How does one use a hand held digital refractometer to read coffee extraction? I found a used one for nothing and it turns on but I have no idea what the readings mean or how to apply it to coffee.

Which model did you get?

There are videos on Youtube and blog posts that can fully explain coffee refractometer use.

To summarize you zero the device with distilled water, then clean the lens, place a coffee sample onto the refractometer's lens and press the button to read. It will then read the TDS of the coffee. You can then use that to calculate the extraction yield using a pen and paper, a free app or a paid app, or a spreadsheet.

I have the Atago refractometer.

Optimal Coffee Extraction is a free (or inexpensive, can't remember) iOS app, not sure if there is an android.
CoffeeTools is a paid app for all platforms, including mobile and desktop
It is just a formula you can use in a spreadsheet if you don't want to use the apps.

-David

The basic formula is quite simple: where
EY = extraction yield
TDS = total dissolved solids (refractometer reading)*
liquid = weight (g) of brewed coffee or espresso
grinds = weight (g) of ground coffee

* Most handheld refractometers give readings in Brix. To convert Brix to TDS, multiply by 0.85.

E.g., 18g dose of ground coffee, 36g espresso (1:2 brew ratio), refractometer Brix reading of 11.0 =>
EY = 11.0 * 0.85 * 36 / 18 = 18.7%

Use = measuring extraction of brewed coffee (against the coffee brewing control chart)

Usefulness = validating what you taste or how you've brewed

Taste always comes first and is more useful, but using a refractometer can help you understand why you're tasting what you taste.

I played with a pharmacy Brix Refractometer a good while ago, and I am pleased to see that discussing the 'secret formula' appears to be allowed now!
For me it added little, unless you need confirmation or support for your olfactorial sensors (exactly as Travis wrote).

Interesting topic that actually motivated me to buy a refractometer.
My espresso shots came out at EY of 20.6% but the Hario brew, that I'm failing to dial in, was 81.6%.

I did 18g of coffee grinds to 272 coffee extract.

I'm apparently hopeless in differentiating sour from bitter. So which direction should I go with the Hario? More fine, more coarse grind size?

I guess my question is what to change when EY is higher and what to change when it is lower than 20%?

81.6% or 18.6?

My Brix was 6.0.

I actually did 17g of beans.

So the formula I used was:
EY=6.0*0.85*272/17= 81.6%

baldheadracing wrote:The 0.85 conversion is not the 'secret formula.' The 0.85 factor is from Bunn IIRC. There are two 'secret formula's' that I know of. One formula uses information from the VST patent - so it isn't secret, although a bit of algebra is needed. The other 'secret formula' converts the Brix measured by a specific model of digital refractometer to the TDS values reported by the VST refractometer.

That seems impossibly high for a (I assume) V60. My V60 this afternoon was 1.41% TDS. That's about 1.19 Brix using the 0.85 factor.

Yes, it was a V60. I will re-measure tomorrow.

If you measured 1.41 in TDS, shouldn't your Brix then be 1.66?
TDS = Brix% *0.85
Brix%=TDS/0.85 = 1.41/0.85 = 1.66

Do I understand the equation correctly?

Assuming I didn't mess up the measurement, how to adjust Hario variables, especially grind size, if EY is >22% and how if EY is <18%?