A rate controller is only as good as the inputs it receives, both from sensors and the settings the operator programs into it. Common automatic spray rate controller inputs are speed, flow, pressure, boom section widths, alarms and the minimum settings.

Always record all controller settings, along with the date, before using the sprayer or starting to check calibration figures, especially if you plan to make any adjustments or changes to the settings.

Speed signal

The speed signal supplied to the rate controller can be received from a wheel sensor, radar or GPS.

When using a GPS or GNSS signal for the speed input, the reliability and accuracy of the system or network will be critical to being able to spray as required. Even when using GPS information for the speed input it is important to have a back-up method for this input, such as wheel sensors, in case the GPS signal is lost for some reason.

Wheel sensors commonly use magnets and a sensor, or a fixed object and a sensor. Some sensors need to have an even number of magnets and/or magnets with alternating polarities.

Check the speed calibration number or factor by marking out a minimum distance of 100 metres, in paddock conditions, with the main spray tank half full and the boom unfolded. If you make an adjustment to the calibration number or factor in the controller, repeat the check again to ensure the input is accurate.

If the speed being displayed is erratic when using wheel sensors at slower speeds, the operator should consider increasing the number of magnets to achieve a calibration figure of 500 or below.

Tips

• A good place to install a speed sensor is on a rigid point near the front-wheel-drive transmission shaft or the four-wheel-drive shaft. You can use a hose clamp on the shaft when using the proximity sensor.

• Magnetic speed sensors can be vulnerable to being damaged or fouled with mud or debris. It can also be hard to have a speed calibration factor below 500, which can be an issue when spraying at slower speeds, producing an erratic speed on the screen. The speed reading can also be compromised when the sensor is on the inside of a turn ‘slower than the actual speed’ and outside of a turn ‘higher than the actual speed’. An option is to install a proximity sensor on a constant shaft, which will give a constant reading when turning. The calibration factor should be below 500.

Flow signal

There are two types of flow meters used in conjunction with rate controllers: impeller or electromagnetic.

The flow meter will typically send several pulses per litre to the rate controller based on the volume that passes through it. The flow meter should be accurately calibrated when leaving the factory and this calibration factor will normally be located on a tag or stamped on the unit.

Be aware that the flow meter will have been calibrated using water, and that changing the density of the spray solution is likely to impact on this calibration for impeller flowmeters, but usually not electromagnetic flowmeters.

Tip

• If you plan to regularly alternate between applying foliar fertilisers and standard tank mixes, an electromagnetic flow meter may be more appropriate.

Density of spray solution

In many controllers it is possible to adjust the density setting for the tank mix being used. Before making such an adjustment, the operator must be sure that they have accurately measured the weight of the spray solution (grams per litre) to determine the density of the tank mix.

Where a density function is not present, the operator may be able to use the measured density and then consult a density conversion chart (Table 1) to adjust the current flow calibration by multiplying this figure by the conversion factor.

Always return the density setting or flow calibration factor to its original setting after completing a spray job.

Positioning the flow meter

The ideal positioning of a flow meter is vertical. Mounting the flow meter vertically will reduce the chance of the flow meter reading air and, more importantly, will help to ensure that solids and debris are not able to settle in the flow-meter housing.

When mounting the flow meter, it is ideal to have a straight piece of hose approximately 10 times the size of the thread on the flow-meter housing before the actual flow meter. This will help to reduce turbulence as the liquid passes through the flow meter.

Find out more

For more information on measuring density, go to Module 8: Calibration of the spray system.

Tips

• Flow meters are designed to operate within specific ranges of flow rates (e.g. 15 to 150L/min) and different models may have different ranges that they can operate between.

• To ensure you have the most appropriate flow meter model for your sprayer, it pays to work out the range of flows that you require for your spraying operations. Use the following formula to compare your lowest and highest application rate at the range of spraying speeds you intend using.

• Total litres per minute through the boom = L/ha x km/h x total spray width (in metres) ÷ 600

Pressure signal

The pressure signal required by the controller is an electronic signal.

The electronic pressure sensor is usually mounted at the back of the sprayer, as close to the nozzle as practical. It is useful to check the pressure sensor by using a calibrated pressure gauge on the nozzle. If the pressure sensor needs to be calibrated make sure there is no pressure in the plumbing system.

To be able to monitor pressure while you are driving it is useful to have a pressure gauge that measures pressure in the spray line mounted outside of the cab.

Pressure and flow rate in combination

Some controllers regulate sprayer output based on either flow or pressure, while other models may be able to use a combination of both (switching from flow to pressure if the flowrate is below the limits of the flow meter).

It is important to understand what functions the controller you have (or plan to purchase) is capable of, so you can take full advantage of the functions it offers.

Tips

• Mount the pressure sensor so the liquid entry point is facing down and use a calibrated pressure gauge on the nozzle body to check the calibration of the pressure sensor.

• A spraying system that can use three-way boom valves allows the flow meter to work in its optimum flow range because the flow through the flow meter does not change if a boom section is turned off.

Multi-step systems and transitions

Some multi-step systems (where the system can change from one nozzle to another to increase the speed and application volume) will require the operator to input into the controller where the transition from one nozzle type or size to another should occur. This may be based on speed, flow rate or pressure.

Find out more

For more information on multi-step systems go to Module 18: Single line and multi- step systems 

Boom width and section widths

Always check and record the exact nozzle spacing measurements for all nozzles on the boom, the number of boom sections and the number of nozzles plumbed into the individual boom sections. This information will give the spray operator the inputs that are required for boom width when setting up the controller.

Make sure that the controller is set in the correct units (usually metric) before starting any programming or before adjusting settings.

If you are working on a sprayer that has just been delivered, complete all of the pre-operational checks listed in Module 6 Pre-operational checks.