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Last Updated on December 3, 2023
The VST LAB Coffee III is the gold standard for coffee refractometers among coffee professionals. It is accurate. It is consistent. It is durable.
It is not cheap.
Things To Consider Before Buying a Coffee Refractometer
Coffee refractometers are scientific tools that allow coffee professionals and enthusiasts to modify their brewing processes based on data. A refractometer gives you information about the quality of your coffee—in particular, it measures the total dissolved solids (TDS) in your brew.
This is basically the percentage of your cup that isn’t water.
When you know this, and you know how much coffee and water you started with, then you know how efficiently your brewing process has extracted from the coffee beans.
Here are a few factors you should take into account when shopping for a coffee refractometer.
Quality and Durability
A refractometer only needs to be consistent with itself. You don’t have to worry if one refractometer gives a slightly different reading from another. You don’t have to decide which one to believe. All that matters is that the same refractometer gives consistent readings for the same coffee, over and over again.
The reason for this is that you will be using the refractometer to compare cups of coffee, so you can improve them. The absolute numbers it gives you are far less important than the change in those numbers from cup to cup, as you tweak your brewing method.
When considering a model’s performance, look through reviews posted by previous customers to get a better understanding of how it performs. Do they have grievances that you find unacceptable? Doing a little bit of this homework increases your chances of finding the product that will suit you.
Ease of Use
It certainly helps to have a refractometer that’s simple to operate. You should consider the product’s design for this. An easy and user-friendly interface is an asset.
Make sure you have a solid idea of how these coffee tools function if this will be your first time using one. A good refractometer should come with an easy-to-understand user manual.
Portability
Most of these devices are fairly portable, but they can vary in size. Think about how easy it will be to transport if you plan to use it in multiple locations, like your home and a coffee shop.
The DiFluid refractometer, the least expensive of the devices we’ll be reviewing, is quite small and fit for transport:
Cost
High-quality goods cost more than average goods. And if we’re being honest, most coffee professionals will tell you that an inexpensive coffee refractometer is a waste of time.
“This isn’t a tool you can compromise on,” said one Reddit user. “You either buy an Atago, a VST, or nothing. Anything less is going to do more harm than good to your understanding of extraction.”
Refractometers are, by their very nature, for serious fine-tuning. So having one that kind of works isn’t much use.
Having said that, we think the more affordable model that we’ll be covering is acceptable for the non-professional.
Types of Coffee Refractometers
You have a variety of choices. However, most coffee refractometers are either digital or optical.
Digital Coffee Refractometers
The digital coffee refractometer, like the Atago PAL, is the most popular tool baristas use to measure TDS. In general, digital coffee refractometers are capable of more than just TDS measurement. For instance, most models can also determine how much sugar is in your coffee.
Optical Coffee Refractometers
Optical coffee refractometers are usually smaller than their digital counterparts (think about a larger-than-normal thermometer) and tend to be cheaper.
An optical coffee refractometer doesn’t measure TDS—it only measures sugar content. This is expressed in a measure known as Brix degrees. A crude way to convert Brix measurements to TDS is to multiply them by 0.85.
Optical refractometers have their limits but can often manage a wider range of sugar concentrations than a digital refractometer.
Best Refractometers for Coffee
You should familiarize yourself with a few products now that you know what to consider before buying a coffee refractometer. I recommend choosing from this list of three.
1. VST LAB Coffee III
I covered the 2022 World AeroPress Championship when it was held in Vancouver, and the vast majority of competitors there used a coffee refractometer.
The vast majority of those used the VST LAB Coffee III.
With few exceptions, these were serious competitors who knew their specialty coffee, and it was clear that the VST was their choice.
The device is about the size of your open hand, or the little credit-card device they hand you at a restaurant after your meal. It’s encased in durable rubber armor, much like those bright-orange external hard drives. It seems pretty much indestructible.
They are professional devices. They are designed to last.
The sample well is a concave, stainless steel circle on the surface of the device that resembles the business end of the Death Star. You use a syringe to place a few drops of coffee in the middle of that well, then close the cover and press Go. Within two seconds, you get a TDS reading on the display.
You need to do a few things to ensure accurate readings from the VST.
Keep the Lens Clean
Alcohol wipes are the best way to ensure that the lens through which the device reads your coffee is kept free of smudges and debris. It’s best to wipe between every reading.
Setting to Zero
Distilled water is ideal for calibrating the machine. Use a few drops of it, as you would coffee, and establish that as your “zero” reading before moving on to measuring coffee.
You may have to do this occasionally, although you can count on the VST to maintain its zero setting much longer than lesser devices.
Mind the Temperature
Room temperature is ideal for taking your readings. Again, the VST will do better than most competitors when liquids stray from room temperature, and its technology includes the ability to compensate for higher or lower temperatures. Still, the optimal temperature range for the device is between 15° and 40° C. Outside of this range, you may not get accurate readings.
You might be thinking, “Wait, I’ve just brewed hot coffee and now I have to wait for it to come down to room temperature?” But you’d be surprised how quickly this happens. Once you’ve drawn a small amount of coffee into your syringe, you can squirt it into a separate cup and it should cool down within 30 seconds. Then you’re ready to transfer it to the VST.
VST recommends that you filter espresso before attempting to take readings, and provides special filters that attach to the syringe. This step is unnecessary when measuring pour-over or AeroPress coffee that has already passed through a paper filter.
The device is powered by two AAA batteries, which should last you more than 4,000 readings before they need to be replaced.
2. Atago PAL-Coffee
One of the simplest instruments to use with coffee is the Atago PAL-Coffee. You can measure coffee samples directly into the primed and cleaned prism, then click the button to obtain the TDS reading. It also reads Brix. Readings are reliable and simple to record.
Specifications for the Atago PAL-Coffee include:
- Temperature accuracy of +/- 1° C and accuracy of +/- 0.15% for TDS
- Measuring range of 22% for TDS
- Resolution of 0.01%
- Automatic temperature adjustments
Like the VST, the Atago runs on two AAA batteries.
It falls short of the VST in a couple of areas: consistency, and need for recalibration.
You’ll have a hard time finding anyone who questions the VST’s consistency, but the Atago may have you doing the occasional double-take when a reading seems off.
Sometimes this is a calibration issue. You may have to set the zero mark again using distilled water. For whatever reason, this seems to be necessary more often with the Atago than with the VST.
3. DiFluid Coffee TDS Refractometer
Finally we have the DiFluid refractometer, which is a relatively new entry to the market and comes in at a lower price point than the other two.
It wouldn’t be accurate to describe the DeFluid as a competitor to the two coffee refractometers described above. It isn’t in the same league. However, it could serve a purpose for the home brewer who wants a little feedback on their brew.
Remember, none of these devices actually improve the taste of your coffee. That part is up to you. So it’s understandable if you don’t want to fork out hundreds of dollars.
The DiFluid TDS refractometer (not to be confused with their Brix refractometer) is about one-tenth the size of the VST, smaller than an Apple TV remote.
You can re-charge it using a USB-C cable, and it connects to an app via Bluetooth. The app will show you whether your coffee falls within the Specialty Coffee Association-recommended TDS readings for certain brewing methods. However, you don’t have to use the app—the device also shows the readings on a display. One cool thing about the app is that it can store your results for future reference.
The drawbacks of this device are:
- It performs quite poorly when coffee is not at room temperature.
- You need to reset it to zero quite frequently.
So, you’ll need to take care to bring your sample down to room temperature first if you want to get an accurate reading.
The device also slips out of calibration quite easily, and if you want to be confident in the results, you should probably set it to zero before each use.
That said, many home baristas credit the DiFluid for helping them dial in their espresso.
The fact that you can take it anywhere very easily is a bonus.
As for specs, the key numbers are:
- Range of 0%–26%
- Precision of +/- 0.03%
- Rechargeable lithium polymer battery, about 30 days use per charge
What Is a Coffee Refractometer?
A refractometer is a tool that calculates the amount of light that is refracted as it passes through a liquid. In other words, it calculates the amount of light bending at the point where the atmosphere and liquid meet. This refraction measurement can give more detailed information about the liquid under consideration.
A coffee refractometer measures light refraction through coffee to calculate the percentage of total dissolved solids (TDS) and/or the sugar content (Brix). With some simple math, you can then calculate the extraction yield—essentially, how much of the coffee within your coffee beans that you managed to pull into your cup.
Why Use a Coffee Refractometer?
You can use the refractometer for coffee to determine the TDS and optimal extraction yield of a superb cup of coffee. This way, you’ll have an easier time achieving it again during future brews.
However, if your coffee tastes odd, charting TDS and extraction yield might help you determine what adjustments you need to make. It might be difficult to tell from taste alone whether your coffee is excessively strong or weak, under-extracted or over-extracted.
How Coffee Strength Is Measured
The roasted and ground coffee components that dissolve in your brewing water make up the total dissolved solids (TDS) in your morning beverage. They are essentially the coffee itself.
TDS significantly influences coffee strength. Therefore, you can work your way toward brewing better coffee by adjusting your recipe to get a different TDS value and extraction yield.
TDS is typically measured as a percentage.
Most coffee refractometers give you the TDS percentage value immediately. To calculate your coffee extraction yield from that, use the following formula:
Brewed Coffee (g) x TDS (%) / Dose (g) = Extraction Yield %
Ideal TDS for Each Brewing Method
- French Press: 1.4%–1.7% TDS
- Espresso: 8%–12% TDS
- Pour-Over: 1.2%–1.5% TDS
- AeroPress: 1.4%–1.7% TDS
How To Use a Coffee Refractometer
Step 1: Calibrate
Calibrate the refractometer with distilled water to start, then set it to zero. Remember that with a quality refractometer you won’t have to do this before each reading, but you will likely have to do it more frequently as you move down in price point.
Step 2: Prepare Coffee
After resetting the refractometer, thoroughly stir your coffee before taking a reading (coffee stratifies very quickly).
Step 3: Add Coffee
Take a tiny coffee sample using a pipette. The coffee sample should be pipetted onto the refractometer glass until it completely covers the surface.
Step 4: Take Reading
Wait for a few seconds, typically 15 to 20. Depending on your model, close the refractometer’s lid and press “Go” or “Run.” Then wait until the gadget displays the same TDS reading three times.
Step 5: Clean Up
The glass can be dried using a tissue. Then use some alcohol to clean the refractometer’s glass.
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Using TDS To Improve Your Coffee
You can alter the coffee brewing procedure to raise the TDS value, increase the extraction yield figures, and obtain coffee of greater flavor and quality. Here are some pointers for improving coffee and raising TDS values on your refractometer:
Adjust Coffee-to-Water Ratio
This is the simplest way to make your coffee stronger and, as a result, raise your TDS number. If you use more coffee in comparison to the amount of water you previously used, your TDS result will be higher. The opposite will happen if you use a higher proportion of water.
The more surface area your coffee has after being ground, the more water comes into contact with the bean and the more extraction occurs.
Imagine cutting a coffee bean in half. Instead of the just outer surface area, you now have the entire outer surface area plus two extra surfaces on each side of where the bean was cleaved. It’s the same bean, but with more surface area.
Now imagine what happens as you keep grinding finer. More and more surface area.
Grinding finer is a sure way to increase extraction and hence, TDS.
Adjust Brewing Temperature
Increased thermal energy during the coffee brewing process increases extraction speed and effectiveness. The result is coffee with more strength and higher TDS.
Adjust Agitation
Coffee grounds will move about in the water, and the coffee bed will become tumultuous if you stir while your coffee is brewing. The number of coffee grounds exposed to the hot water will vary depending on how quickly you stir, which results in better extraction and stronger coffee. To put it another way, stronger coffee results from more turbulence.
Brewing a specialty coffee that scores above 80 points on a 100-point scale can be challenging and requires some consistency in order to fine-tune and get the most out of your brew. TDS can be a crucial tool for understanding how your coffee is being extracted.
Maybe it’s too technical for the ordinary home barista, but if you are keen to learn how the changes you make in your process affect the coffee in your cup, a coffee refractometer can give you a lot of information. If you aren’t ready for the VST or the Atago, perhaps the DiFluid is enough to start you on the path.
Thumbnail image: © AWasteOfCoffee
