In this article we will break down some of the key considerations for the process of calibrating spray systems in agricultural drones.

Introduction

Calibrating spray systems for agricultural drones is crucial to ensure accurate application rates, efficient use of chemicals, and optimal coverage.

In this article we will break down some of the key considerations and demonstrate some of the mathematical formula used in the process of calibrating these systems.

Understand the RPA and Spray System Specifications

Payload Capacity

Know the maximum payload capacity of your system to prevent overloading. Whilst this is not a critical component of the calibration process, knowing the tank capacity will be useful when working out the amount of time it will take to empty the tank at various settings.

Nozzle Type and Configuration (TeeJet style nozzles)

Different nozzles produce varying spray patterns and droplet sizes. These parameters are not only dependant on nozzle type but also pump flow rate and pressure. Ensure nozzles are selected as appropriate for the intended application as it may be necessary to recalibrate equipment after a change in nozzle specifications.

Pump Flow Rate

Determine the flow rate for your pump to ensure it meets the requirements of the spray system. Ensure that the output is consistent and note any situations that may cause changes (i.e. varying pump voltage).

Software Optimisation for Spray System Calibration

Different RPA platforms operate on various manufacturer-specific software applications. These applications are designed to optimise the calibration process through automation. Most software solutions not only handle calibration but also measure critical performance metrics such as voltage settings for pump efficiency, temperature reading for motors, tank volume, and output levels. This comprehensive approach ensures accurate and efficient operation of the spray systems.

Preparation

Ensure that you have appropriate safety gear available during the calibration process. Even when calibrating using water it is worth wearing PPE such as protective clothing, gloves and eyewear, especially if there is a chance of residual chemical being present in the spray system.

Plan to calibrate your system in a flat, open area. Preferably choose somewhere sheltered from the wind and have no obstacles in the way.

Measure the Flow Rate

The first task is to calculate the flow rate of your system – that is the amount of product ejected by each nozzle per minute.

We have broken down the simple steps below for manual flow rate calculation:

Fill the spray tank with fresh, clean water and run the system at the intended operating pressure.

Collect the output from all nozzles for a set period of time.

Try to be accurate with your timing and make sure to collect ALL spray output.

Measure the total volume of liquid collected.

At this stage you can decide wether to calculate the flow rate of the entire system or test each individual nozzle.

For individual measurement, divide by the number of nozzles installed to get the average flow rate per nozzle.

Use the formula below to determine the flow rate per minute:

Flow Rate = Total Volume Collected / Time

Example:

  • We measure 6 litres of water over a period of 2 minutes.
  • There are 2 nozzles in the spray system on the aircraft so we collected 3 litres per nozzle (6l / 2 nozzles)
  • 3 litres / 2 minutes = 1.5l/min per nozzle or 3l/min for the total output.

Adjusting Spray Pressure and Flow

After performing the calculations above, if the flow rate is not within the desired range for application you may need to adjust the pump pressure. Repeat the flow rate measurement and make adjustments until the correct flow rate is achieved.

Determining Application Rate

The application rate defines the actual amount of chemical we need to apply to a given area to achieve a successful outcome. This rate is usually defined on the product label and should be strictly followed to avoid over-spraying and off-target application.

Determine the desired application rate for your treatment area. (e.g. litres per hectare).

Use the following formula to calculate the required flow rate per minute for the desired coverage area:

Required Flow Rate =

(Application Rate x Drone Speed x Swath Width) / 60

(litres per hectare x meters per second x meters) / 60

EXAMPLE:

(10l/ha x 6m/s x 7.5m) = 450 / 60 = 7.5l/min

Required Flow Rate = 7.5l/min

Adjust the drone’s flight speed and altitude and recalculate to achieve the desired swath width and coverage.

For example in our previous calculation:

(10l/ha x 6m/s x 7.5m) = 450 / 60 = 7.5l/min

If we needed to achieve a flow rate of 5l/min we could reduce speed and height:

(10l/ha x 5m/s x 6m) = 300 / 60 = 5l/min

Field Testing and Adjustment

The final step in calibrating the system, particularly a new setup or aircraft, is to perform field testing.

Test Flights

Conduct test flights over a known area to verify the application rate and coverage. Have a set field planned in your software to ensure the test can be repeated whilst fine-tuning settings.

Collect Data

Use onboard sensors and ground-based measurement tools to collect data on spray distribution and coverage.

Measuring swath width can be difficult. A single pass over a flat surface that is easily marked is the easiest method. Sometimes using a dye in the water will help with visibility (although be careful with dye as it can ruin the look of your nice new aircraft!)

Make visual observations of spray pattern and consistency to look for signs of blockage or damaged nozzles.

Adjust Settings

Based on your test results, adjust the drone’s speed, height, and nozzle settings to fine-tune the application.

Record Keeping and Documentation

Detailed records must be maintained when performing manual calibration of spray systems. Keep notes of current calibration settings, test results, and adjustments made.

This information is not only useful for future calibrations and troubleshooting problems, but may also be used to ensure compliance with local regulations and industry standards.

Equipment Maintenance

Maintenance is essential for spray system efficiency and performance. Regular inspection of spray assemblies is mandatory to ensure optimum spray system performance:

  • Spray pumps
  • Arm/boom assemblies
  • Hoses and clamps
  • Nozzles (TeeJet/CDA)

Remember that all maintenance, whether routine or on-condition, must be documented within the RPA Technical Log as per CASR Part 101 regulations.

Software Updates

Most modern agricultural RPA use software based applications to calibrate spray systems for optimum performance. Operators should follow the manufacturer operator manual on the procedures to calibrate systems prior to use.

Conclusion

Calibration of spray systems differs between aircraft type and manufacturer. Operators are advised to follow manufacturer recommendations specific to the aircraft type when calibrating spray pump systems.

Calibrating spray systems using water is a standard industry practice. Operators must recognise that the liquid volume may differ between water and chemical mixtures, impacting the application rate per hectare.

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