Static is the reason ground coffee clings to a grinder’s chute, jumps out of the dosing cup and clumps in the basket. RDT is the cheapest fix people use for it: a little water on the beans. It started as a forum tip, and in 2024 a team of physicists and chemists measured what it does.
Where the name comes from
The name comes from a December 2012 thread on Home-Barista.com. Its first post credited the trick, “one or two droplets of water” on the beans before grinding, to a 2005 post by David Ross on the alt.coffee forum, and the thread’s title was later changed to “Ross Droplet Technique (RDT)”. The 2024 study describes the technique the same way: a method “posited to have been originally presented on an online message board.”
Why grinding makes static
The study is Joshua Méndez Harper, Christopher Hendon and colleagues’ “Moisture-controlled triboelectrification during coffee grinding,” published in Matter in January 2024. They ground about 30 commercially roasted coffees, and coffees they roasted themselves, on a Mahlkönig EK 43 with aligned stock burrs, and caught the grounds in a cup wired to measure their charge. What they found:
- Breaking the beans makes most of the charge. Coffee charges by friction, but grinding already-ground coffee a second time, without breaking it further, cut the charge by up to 90%. The authors conclude that fracture, not rubbing, does most of the charging.
- Finer and darker means more charge. Finer settings charged the coffee more. Darker roasts charged negatively and more strongly, up to about 120 nanocoulombs per gram, while positively charged coffees generally stayed under 50. Fine, dark roasts reached charge-to-mass ratios comparable to those inferred for particles in volcanic plumes and thunderclouds.
- The bean’s own moisture predicts the charge best. Coffees charged positively only above about Agtron 70 on the roast-color scale, and switched to negative charging when the bean’s internal water fell below about 2% by mass. Moisture predicted the charge better than color did.
- Origin and processing didn’t seem to matter. The authors found no apparent dependence on either.
- Room humidity only helped when it was high. In their tests, humidity affected charging only above about 60% relative humidity.
That fits what Jonathan Gagné, an astrophysicist who studies coffee, wrote in 2018: dark roasts “seem to produce a lot more static electricity,” so their grounds stick to the grinder, and electrostatic forces hold fines onto larger particles.
What adding water did in the study
The researchers added water to whole beans with a pipette, shook them in a sealed container to spread it evenly, and ground them right away.
- Charge fell as water went up. Every coffee they tested charged less as more water was added, and the charge approached zero at about 20 microliters per gram of coffee.
- Clumps broke up at small amounts. As little as 5 microliters per gram immediately reduced the clumps of fines stuck to larger particles, measured by laser diffraction.
- Salt in the water made no difference. Water with sodium chloride dissolved in it cut the charge just as pure water did.
- Retention dropped, but wasn’t measured. The authors note that, “in our hands,” wetting the beans also left almost no grounds retained in the grinder. They didn’t put a number on it, and say they may return to it in a future study.
Those amounts are small. A microliter of water weighs about a milligram, so 20 microliters per gram on an 18 g dose is 0.36 g of water, and 5 microliters per gram is 0.09 g (calculated).
What it did to an espresso shot
To see the effect in the cup, the team pulled espresso with and without water on the beans and changed nothing else: 18.0 g of coffee to a 45.0 g shot, grind setting 1.0 on the EK 43, tamped at 196 N (about 44 lb of force, calculated), 201°F (94°C) water at 7 bar with a 2-second pre-infusion, on a Victoria Arduino Black Eagle.
| Without water | With water on the beans | |
|---|---|---|
| Shot time | Baseline | Nearly 50% longer |
| First drops | About 10 s | About 10 s |
| Strength (TDS) | 8.2% | 8.9% |
| Extraction yield (calculated) | About 20.5% | About 22.3% |
The extraction figures are ours, calculated from the paper’s numbers: 45 g × 8.2% ÷ 18 g, and 45 g × 8.9% ÷ 18 g. The authors explain the slower shot as a denser bed: with fines no longer clinging to larger particles, the puck packs more tightly. They also caution that some of the extra strength may simply come from the longer contact time. And they suggest that anyone trying it should change the dose and grind too, rather than expect the same recipe to run the same way.
So RDT changes the shot, not just the mess. If you start using it on a recipe you’ve already dialed in, expect it to run slower. Our extraction yield guide explains the calculation.
What grinder makers say
Research conditions aren’t the same as your grinder’s instructions, and makers disagree.
- Turin (DF54, DF64 Gen 2, DF83 Gen 2) recommends it. Its troubleshooting guide suggests RDT, “1–2 misted drops per 18–20 g,” for static and clumps in dry air, and also suggests dipping a small spoon handle in water and stirring the beans with it.
- Flair says no for its Royal hand grinder: “Do not use this method with the Royal: It is not necessary and can cause the steel burrs to oxidize.”
Your grinder’s manual or support pages decide. If the maker warns against water, don’t use it, however well it works for others.
How to try it
If your grinder’s maker allows it:
- Weigh the dose first, so the water doesn’t count toward it.
- Add a drop or two. Turin’s amount is one or two misted drops for 18 to 20 g. Even the study’s highest amount, 20 microliters per gram, is only about a third of a gram on an 18 g dose.
- Spread it. The researchers shook the beans in a sealed container; Turin suggests stirring them with a wet spoon handle. Either way, aim for every bean slightly damp, not wet.
- Grind straight away. In the study the water went on just before grinding, so it stayed on the bean surfaces.
- Re-dial espresso. Expect a slower shot, and adjust the grind to bring it back.
What RDT doesn’t do
- It doesn’t replace puck preparation. Clumps also form when fast particles hit each other and the chute, static or not, as Gagné noted in 2021. Breaking them up in the basket is the job of WDT. If you’re fighting channeling, RDT alone may not fix it.
- It doesn’t suit every workflow. In his 2021 low-fines espresso experiments, Gagné recommended against RDT because “humidity may increase clumping even if it eliminates static”; in his preliminary tests, clumping seemed slightly worse with it. The 2024 study, on an EK 43, found that water broke clumps up. Both can be true for different grinders and grind sizes, so watch what happens in your own basket.
- It doesn’t fix the grinder. Water changes how the grounds stick together, not the burrs that cut them. For what the burrs do, see our conical versus flat burr comparison and the grind size micron chart.
Sources
- J. Méndez Harper, C. S. McDonald, E. J. Rheingold, L. C. Wehn, R. E. Bumbaugh, E. J. Cope, L. E. Lindberg, J. Pham, Y.-H. Kim, J. Dufek and C. H. Hendon, “Moisture-controlled triboelectrification during coffee grinding,” Matter 7, no. 1 (2024): 266–283, sections “Electrifying coffee” through “Grinding with supplemental external water and its impact on brewing,” Figures 2–7, and Experimental Procedures: doi.org/10.1016/j.matt.2023.11.005
- yakster (Chris), “Ross Droplet Technique (RDT)-Eliminating Grinder Static,” Home-Barista.com forum, first post, December 18, 2012: home-barista.com
- Jonathan Gagné, “How to Brew Better Coffee with a V60,” Coffee ad Astra, 2018, §§4.2–4.3 (static and dark roasts): coffeeadastra.com
- Jonathan Gagné, “Pulling Low-Fines Espresso Shots,” Coffee ad Astra, April 14, 2021 (RDT and clumping): coffeeadastra.com
- Turin, “Troubleshooting Guide for the Turin DF54, DF64 Gen 2 and DF83 Gen 2,” August 20, 2025, “Fast fixes” and “Clumps / static mess,” checked October 15, 2026: turingrinders.com
- Flair Espresso, “Can I use RDT (Ross Droplet Technique) with the Royal?”, support knowledge base, checked October 15, 2026: support.flairespresso.com