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| Excellent question.
Tough thing is adjuvants can make differences between different actives (e.g. Roundup+Dicamba made worse drift with Interlock - https://www.producer.com/crops/wind-tunnel-and-lasers-help-reduce-sp...).
Sprayers101 has a few good articles highlighting info, especially to that question.
https://sprayers101.com/spray-wind/ - [Regarding low-drift adjuvants] "Specific products such as Interlock or Valid have been shown to reduce driftable fines (<150 microns) by between 40 – 60%, without adding significant volume in coarser droplets. The response will depend on the nozzle and the tank mix, but can be very noticeable."
For WinField's info that they provide, I think they run the number of ~50-70%, and with right water conditions/etc. AGAIN, bear in mind that their claims on many situations are based on a conventional flat fan nozzle (so like a Wilger ER/Teejet XR kind of nozzle without any drift reduction technology built into it).
https://www.winfieldunited.com/products/built-for-it/interlock
WinField does a lot of lab testing with actives, as Interlock is a core part of their business, and would have the tools to do so consistently, so despite being a seller, seems like there is reputable info.
The main question of 'Does it make all the volume too coarse now' is a good one.
I don't think I've seen a data set of one nozzle (without interlock) and a second with interlock to see how the droplet sizes stack up against eachother.
Again, if we were talking like a VMD droplet size, if the middle point of the spray VOLUME doesn't change, then you could determine that your spray droplets have not shifted terribly to the coarser side of things, and the VOLUME is the same for droplets that are smaller than VMD.
The perfect answer to the question would be pretty much the catch bin data (when they test nozzles a laser counts every droplet that comes out of a nozzle), and then seeing whether there is a huge shift ONLY in the % of spray volume made up of droplets smaller than ~141µ (where we call droplets smaller than that as 'driftable fines') and not a huge shift in the VMD (which is the Dv0.5 or droplet size at the 50% mark of the spray volume coming out of the nozzle).
If the situation is that the <141µ does get smaller (drift is less), and the VMD moves to be a lot coarser, then for sure the spray droplets are getting coarser on average, which would mean less droplets for each gallon of spray.
Again, there are other factors of the Drift-reducing agents/adjuvants/etc, that the characertistic change of the liquid can make the droplets 'stick' to the plant and absorb through waxy membranes/etc, then it once again makes it tough to compare apples to apples. If one droplet of Interlock liquid is better than one droplet of non-Interlock liquid, due to other characteristics, it does complicate things.
Anyways, my '40%' rule is generally garnered from experience with weed scientists, and based on even what Winfield is promoting is on their lower side of the range, so should be some merit to it as a 'general rule of thumb', but it can't be assumed to be valid for every type of active chem in the tank.
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Drift retardants. Do they do any good?