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What’s a Refractometer?
A refractometer is a device that measures the deflec-
tion of light as it passes through something. You know the whole
‘aim below the fish when throwing a spear’ thing you learnt in
that movie sometime? That’s refraction at work.

When dealing with coffee, there’s a specific rela-
tionship between RI and the Total Dissolved Solids (TDS or
strength) of the coffee, but it varies over temperature.

One way to measure TDS is by dehydrating the brewed
coffee in an oven and weighing what’s left, but it takes ages,
requires incredibly accurate scales and costs way too much.
Luckily, VST’s refractometer measurements are as accurate and
repeatable as a traditional dehydration oven paired with scales
Once you measure how far the light was bent, it’s accurate to +0.0001g. These kinds of accuracies were previously
communicated as ‘refractive index’ or RI. RI is a super useful only possible with laboratory apparatus costing several tens of
number in all kinds of fields like agriculture, pharmaceuticals, thousands of dollars. So don’t complain that it’s too expensive.
photography, gemology, process control, and now coffee (it’s When a brew is stronger, it will bend the light more. If
also how your car’s wipers know if it’s raining. I thought that it’s weaker, it will bend the light less. This is super useful, be-
was magic, but science wins again). cause it means that with a coffee refractometer, we can measure
the strength of a brew with exceptional accuracy, provided the
refractometer employs a sensor with enough resolution, and
the requisite accuracy and precision (ability to repeat the same
measurement). To get meaningful, accurate results from coffee
with a refractometer you need incredible levels of accuracy.
This thing is far from a sundial.
Reminder: when talking about Strength and TDS, I’m
referring to the proportion of the beverage that’s made up of dis-
solved coffee flavour; NOT the perceived intensity. eg. My espresso
is 10%TDS but my filter coffee is 1.5%TDS. (For more on strength
read this link www.baristahustle.com/analyzing-espresso-recipes-
strength/)
Once you know the strength of a coffee, you can also
calculate how much flavour you extracted from the dry coffee
grounds. This is called extraction yield. (for more on extraction
read my first post: www.baristahustle.com/coffee-extraction-
and-how-to-taste-it/). Extraction is even more useful than
just TDS on its own. In combination, TDS and Extraction can
tell you an incredible amount about your coffee, your brewing
equipment, your brewing method(s), your roasting and even
This is VST’s LAB Coffee Refractometer. It’s the only your technique. Herein lies the incredible value of refractom-
refractometer I recommend if you want accurate and consistent eters for coffee.
results. Now that I’ve covered what a refractometer is and how
The RI (Refractive Index) of a brew is measured by it works, it’s important to understand what it can and cannot do.
placing a small sample of coffee on a refractometer’s prism. The There’s a lot of incorrect assumptions and opinions about these
refractometer will shine light at the sample through this prism. devices that I find myself constantly battling. Many baristas are
The light doesn’t pass through the coffee, it only touches the understandably concerned about a device that might measure
boundary layer between the glass and the liquid and bounces their effectiveness or replace their tastebuds. So this should set
back. This phenomenon is called the ‘Total internal reflection’ the record straight.
(you know when you’re underwater the surface looks like a mir- Here are some erroneous statements followed by my
ror? That thing.). A linear detector then receives the light, and usual rebuttals.
sends a signal based on where it gets hit, which can be used to “Extraction has nothing to do with the real taste of
calculate RI. Here’s what it looks like from the side. the coffee.”

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