What is RDT? The Ross droplet technique for grinder static

How do you do RDT?

Spray or drip a little water onto the beans, mix, then grind. Kaffeemacher's method is three steps: weigh the coffee, add "2-3 drops of water" with a spray bottle, shake lightly and grind.

Kaffeemacher puts the amount at about 0.1 to 0.2 g of water per 18 g of coffee. Wild Kaffee describes it as "1-2 sprays." Kaffeemacher uses a spray bottle for even distribution.

Start with the smallest amount that stops the static, and add more only if grounds still cling.

Does RDT actually work?

For static, yes. In a 2023 paper in Matter, Joshua Méndez Harper and Christopher Hendon measured the charge on ground coffee. Adding water to whole beans before grinding reduced the charge "by a minimum of 50-60%."

The paper calls the practice "the so-called Ross droplet technique." It found that water at less than 0.05 mL per gram prevented clumps and freed fine particles that would otherwise stick to the grinder. Of four static-control methods the authors tested, only water significantly improved the brew.

The press release on the study adds that water lengthened extraction time and raised brew strength. Shots were also more consistent between batches.

The researchers also explain why some coffees are worse. Residual moisture in the beans was "the primary determiner of charge polarity." Dark roasts charged largely negative, while lighter roasts charged positive.

Does RDT improve extraction or flavor?

Possibly, for some coffees. This part is less settled than the static result.

The Matter paper found "at least 15% higher coffee concentration" for espresso made from darker roasts with water added. Light roasts showed no significant improvement. The authors suspect a similar effect for other percolation methods, such as pour-over, but did not test them.

Kaffeemacher summarizes the wider evidence as "inconclusive": some experiments show more extraction, others show no significant difference. A fair reading is that RDT reliably cuts mess and clumping. Whether it changes the taste of your cup depends on the coffee and the method.

Will RDT rust or damage my grinder?

Small amounts are generally considered safe, but check your manufacturer's advice. Kaffeemacher argues the water evaporates quickly and that roasteries add more water to coffee than RDT does.

Some makers are cautious about water in general. The 1Zpresso J-Max manual says, "Do not clean the grinder with water," and advises keeping the grinder in a dry environment. That is about cleaning, not RDT, but it is a reason to keep the amount small.

If you use RDT, spray the beans, not the grinder. Do not let water pool in the dosing cup.

When is RDT worth doing?

When you see static: grounds sticking to the chute, spraying out of the catch cup, or clinging to the cup walls. It is especially common with single dosing, where every gram that sticks shortens the dose.

Static also raises grind retention, because charged grounds cling inside the grinder. Kaffeemacher says static is most common with single-dose grinders, and that RDT mainly helps people who see grounds sticking to the grinder or chute.

If your grinder produces no visible static with your usual coffee, there is less reason to bother. You can still try it with a dark roast, where the research saw the clearest brew effect.

Does RDT replace an anti-static grinder?

No, but it narrows the gap. Some grinders build static control in. Turin's DF64 Gen 2.7 includes a plasma ion generator, and Fellow advertises "anti-static technology" on the Ode Gen 2.

The Matter paper tested ionizers too. They cut charge, but only water significantly improved the brew. So a grinder with an ionizer may still benefit from a light spray.

When you compare grinders, treat static as one factor among many. The comparison index lists grinder pairs side by side, and the grind converter translates settings between models.