All about nozzle wear!
Apart from depending strongly on the nozzle material, the rate of nozzle wear depends on the combination of a number of different factors including the abrasiveness of the products being sprayed, operating pressure, nozzle size and the adequacy of filtration.
Spray nozzles do not wear evenly, tending to wear more in the center than at the edges. Consequently, the evenness of spray distribution begins to break down and so the ‘Coefficient of Variation’ begins to rise.
Coefficient of Variation (CV) explained.
The CV is a measure of the irregularity of your spray pattern represented as a percentage variation from perfectly even. I prefer to think of the CV as an inverse indicator of the evenness of spray distribution, nozzle by nozzle along the full length of the boom.
Here are some examples. Note that the CV %age value increases with the level of wear or damage.
The practical consequences of nozzle wear.
These will differ depending on whether or not your sprayer is fitted with a spray rate controller.
Manually Controlled Sprayers
On a manually calibrated and controlled sprayer, the effect of worn nozzles will be that the amount of product being sprayed per minute increases and so chemical will be wasted. For example;
Let’s say due to wear, your nozzles have increased in output by 10%. If 500 hectares is sprayed, you will waste enough pesticide enough to spray a further 50 hectares.
In this instance, it will be easy to recognize if your nozzles are worn out as you will simply run out of spray mix before the end of the job.
Automatic Spray Rate Controlled Sprayers
On the other hand, if your sprayer has an automatic spray rate controller (as I expect most of my readers will have) you will not waste any pesticide at all, however it will start and continue to be applied less efficiently.
As your nozzles wear your spray rate controller will progressively reduce the spray pressure in order to maintain the desired rate per hectare, all the while having negative effects on your CV and also throwing your droplet distribution out of whack! As these factors deteriorate over time, the efficacy of your spray mix may begin to suffer.
Neither of these effects on your CV and droplet distribution will be measurable or even visible to you other than the relentless miniscule incremental drops in spray pressure! So, apart from physically testing nozzle output against the manufacturers table from time to time the pressure will be your only guide. Since the small drops in operating pressure will hardly be noticeable, get into the habit of writing them down on a regular basis.
The take home message is that nozzle wear is the unseen enemy!
Keep a track of spray pressure in a notebook for each of the rates you use and check the output of at least some of your nozzles from time to time!
Article by David Young