Guide · Grinder accessories
Does RDT work?
RDT, a little water on the beans before grinding, is used to cut static. One controlled test found no extraction gain. What that means and how to try it safely.
Short answer
RDT (the Ross Droplet Technique) means adding a tiny amount of water to whole beans just before grinding. Its purpose is to cut static, so grounds cling and scatter less. Socratic Coffee's controlled test found no significant change in extraction strength (TDS) with RDT, and some sign that shot times got less consistent.[1] Use it if static makes a mess; do not expect a better shot from it.
What RDT is
Grinding builds up static charge. Charged grounds stick to the grinder's chute, cling to the dosing cup and jump onto the counter. Some stay behind in the grinder, which is part of what baristas call retention.
RDT tries to reduce that charge by adding a trace of water to the beans before they go in. In practice that means one light spray from a fine mister, or a spoon handle dipped in water and stirred through the dose. The beans should look barely damp, not wet.
RDT fits naturally with single dosing, where you weigh each dose before grinding, because you can treat the beans in a cup first.
What the evidence says
| Question | What was found | Source |
|---|---|---|
| Does RDT raise extraction strength (TDS)? | No significant difference in Socratic Coffee's test | [1] |
| Does it change shot time? | It may have made shot times less consistent. Spritzing before grinding was the most variable condition (standard deviation 2.35 s, p = 0.0523) | [1] |
| Is there any evidence of a gain? | Socratic notes a 2023 paper by Harper, Mendez and colleagues that reported a possible increase in brew efficiency, in a very small sample (n = 3) | [1] |
| Does it reduce static or retention? | We found no controlled measurement we could verify | None |
A p-value of 0.0523 sits just above the usual 0.05 cut-off for statistical significance,[1] so the shot-time finding is a warning sign rather than a firm result. Even so, it points the wrong way for anyone hoping RDT makes shots more consistent.
The honest summary: RDT has a plausible job, reducing static, and one controlled test that found no extraction benefit. The static benefit itself is reported by users rather than shown in a study we could check.
Why "no extraction gain" is not the whole story
Static mess and extraction are different problems. A shot can extract identically whether or not grounds jumped onto your counter on the way. If RDT keeps your workflow cleaner and your dose weight steadier, that can be worth the step, even with no change in TDS.
What RDT should not be is a fix for uneven shots. For those, look at the puck first: grind, distribution and tamping. A peer-reviewed study found that at very fine grind settings water flows unevenly through the puck and shots become harder to reproduce.[2] RDT does not address that: in Socratic Coffee's test it did not raise TDS, and shot times may have become less consistent.[1] For clumps in the basket, a needle tool does the work; see what is WDT.
Should you use it?
Try RDT if:
- grounds cling to your grinder's chute or cup, or spray onto the counter;
- you weigh beans before grinding and the dose that comes out keeps coming up short;
- you grind into a dosing cup and lose grounds when you pour.
Skip it if:
- you have no static problem to solve;
- your grinder's manual warns against moisture;
- you were hoping for a stronger or more consistent shot. The published test found neither.
How to try it
- Weigh your beans into a small cup.
- Add a very small amount of water: one light mist, or a spoon handle dipped in water and stirred through.
- Shake or stir so the moisture spreads over the beans.
- Grind straight away. Do not store treated beans.
- Keep everything else the same while you compare: the same beans, dose and grind setting, with and without RDT.
How to tell whether it helps you
Static is something you can check at home with the scale you already use:
- Weigh the beans going in and the grounds coming out. If the output keeps coming up short, some of the dose is staying in the grinder or landing on the counter.
- Repeat with and without RDT over several doses at the same grind setting, and compare the average shortfall.
- Look at the chute and the cup. Less clinging and less scatter is the benefit RDT is supposed to deliver.
If the shortfall and the mess do not change, RDT is not solving a problem you have, and you can drop the extra step.
Next
Questions
Does RDT increase extraction?
Not in Socratic Coffee's 2024 test, which found no significant difference in TDS with RDT. Socratic notes that a 2023 paper reported a possible gain in brew efficiency, but in a very small sample (n = 3).[1]
Why do people use RDT, then?
To cut static, so grounds cling less to the grinder and scatter less on the counter. That is its stated purpose; we found no controlled measurement of the static effect that we could verify.
How much water should I use for RDT?
Very little: a single light mist, or a spoon handle dipped in water and stirred through the beans. No source we verified gives an optimal amount, so start small.
Can RDT damage my grinder?
We have no verified evidence either way. Check your grinder's manual, keep water off the grinder itself, and do not use RDT if the manual warns against moisture.
Does RDT make shot times less consistent?
Possibly. Socratic Coffee saw more shot-time variability with RDT; the pre-grind spritz was the most variable condition, a result just short of statistical significance (p = 0.0523).[1]