RDT – why a single drop of water before grinding makes the difference

RDT – why a single drop of water before grinding makes the difference

Dry coffee grounds that pick up a charge and cling to the grinder chute instead of dropping cleanly into the portafilter. A fine veil of dust on the worktop that barely wipes away, because it practically sticks to the metal.

For exactly this small, everyday detail there has been a very simple answer for over twenty years: a tiny drop of water on the beans before the grinder even starts up.

Where the charge comes from

While grinding, bean and burr rub against each other hard enough that electrons shift between the surfaces – an effect called triboelectric charging, the same one that makes a wool jumper crackle in winter. Darker roasts in particular intensify it noticeably, because the beans are drier inside. The result: fine grounds cling to the grinder wall, the chute and the portafilter fork instead of falling completely into the cup or the portafilter.

One or two drops or fine spritzes of water on the beans before grinding raise the surface conductivity just slightly – enough for the charge to dissipate again rather than build up. The beans themselves stay dry; it really is only a wafer-thin film of moisture.

The technique is named after David Ross, who is credited with a 2005 post in the old Alt.Coffee newsgroup. It became widely known above all because Andy Schecter later picked it up again on the Home-Barista forum and spread it further – since then "RDT" has established itself in the coffee scene as shorthand for Ross Droplet Technique.

Our opinion is one of many, but here it is honestly: the research is mixed. One test with a realistically small amount of water found no significant difference in extraction, while another study published in 2024 showed a measurable effect on extraction and shot time with a considerably larger amount of water. At the minimal dose of one to two drops usual in practice, a noticeable advantage in taste cannot reliably be demonstrated so far. What RDT does deliver reliably is less static, a tidier worktop and less ground coffee left hanging in the burr chamber instead of ending up in your cup.

At a glance

	Water-based (RDT)	Electronic
Principle	Film of moisture before grinding	Ion beam neutralises the charge
Effort per grind	1–2 spritzes or drops	Switch the device on, it runs alongside
With us	Normcore RDT spray bottle 30 ml	Acaia Ion Beam Antistatic
Demonstrated benefit	Less static, less retention	Less static, less retention

Who it is worth it for

You notice the difference most on grinders without a large hopper buffer – single dose grinders such as the Eureka Mignon Single Dose 65 from our Eureka coffee grinders collection, for instance, or hand grinders like the models in our Comandante collection. There every gram of retention shows up directly, because no large supply is buffering in the background.

On dosed grinders with a hopper the effect is noticeably smaller, because retention spreads itself across many grinds – and yet plenty of people in our community happily reach for the spray bottle here too, simply because a clean worktop is nicer than a dusty one.

Neither approach is the only right one here: if you have no appetite for spraying before every grind, you reach for the electronic solution such as the Ion Beam Antistatic from our Acaia collection. If you like the hand movement, you stay with the drop. Both lead to the same clean result, in Switzerland as anywhere else.

What else belongs with it

With the grinder itself, it is always worth looking at the bigger picture: our Eureka coffee grinders collection for anyone who wants to grind dosed or by weight, or our Comandante collection for anyone who would rather crank by hand.

For a clean transfer from grinder to portafilter, a dosing funnel fits in nicely too: affordable from the Normcore collection, or in aluminium with an even more precise fit from our Weber Workshops collection.

We deliver the spray bottles and the accessories throughout Switzerland and to Liechtenstein in 2 to 4 working days – usually together with the grinder you pick to go with them.

Frequently asked questions about RDT and static charge

What exactly does RDT mean?

RDT stands for Ross Droplet Technique, named after David Ross, who is credited with the idea in a post from 2005. One or two drops or fine spritzes of water on the beans before grinding reduce the static charge that builds up during grinding.

Does RDT change the taste of my coffee?

The research is mixed: one test with the small amount of water usual in practice found no significant difference in extraction, another study with considerably more water showed a measurable effect. At the typical, minimal RDT dose, a reliable advantage in taste cannot be demonstrated so far. The benefit that is clearly established in any case lies in less static and less retention in the grinder.

How much water do I really need?

Common practice is one to two drops or spritzes per grind – enough to moisten the bean surface minimally, without the beans themselves getting wet. As a rule, no more than that is needed.

Which grinders benefit most from RDT?

The effect is especially noticeable on single dose grinders and hand grinders without a large hopper, because every bit of retention shows up directly there. On dosed grinders with a hopper the difference is smaller, but still noticeable for the sake of a clean worktop.

Can I see the spray bottles before buying?

Yes, in our showrooms in Bern (Gerberngasse 44) and Zurich (Hagenholzstrasse 50b) you can have the accessories shown to you directly and experience how they handle.