The problem with camera/spot-spray systems that nobody wants to talk about

When you take a spot spraying system and attach it to a boom sprayer, the expectation is that it will just work like normal. Well if two or more adjacent nozzles are firing, it probably does.

By normal, I mean that the rate per hectare being managed by the sprayers rate control software is calculating the rate based on the standard formula. That formula is a function of the nozzle spacing and accounts for overlapping nozzles. It looks like this;

L/ha = (600 x lit/min per nozzle) / (speed x nozzle spacing)

This is the case on a Bilberry system for example, when it is spraying in side safety mode. That is where an extra nozzle is engaged at either side of a single nozzle or group of nozzles to help deal with windy conditions or twists and turns. It is similar to the triple nozzle mode on the See & Spray.

Typically, manufacturers supply their integrated systems with nozzles that suit these multi-nozzle modes. Sometimes even with 110° nozzles!

I’m assuming they do that because it syncs with the rate control system. I.e. based on nozzle spacing as shown in the formula above.

The problem that nobody wants to talk about is what happens when the operator, usually in the drive to reduce chemical use, switches from multiple nozzle mode to single nozzle mode. We call it single-spot, and it's where every nozzle is capable of operating in isolation.

When you switch to Single Spot

All of a sudden a different calibration formula comes into play. In this formula, the nozzle spacing is replaced by the spray-width for the individual nozzle.

The spray width is derived from the angle of the nozzle and the height of the nozzle. An 80° nozzle, 70cm off the ground has a footprint of approximately 1.2 metres. So the new calibration formula would look like this;

L/ha = (600 x lit/min per nozzle) / (speed x spray width).

Using the same nozzle. Let’s say an 80° 04 nozzle at 3BAR (1.6 l/min), 18 km/h, 50 cm nozzle spacings at 70 cm nozzle height (spray width 1.2 m).

Side safety mode

L/ha = (600 x lit/min per nozzle) / (speed x nozzle spacing)
= 600 x 1.6 / 18 x .5m
= 960 / 9
= 106 l/ha

Single spot mode

L/ha = (600 x lit/min per nozzle) / (speed x spray width)
= 600 x 1.6 / 18 x 1.2m
= 960 / 21.6
= 44 l/ha

Can you see what happens when you switch from side safety to single? That is why you may have been told to double the rate when you go single spot. The options available for doing that are either halving your speed, doubling up your batching rate or changing the size and/or angle of your nozzles.

I know from the many conversations I have had with spot spray system owners of all types, that this information comes as a complete surprise.

That is why we have gone to the trouble to provide you with an App that works all this stuff out for you.

Go to appcentral.online and register for the app for your own machine.

Now let’s return to your sprayers spray control software. When you switch from multi to single, does it make the necessary change in the calculation, replacing the nozzle spacing with the spray width? Generally, the answer would be no. However that would be something you would need to enquire about with your system supplier.

If the answer is yes, all well and good.

There is high likelihood though that the answer will be no. That would mean the rates being displayed on your screen when you are in single mode are presented as if they are in multi mode. If a lot of adjacent nozzles are firing, the screen display may even be close to being correct!

If targets are sparse however. and there are mostly single activations, the screen display and recorded data will not be correct. Not forgetting that the rate per hectare actually being applied by isolated nozzles on those sparse targets will be way less than you think. (Refer example above)

That is why in addition to our Spot Spray App’s, we’ve developed a soon to be released batching and reporting companion App that will help you work the numbers out independent of your controller. All you’ll need to have is accurate measurements of your tank volumes before and after your spot spray job. We can even help you with that too with a VISIO tank level monitor ;) So keep an eye out for Batch App!

Conclusion

The starting place is to know your numbers accurately in advance. The App will take care of that! If you can you find a nozzle that will do both single and multiple according to the correct methods of calculation. Pretty tough though without major compromises.

So for most of you, it's one set of single spot spray nozzles for sparsely distributed hard to kill gnarly weeds taking the "bomb the crap out of them" approach, and another set of side safety nozzles of 80° or less (minimising wasted edges) to deal with high weed concentrations.

And lastly having a way to calculate your chemical usage and coverage percentages even if the information for the control system is wrong. Batch App!

More to follow.

Regards,

Dave