Diagnosing Airblast Coverage

Assuming there are no mechanical or maintenance problems, water-sensitive paper can be used to diagnose sprayer performance. Go here to read more about water-sensitive paper. Interpreting the results and knowing what changes to make is the critical part of the process. Observing no coverage, or a sodden paper, make for obvious conclusions… but what about everything in between? Here are the ground rules:

First: Only ever test coverage in environmental conditions you would normally spray in. Temperature, humidity and wind speed can make or break an airblast calibration.

Second: When altering sprayer settings, only make one change at a time for each test pass so you can isolate what’s wrong.

Third: Each pass requires a new set of papers located in the same place, oriented the same way, distributed throughout the canopy. Mark their locations with bright flagging tape and write the pass number and canopy position on the back of paper prior to placement. This helps you to compare the passes later on. Don’t collect papers until they’ve had an opportunity to dry a little, or they will smear and stick together.

Fourth: Pass down one alley first. Have a look at the papers without removing them. Then, spray the target canopy from the other side. Now the papers can be removed for analysis. This order is important because it reveals the impact of wind direction and the cumulative effect of spraying from both sides. In some cases, the sprayer operator may wish to travel an additional upwind alley to reflect the cumulative coverage on a typical spray day. Alternate row applications are not recommended.

This Turbomist has been outfitted with sensors that detect the presence of a canopy. Each eye corresponds to a boom section, turning the section on and off as required and improving efficiency. If it’s not there, why spray it?

Once the papers are retrieved, it’s time to diagnose the coverage. The following situations are typical in calibrations, and possible fixes are suggested. Remember, this is a process that takes time. Several passes may be required before satisfactory coverage is obtained. Once the correct settings are determined for the block, continue to use them until there is a significant change in the crop staging or weather. At that point, repeat the process.

Situation One:

<15% coverage and <85 Fine/Medium droplets/cm² at top of target (e.g. tall targets such as hops or trees). Suggested Fixes:

Wind might be stealing fine droplets. Try Coarser droplets (e.g. using air induction nozzles). Be aware that you may have to increase volume to compensate for reduced droplet counts and that they may fall out of the airstream before reaching distant targets.
Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.
Fan may have to be set to higher gear, or if using GUTD, return to 540 rpm to increase fan speed. If still insufficient, you may need a sprayer with higher air capacity.

Situation Two:

<15% coverage and <85 Fine/Medium droplets/cm² deep in canopy – sometimes papers on outside of canopy are visibly wet. Suggested Fixes:

Ground speed may be too high. Use flagging tape indicator on far side of target and see if air is getting through.
Canopy maintenance may be required (e.g. pruning, hedging, leaf stripping, etc.). No sprayer can consistently penetrate really dense canopies.
Fan may have to be set to higher gear, or if using GUTD, return to 540 rpm to increase fan speed. If still insufficient, you may need a sprayer with higher air capacity.
Increase carrier volume.

Situation Three:

Papers are drenched, dripping or show channels of running liquid. Suggested Fixes:

Reduce spray volume, either overall or in key locations on the boom corresponding to the drenched papers.
Ground speed may be too low. Use flagging tape indicator on far side of target and see if too much air is getting through. If so, increase ground speed.

Situation Four:

Considerable overspray beyond target row. Suggested Fixes:

Turn off upper nozzles until spray JUST clears target.
Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.

Situation Four:

Considerable blow-through beyond target row. Suggested Fixes:

Slow the fan speed by shifting to low gear, or using GUTD method
Ground speed may be increased as long as coverage is not compromised. Use flagging tape indicator on far side of target and see if air is getting through.

Situation Five:

Ground under target row is drenched. Suggested Fixes:

Rotate lower nozzles slightly upward, but do not shut them off. If ground remains drenched, turn them off entirely. Each hollow cone produces up to an 80º spray angle, so the next higher nozzle often compensates by spraying lower than expected.
Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.

Situation Six:

<15% coverage and <85 Fine/Medium droplets/cm2. Remember that this coverage threshold is only a point of reference, not a hard fact. It does not apply when using Coarser droplets. Suggested Fixes:

Increase spray volume, either overall or in key locations on the boom corresponding to the under-sprayed papers.
Wind might be stealing fine droplets. Try coarser droplets (e.g. using air induction nozzles). Be aware that you may have to increase volume to compensate for reduced droplet counts.
Ground speed may be too high. Use flagging tape indicator on far side of target and see if enough air is getting through. If not, decrease ground speed.
Canopy maintenance may be required (e.g. pruning, hedging, leaf stripping, etc.). No sprayer can consistently penetrate really dense canopies.

Situation Seven:

Inconsistent coverage on outer edge of canopy (e.g. one spot never seems to get spray.) Suggested Fixes:

Nozzle spray angle may be too acute (e.g. full cones), and spray is not overlapping before reaching target. Try wider spray angles.
Some tower sprayers have ‘dead spots’ in their air. Check for limp or flagging ribbons tied to nozzle bodies and/or deflectors. Deflectors may need to be adjusted, or adjacent nozzle body angles repositioned to compensate. Try an air induction nozzle in the dead zone.
Canopy may be brushing against nozzles as the sprayer passes, temporarily blocking them. Canopy management required.

Some sprayers, such as Rears, Turbomist, FMC or this Durand Wayland have an option for electronic ‘eyes’ that detect spray targets. The boom will shut off completely if there is a gap in the planting. This can save a great deal of wasted spray. It is less applicable in trellised plantings where it has been known to be “fooled” by wires and posts.

If you still are unable to achieve satisfactory coverage, you may have to consider more extreme solutions. You may have an under- or over-powered sprayer. You may have to perform significant canopy management. Or, you may be trying to spray in poor weather conditions.