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Chemical injection a perk of some chemical rate controllers

Several chemical rate controllers feature chemical injection. In these systems, a large tank houses only water. A small amount of chemical is injected into the lines carrying water to spray booms.

These units have several advantages, says Daniel Humburg, South Dakota State University Extension agricultural engineer. They include:

  • Constant flow rate of the carrier.
  • More consistent droplet sizes.
  • A varied chemical injection rate that matches ground speed and changes in the map-commanded rate.

Any downside? It’s more complicated and expensive. An occurrence called “transport delay” also results.  “When you slow down suddenly, (like the end of the field) the injection controller ramps back the chemical injection to match decreasing speed,” explains Humburg. “However, your plumbing from the injection point to the nozzles is full of mix that was matched to a higher ground speed.  This results in a ‘transient’ of the wrong mix for a few seconds.

“The same thing happens when you accelerate after the turn,” adds Humburg. “A similar error results for a few seconds if the map commands a new rate. The plumbing is full of the old rate. It has to work its way through.”

Good fit for large acreage

These systems are preferred by commercial applicators, says Humburg. 

“No tank mix means less tank rinsing, which enables them to quickly move onto the next field and chemical,” he says. “No leftover tank mix results at the end of the field. Operators can simply remove the bulk container for later use.

“These systems can also be repeated on the machine for multiple variable rate products,” adds Humburg.

“While expensive, chemical injection may be a good fit for larger farmers or operators who do custom work in addition to their own farm with a high-clearance, self-propelled sprayer.” 

Controllers keep chemical rates constant, but not droplet size

Chemical rate controllers do a good job in keeping rates constant across a field despite speed changes.

“If an operator applies a chemical at 10 gallons per acre and speeds up and goes twice as fast as before, the rate controller demands the pump double the output that is needed,” says Bob Wolf, Kansas State University Extension agricultural engineer. Conversely, the rate controller cuts chemical output in half if the operator halves the original speed.

There’s one complicating factor, though, and that’s changes in pressure.

“As the pump increases or decreases pressure, the droplet size is affected,” says Wolf. “With higher pressure, you get finer droplets, and at lower pressure, you get bigger droplets. Thad’s due to the fact that most sprayer nozzles are fixed orifice nozzles. The hole is a given size. It doesn’t change.”

Droplet size changes impact efficacy. More and more chemical labels specify the droplet size for optimum performance. For example, the label for Ignite 280 used in LibertyLink systems specifies medium droplets be used.

Droplet size change also increases drift potential. “When you double speed from 8 to 16 miles per hour at a chemical you’re applying at 30 psi (pounds per square inch), the pressure has to increase four times to meet that demand,” says wolf. Going from 30 to 120 psi knocks droplet size out of the ballpark in most cases. We create driftable material.”

Watch speed changes

As of now, there’s not a good answer on the technology side to curtail this. Work is underway on variable orifice nozzles. They are being tested for the ability to adjust flow rate without affecting droplet size.

In the meantime, Wolf says one way you can stay close to optimum droplet size and nix drift is by avoiding rapid speed changes, particularly at ends of the field or near sensitive areas.

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