounces you collect equal gallons per acre. On 20 in spacing the course is 204 ft.
Why ounces caught equal gallons per acre
A gallon holds 128 fluid ounces. If you measure what one nozzle sprays over 1/128 of an acre, every ounce stands for one gallon over a full acre. No GPM conversion and no speed maths: the jug reads GPA.
The course length depends only on nozzle spacing. 1/128 acre is 340.31 sq ft; divide that by the spacing in feet, or use the short form 4,083.75 ÷ spacing in inches.
| Nozzle Spacing | Course to Drive |
|---|---|
| 15 in | 272 ft |
| 20 in | 204 ft |
| 30 in | 136 ft |
Do the 1/128 acre calibration with SprayMaster
- Open Sprayer Calibration from Pro Calculators and set Units to US.
- Set Method to 1/128 Acre and type your Nozzle Spacing in inches. The app shows the Course to Drive in feet.
- Measure and mark that course in the field you will spray, with the sprayer partly filled.
- Drive the course at spraying gear and throttle with a running start, and time it in seconds with your own watch. The app has no timer.
- Park, keep the same pressure, and catch one nozzle in a measuring jug for that many seconds.
- Type the fluid ounces in Amount Caught. The Application Rate in GPA and the Same rate in figure in L/ha appear.
Worked example
| Step | Value |
|---|---|
| Nozzle Spacing | 20 in |
| Course to Drive | 204 ft |
| Amount Caught | 22 fl oz |
| Application Rate | 22 GPA |
| Same rate in | 205.79 L/ha |
The big mistake: boom width instead of nozzle spacing
The method is built on one nozzle's strip, not the whole boom. Type the boom width instead, say 240 in for a 20 ft boom, and the course shrinks to 17 ft — twelve times too short. You then catch for a fraction of the right time, the GPA reads far too low, and every tank you mix afterwards comes out too strong. This mistake is easy to make with a borrowed boom, and the result is a scorched lawn with no obvious cause. That's why the screen warns about it.
More tips for a good catch
- Time the course two or three times and use the average.
- Catch several nozzles. If one is well off the others, replace that tip and repeat the catch.
- Calibrate with water on the ground you will spray: soft soil and slopes change speed.
- Use a jug marked in fluid ounces so no conversion is needed.
Metric version: Measured Run
In Metric, the method becomes Measured Run. Enter Nozzle Spacing in cm, the Course Driven in metres (100 m by default) and the Amount Caught in mL; the app computes L/ha as 1,000 × mL ÷ (metres × cm). With 50 cm spacing, a 100 m course and 900 mL caught, the rate is 180 L/ha, or 19.24 GPA.
Questions and answers
How long is the 1/128 acre course for 30 inch spacing?
Why do ounces caught equal gallons per acre?
Does SprayMaster time the run?
Is the 1/128 method in the free version?
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