Specific Gravity Measurement – What is happening to our Fermentables?
“You can’t manage what you can’t measure.” – Peter Drucker
In brewing beer, we are converting starches to fermentable sugars in the mash, then adding yeast to convert those sugars to alcohol and CO2. Wouldn’t it be nice to know how much fermentable sugar we have at the various stages of the brewing process? Measuring Specific Gravity is how we do it!
Fermentable sugars can be measured as Specific Gravity (SG), or the density in comparison to water. Water at 60F is assigned a specific gravity of 1.00, and our pre-boil wort for a New England IPA style home brew coming out of the mash and lauter might be a specific gravity of 1.066. We will lose some water to evaporation during the Boil process, so the wort we put into the fermenter might increase to 1.077 (this is our Original Gravity (OG)). After the yeast have done their job converting sugars to alcohol and CO2 the Final Gravity (FG) might be something like 1.015 and our Brewfather app tells us we have an ABV (Alcohol By Volume) of 8.1%.
We measure at the end of the mash to verify we have fully converted the grain starches to fermentable sugars–the pre-boil SG. We measure again near the end of the boil to verify we have hit our original gravity (OG). And we will measure again at the end of, and sometimes throughout, the fermentation to verify we hit our final specific gravity target. There are different methods and tools available for taking the measurements, and the different methods and tools work better at different points in the brewing process.
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Hydrometers are one of the most basic and necessary tools in homebrewing. They are inexpensive, and tell you the specific gravity of your wort or beer. To use a hydrometer, you fill the hydrometer jar with the wort or beer cooled to room temperature (temperature affects volume, so you will need to know the actual temperature) and if any foam develops, you’ll need to wait for that to dissipate so you can see the readings. Then you gently drop the hydrometer into the jar and give it a little spin. When the hydrometer stops spinning, you read the SG marking at the point where the hydrometer pokes out of the liquid. If your test sample is exactly 60F, you have your reading. But usually we just try to get it into the low 70’s and then use a temperature conversion chart to calculate the actual SG.
There are few types of scales used on different hydrometers, but this is just a preference thing. You’ll need a Hydrometer Jar ($6) as well to hold the sample. Triple Scale Hydrometer ($13) is probably the most common, and this measures your specific gravity, potential alcohol content, and brix. A Final Gravity Hydrometer ($22) is useful for FG readings, it uses a smaller portion of the scale, but in much larger increments so it’s easier to read – think zoomed in. It’s better for getting a FG reading, but OG readings will probably be out of scale. And my favorite, the digital hydrometer!
Tilt Digital Hydrometer
Want live data to track your fermentation process on your smartphone or computer? If you like “set it and forget it” tools, this one is for you! With the Tilt Hydrometer and Thermometer ($135) you just drop it into your fermenter and you can watch real-time data on the current specific gravity and temperature of your wort.
DANGER: watching beer fermentation is highly addictive; you may find that your screen time dramatically increases as you watch the krausen drop your SG fast, or wait to get those last few 1/1000’s to reach your FG.
Tilt hydrometer lets you instantly read your brew’s specific gravity and temperature on your compatible smartphone or tablet. Reading SG and temps from the app is super simple and pretty much anyone who uses a smartphone will get it immediately. But there are some more advanced things you can do as well via the cloud.
With a little more configuring you can use the Tilt Pi to get your data onto your computer. Most Bluetooth 4.0+ devices will work with the Tilt. You can then optionally log data to the cloud using their free Google Sheets template or other 3rd party cloud platforms. We set up a Bluetooth Raspberry PI 4 device and Case in our brewing areas (mine in the garage, Tim’s in his basement) that picks up the Bluetooth signal and transmits it via WIFI so our pc’s can pick it up. That’s the tricky part and there are instructions on the Tilt website on how to do it. After that, you just connect it to Google Sheets and you’ve got data in a spreadsheet. It also works with the Brewfather app and they produce a pretty sweet looking graph.
Just to level set:
- Using the app – Super Simple, pretty much anyone can do it with minimal effort
- Using the cloud features – a little challenging and requires some additional equipment. If you’re not a techie you’ll want to have your nerdiest friend nearby to help you set it up.
Tilt provides you more data, in an easy to use (and easy to read) tool , to help you make informed decisions that lead to better beer. You can monitor the specific gravity and temperature of your beer while it’s fermenting, without ever having to open your fermenter or pull samples of your beer. This makes brewing more consistent and easier to track.
The Tilt comes in several colors, and while colors are fun, it’s for a more practical purpose. Each color has a different signal, so if you’re using multiple Tilts, you know which is which. I have two conical fermenters, so I have an Orange and a Green Tilt, but they also come in Red, Blue, and Black. We love these things and strongly recommend them after you get your basic brew setup going. There is also a bigger/heavier Pro version available in Red or Blue for $250 if you need more stability, but the regular version has worked great for us.
Quick Results from a Small Sample -- Refractometer
A refractometer is used to measure the density of a solution, in our case how much malt sugar is in solution. While a hydrometer is generally more accurate than a refractometer, it is also VERY temperature sensitive, whereas a refractometer is not temperature sensitive–making it useful to test hot wort. And the sample size for a refractometer is just a few drops of wort. When you are brewing beer, the critical specific gravity (SG) readings are the Original Gravity (OG) going into the fermenter, and the Final Gravity (FG) coming out of the fermenter, so that we can determine if fermentation is complete and calculate what our ABV is. But there are some other significant SG points we would like to know as well.
We want to know if we have converted enough sugar in the mash process. One way to do this is to take an SG reading. We could take a sample, cool it down, and use a hydrometer to take a reading–but this takes time and we lose wort. Similarly, after we have lautered, we want to know what our pre-boil SG is so we can calculate how much water we need to boil off to hit our OG target. Neither of these measurements is used in calculating ABV or determining if the fermentation is complete, so we just need a close approximation, and we would prefer not to have to keep taking samples and cooling them down so we can test them. Using a refractometer, we sample with an eyedropper and have a result in seconds. Problem solved!
It is important to note here that a refractometer does not completely replace the hydrometer–once alcohol starts to be produced, the refractometer will no longer give an accurate reading. You’ll still need your hydrometer or some other device for your FG readings. That said, they are very handy to have for a quick reading!
MoreBeer! caries 3 different models with the higher priced version being easier to read. At the low ent is the Dual Scale Refractometer w/ ATC & LED Light for $40. Slightly better is the Brewing Refractometer – Dual Scale for $60. And best of the bunch is the Vee Gee Deluxe Refractometer w/ ATC for $160.
Best of Both Worlds – The Digital Refractometer
You want accurate measurements even when alcohol is present, but you also want the benefits of small sample size, and easy to read instant results? A digital refractometer may be your solution!
MoreBeer! carries The New EasyDens by Anton Paar for $350. The price may sound a little rich compared to the other devices we’ve covered, but it has some really great functionality. First, you get the benefit of quick reading from a small sample with a maximum temperature of 176F–that’s a huge leap from having to pull a hydrometer sample and cool it down to 70F, wait for the bubbles to dissipate to get a reading, and then calculate the conversion. And if you’re a techie or data-junkie, the New EasyDens is Bluetooth compatible, so you can get the readings on your smartphone and set up profiles to track the data. While I would have snubbed my nose at this a few years ago, with the improvements in technology in homebrewing and the software tools to grab, store, and analyze data, I’m really loving this feature now!
If I got you excited about using the digital refractometer, but $350 is not in your budget, there is the Hanna Digital Refractometer for almost half the price at $189. Hanna is well known for making laboratory equipment and has a pretty solid reputation. It will give you quick and accurate reads at high temperatures with a small sample size. But as you would expect, it’s more complicated to use (more frequent calibrations required) and doesn’t have the Bluetooth capability of the EasyDens. It is waterproof and has a more compact design, so there are some added advantages. For me, if I’m spending over $100 on testing equipment, I want the digital data capture and transfer, whether that’s Bluetooth or WIFI or whatever, but I could see a use-case for the Hanna model.
Read More About the Beer Lab and Data
Merritt Waldron has a great book for home brewers intersted in the testing and analysis of beer called Quality Labs for Small Brewers: Building a Foundation for Great Beer (Paperback $95) that we highly recommend. There is also a Kindle version available for $90.
Keep Checking Back -- More Lab To Come!
- PH Meters
- Microscopes for Brewing
- Disolved Oxygen (DO) Meters
- Brewing Software