Explaining the Spray Quality Standard ASABE S572.1
Have you ever looked at the Spray Quality tables from different manufacturers and wondered why the droplet size numbers for the different classifications are different (if they publish them)? After all, you have to be able to compare apples with apples - right?
Well, not exactly! The best way to describe ASABE S572.1, is that it helps you compare apples with oranges.
As you all know, Spray Quality or Droplet Size Category Standards, have been in place for some years.
The Spray Quality ratings such as Fine (F), Medium (M), Coarse (C), Very Coarse (VC) etc, are used by chemical manufacturers on their labels. The Spray Quality ratings are also used by nozzle manufacturers to rate their nozzles, so that the nozzles you choose at a given rate and pressure, can be matched up with what the chemical label demands.
You would expect that if you obtained a copy of the Standard, that it would provide a nice tidy table or graph that shows the droplet sizes and ranges for each Spray Quality. If this is your expectation however, you are wrong.
The reason for this is quite simple, and it is because of the variety of different testing equipment that is used around the world, which all produce different results, so it is impossible to select droplet size A or B, and expect everyone’s measurement to be consistently the same.
To solve this problem, Standard ASABE S572.1 is a comparative Standard, not an absolute Standard. It sets out the procedures for testing laboratories to apply, on how to create a reference graph of classification droplet spectra from a set of reference nozzles using their own measuring equipment. The results from the testing of all the other nozzles from the manufacturer, are compared to that. Consequently, the standard document contains an "Example Reference Graph" to demonstrate the process.
Take care not to use the numbers in this graph for nozzle comparison purposes. They are example numbers only and not relevant reference values.
The important issue for consistency in each case, and in each laboratory, is that all of the testing, including the reference nozzles themselves, is conducted on the same day, with the same equipment, under the same conditions, by the same laboratory technician and that it is all done as outlined in the Standard.
It may take some radical thinking to wrap your head around the fact that there is no single set of droplet size figures, from which Spray Quality ratings are derived, or can be compared, but that is the case.
Here is a snappy description of the process;
"Both the ASABE and BCPC standards are comparative. A set of reference nozzles defining the category borders are measured, and the DV0.1, 0.5, and 0.9 are noted. It simply does not matter at all what those values are. What matters is where the nozzles of interest fall in relation to the values observed for the reference nozzles. That alone determines their spray quality category."
"Somewhere I have an older chart that shows values from three different labs for the same nozzles. As you might expect, the values are quite different. And that fact is the main reason why the standard was developed as comparative." - Tom Wolf, Ph.D., P. Ag. AgriMetrix Research & Training
Therefore, if the nozzles have been tested under Standard ASABE S572.1, then the actual droplet size numbers produced, are of no relevance. Different manufacturers may have completely different numbers, and their respective Quality Ratings will still all be correct.
So, the take home message is this. When you compare nozzles from different manufacturers under Standard ASABE 572.1, what the actual numbers in a droplet size table show doesn't matter. The only relevant information to make comparisons by, is the Spray Quality rating published by the manufacturer, either Very Fine, Fine, Medium, Coarse, Very Coarse, Xtra Coarse or Ultra Coarse as the case may be.
That is how you compare apples with oranges!
Article by David Young
14 March 2017
# The Standard document itself is a copyright document and may be purchased online from the ASABE web-site. http://www.asabe.org/