How Fast Should I Drive My Sprayer?

It seems simple: The faster you drive the sprayer, the more area you cover. This makes higher travel speeds a seductive method for improving productivity. Sprayer manufacturers knew this 25 years ago when pull-type sprayers first received bigger, suspended outrigger wheels. Since then they’ve delivered more powerful engines, better hydraulic motors, smoother suspension and cruise control.

Each of these innovations still required the operator to consider the relationship between travel speed, pressure, nozzle choice and the desired output per acre. But now we have rate controllers, and we don’t have to think about such mundane things anymore… do we? Do we still do a good job if we go faster? What exactly happens when we speed up?

Before considering the role of the rate controller, you have to decide on an overall target-speed range. Charts, apps, or online tools can help you select nozzles sized to deliver your application volume at a given speed and pressure. This initial travel-speed decision requires an understanding of how spray gets delivered to the target. Let’s start with the spray boom.

As the boom moves through air, the oncoming air does three things to the spray:

  • It shears the spray, making it a bit finer.
  • It scrubs the smallest droplets from the pattern, leaving them in the wake of the boom.
  • Finally, negative pressure behind the pattern sucks even more fine spray into the sprayer wake.

Collectively, these create the dreaded “spray plume” that hangs behind the spray boom… and we’ve lost control over it. The faster we move, the greater the proportion of the spray that ends up in the plume. This can be anywhere from one to 15% of the spray. Once formed, that plume moves with the prevailing winds.

Today’s sprayers have wide booms, and faster speeds often require us to keep these booms higher than we have in the past to prevent impacts. But higher booms reduce our control over the spray’s direction. For example, when spraying vertical targets (e.g. wheat heads) we have begun to employ angled sprays. But droplets lose momentum quickly. The further they are from the target, the more likely they are to slow or even fall vertically before they reach the target. That means that higher booms often negate the benefit of angled sprays.

Still not convinced of the perils of high speeds? Well, think about the aerodynamics of the sprayer itself. As travel speed increases, the sprayer, the boom, and even the spray pattern itself disrupt the air around it.  Visualize a sprayer in a wind tunnel with smoke tracer lines. The nice pattern created by the boom gets really messy in a turbulent environment. This can cause a loss of deposit uniformity, resulting in a reduction of overall effectiveness.

So far, we’ve talked about average speeds – choosing to travel eight, 12 or 16 mph overall, and then choosing the nozzle that will suit. Now let’s talk about changes in your travel speed within your target-speed range.

Operators know that even small travel speed changes can result in large pressure changes.  That’s because travel speed and pressure enjoy a “square-root relationship”. If you double travel speed, your rate controller needs to quadruple the spray pressure to meet the new flow need!

Even minor changes in speed (to adapt to field conditions) can lead to big fluctuations in pressure, changing average droplet size, and affecting coverage and drift potential. Severe pressure fluctuations are more likely with a faster average travel speed. That’s perhaps why pulse-width modulation, which decouples spray pressure from travel speed and replaces it with a solenoid duty cycle, has a growing role in fast self-propelled sprayers.

To minimize pressure fluctuations, use the pressure gauge as your speedometer. Have the boom pressure displayed prominently in your sprayer cab, and try to operate at speeds that result in a pressure which is optimal for the job you’re trying to do.

So, let’s summarize the effects of fast travel speeds.

Pros:

  • More area covered per hour
  • Better contact with vertical targets (if the booms are kept low)

Cons:

  • More drift
  • Less uniform deposition
  • Wider pressure fluctuations

So, how fast is too fast? We won’t draw a line in the sand, but we will emphasize how important it is to consider as much information as you can before deciding on a travel speed. Don’t rely on the rate-controller to think for you – it doesn’t have all the information.