Hydrometer VS Refractometer: Measuring Your Wort
Although it seems like a very simple, very necessary step when fermenting your favorite homebrew, wine, mead or cider, what are you really measuring when you measure your favorite libation? This week we’ll go a little more in depth as to how to use your hydrometer and refractometer, as well as some of the math behind specific gravity, brix and Plato and how to easily move between them.
What Is Specific Gravity?
Simply put, specific gravity is just a measurement of density of solution, which is a measurement of mass per unit volume, specifically the ratio of the density of a specific substance to that of a standard. It is a common measurement tool in petroleum products and in the medical and veterinary fields as well as well as in brewing, wine and cidermaking. In our case as homebrewers, our substance is sugars (up to carbohydrates) to the standard density of water, which is 1.000. So when we use our hydrometer to take a reading, we are measuring how much sugar is in there by reading anything over 1.000 as sugar, which will make your hydrometer float.
This is why we always take a reading both of wort going into the fermenter, and beer coming out of the fermenter. We express these measurements in ‘Gravity Points’, and these measurements are what we use to determine what our alcohol by volume and apparent attenuation are. While I usually refer to an online calculator to save myself from doing any actual math, the equation for ABV is actually pretty simple:
ABV = (OG - FG) x 131.25
Original Gravity (OG) is what you measure going into the fermenter, and Final Gravity (FG) is what you measure coming out of the fermenter. This simple measurement is widely used by homebrewers, and while it’s not as accurate as measuring your alcohol by weight, using alcohol by volume is more familiar and relative to what we understand in general because it’s the same measurement that we see daily in our commercial beer.
Why Is A Gravity Reading Important At All?
There are many homebrewers out there who tell us that they don’t measure their gravities at all, and that they’re happy with the finished product no matter what the alcohol is. Let me say that this is AWESOME, especially if you’re brewing pre-made Beer Recipe Kits where the math has already been done, and you don’t want your hobby to get super technical. More often than not, if you’re a more casual brewer, you can skip the hydrometer measurement and enjoy your brews.
If you do want to be more technical, troubleshoot fermentation hiccups, perform quality control and plan and develop your own recipes, the alcoholic content and gravity measurements of your beer are an integral part of all of these.
One issue that we see repeatedly is that a fermentation has gone awry. Either the yeast didn’t start, there were no visible signs of fermentation, or the fermentation has stalled. The only true and scientific method to determine if there is a problem is by checking the gravity and seeing if it has changed, because that’s the only way that you truly know that something is going on, or that things are going as they should.
Many times fermentation is active and happy, but you can’t see that through the bucket and if there’s no action in the airlock, it’s hard to know what’s happening. More often than not when this happens, the CO2 that the yeast have created has found an easier way out of the fermenter, and even though it’s plugging away at your sugar, you may not be able to tell. If your beer stalls, the only way to know that you’ve fixed the problem is by taking multiple readings and monitoring it.
When you’re developing a beer recipe, gravity is always important because alcohol content, residual sugar and apparent attenuation (how much of the sugar the yeast actually ate) are all going to play big parts in the overall balance of your finished beer. All of these factors come back to determining the specific gravity.
If Specific Gravity Is So Important, What Are Brix and Plato?
The brix scale is a similar type of measurement used primarily in the wine industry, although once you’re used to it, it’s just as easy to use in homebrewing or any other type of fermentation. Degrees brix represent the amount of sugar in solution measured as percentage of solution by mass. 1 degree brix is 1 gram of sucrose in 100 grams of solution. Think of using brix like using the metric system, but for beer and wine!
To switch between brix and specific gravity on the fly, the easiest way is to just multiply the brix measurement by 4, which will give you the gravity points. For example:
18 x 4 = 72, so SG is 1.072
It works the other way too, just divide by 4. So 1.072 would be:
72 / 4 = 18, so 1.072 is 18 brix
This is reasonably accurate up to about 1.080, or 20 brix, but from there it’s not quite as linear, so using a Conversion Chart will give you a better number.
Plato is the same type of measurement as brix, but the measurement is done by weight instead of measurement of refractive index, even though it’s still expressed as percentage of solution. The math and conversion is still the same, but commercial brewers will use a pycnometer to measure the weight instead, so final gravities will still be accurately expressed in the same scale. This is why you always see final gravities on craft beers expressed as 3° Plato (1.012) instead of the OG being in Plato and the FG expressed in Specific Gravity.
Plato is a very easy scale to work in, and the rough rule of thumb is that 1° Plato is about 0.4% ABV. This means that when you’re calculating a recipe and you know you want to be about 7% ABV, it’s very easy to do the math on the fly by dividing 7 by 0.4 and have an idea of how much base malt to add to get to a desired Plato. Of course, attenuation and yeast strain, as well as mash temps, etc will play a big part in that, but it’s an easy and quick rule of thumb for commercial brewers during recipe development.
How Do I Use A Hydrometer?
While it’s an extra step, we now know why we’d want to use a hydrometer, but how to do it? It’s VERY quick and easy! First and foremost, standard hydrometers will usually have three scales of measurement on them. The only one you want is Specific Gravity. The other two are not what we’ll use to calculate our numbers, although balling is essentially the same measurement scale as Brix and Plato, just more antiquated and made at a different temperature standard (17.5 ° C instead of 20° C). The potential percent alcohol scale is an easy scale at a glance, but there are too many variables for it to be a reasonable scale for us to use as home brewers.
For all hydrometer measurements, remember to look on your hydrometer to see what temperature it’s calibrated to. Depending on the manufacturer, it will be either 60° F or 68° F. Make sure that you take a temperature of your sample and account for it on your reading. The temperature adjustments are on the sheet of paper that comes with your hydrometer.
Other than that simple temperature calibration, simply use a 10” Hydrometer Cylinder, or something of similar size, fill it with wort, and slowly put your hydrometer in it! At the point where the hydrometer floats, mark down the reading (I always measure at the meniscus for consistency) on the specific gravity scale. That’s really it!
High precision hydrometers are an upgrade from the standard hydrometers, and they definitely make a difference in your measurements. Precision Lab Grade Hydrometers are specific to Starting Gravity and Finishing Gravity, and conveniently come in Specific Gravity for home brew and Brix for wine making. The Precision Lab Grade Hydrometer - Starting Gravity only has values from 1.060 to 1.130, and is ideal for any fermentation. The Brix Version of this hydrometer goes from 9 to 21 Brix, and has an integrated thermometer so you can get an exact measurement, temperature corrected, instantly! For starting gravities, these tools are a must have, especially for brewers that want insanely accurate precision in their readings.
The Lab Grade Precision Hydrometer - Finishing Gravity has values from .980 to 1.020, making it an easy, essential tool for measuring any fermentation, including fruit based fermentations that will fall below 1.000. The Brix Version of this hydrometer has values of 0 to 12, and also has an integrated thermometer for easy measurement readings. These are usually easiest to pick up as a set, in either Specific Gravity or Brix, but be careful! Because these are all such high precision tools, they are even more fragile than regular hydrometers, if you can imagine that!
Why Use A Refractometer?
Specific Gravity is the basis for measuring density, but if you’re doing 1 gallon batches or trying to test your grapes to see if they’re ready to crush up and turn into grape juice, you may not always have 200 mL of available juice. This is where specific gravity and brix refractometers come in. Using the refractive index to determine the sugar content is very easy, and only takes a drop or two of whatever you’re trying to measure.
The concept is ridiculously complicated and surprising simple at the same time. Simply put, a light source passes through the sample and the refractometer measuring the refractive level in that sample and automatically comparing it to the standard of water. The amount of refraction is translated, and you have your density! Most types of refractometers, especially the more common handheld analog refractometers widely available for home brew, now have both scales built in, which makes it very easy to move in between them, and always purchase a refractometer with ATC, or Automatic Temperature Compensation, because temperature changes can happen quickly with such a small amount of liquid, and it’s a pain in ass to get an accurate temp reading without it being built into the refracto!
The one tricky part about a refractometer is using it to make measurements after fermentation. Ethanol refracts light differently, and therefore throws off the measurement. To get an accurate reading, you’ll need to either use a foot long equation to calculate the adjusted refraction, or use one of the handy online calculators and skip all of the math! I’ll usually just google ‘Refractometer Adjust’ and use the first one that comes up. For those interested, check out Sean Terrils stellar article with the equation and some solid scientific experimentation he’s done on taking your FG with the refracto.
There are a few types of refractos available , but one of the more accurate and convenient, since you don't have to hold it at the perfect, critical angle to read it, are abbe refractometers, or benchtop refractos. These higher grade digital versions automatically project an ideal amount of the perfect light, from their own integrated light source, and digitally read the sample. They are more accurate, and also more expensive, but you can't beat the precision, especially if you are a winemaker and need super accurate samples to ensure you're doing your harvest at the right time!
For us, we use the refractometer throughout the entire brewing process, especially when we’re brewing our monthly recipe kits. This helps us ensure that numbers are on point all the way through brew day and helps us keep our process consistent and deliberate.
That being said, we generally skip the math when it comes to the Final Gravity measurement and just use the hydrometer. This gives us a quick reading, as well as gives us the opportunity to perform stringent ‘Quality Assurance’ tests on our finished, pre-carbed beer! Quality Control is a very important part!
Thank you all for reading, and we hope that this helps you understand better the relevant measurement scales in the beer and wine and how to use them to their full effect for your beer! Let us know your process in the comments, and post any questions you have!
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We added in some more information about Lab Grade Precision Hydrometers, due to popular demand, as well as some updated information about bench top refractometers and why you would use them instead of a standard, handheld analog refractometer, as well as our Hydrometer video on BrewChatter TV! Brew On!