specific gravity – Learning to Brew

On the surface both hydrometers and refractometers may seem similar, but there are several differences between these two tools. While both tools measure the specific gravity of your beer, they each do it using a different technique. Both of them are based on the sugar content of the beer, however, they get this data through two very different ways. It is up to the brewer to decide which one they use. I personally own and use both as they both have their own pros and cons. For those just looking to purchase one, there are several factors you should take into consideration before making your final decision. What is a Hydrometer? A hydrometer is a simple tool used to determine your beers specific gravity. They are a fairly inexpensive piece of equipment and you do not need to spend a lot for a reliable one. For example, this Brewer's Elite Hydrometer on Amazon is a high-quality choice and will get the job done, without breaking the bank. A hydrometer looks like a glass thermometer and works by floating it in a cylinder filled with your brew. The height at which the hydrometer floats is dependent on the specific gravity of the beer. Specific gravity is the ratio of the density of the brew compared to water as a standard. So, the lower the density of the beer, the lower the lower the hydrometer will float and the reverse is true for a liquid with a higher density. Hydrometers and labeled with a scale to measure the specific gravity of the liquid. A hydrometer floating in a graduated cylinder filled with beer. How to Calibrate a Hydrometer If you want to guarantee you are getting accurate hydrometer results you should first calibrate your hydrometer. To do this you will need a sample of pure water, either distilled or highly filtered water will work. Place your hydrometer in the water and it should read 1.000. Note: The specific gravity of water changes with temperature. Most hydrometers are made to be used at a certain temperature. The temperature for your hydrometer is most likely printed somewhere on it. If your hydrometer reads 1.000 you are good to go, if not just factor the difference when measuring your wort. For example, if you hydrometer reads 0.999, you would add .001 to your wort reading. Pros and Cons of Hydrometers Pros: Easy to Use - just fill the cylinder with your beer and put the hydrometer in Inexpensive - compared to other pieces of equipment these are relatively inexpensive Can use before and after fermentation - provides accurate results in both situations Cons: Fragile - since they are made out of glass, they are easy to break Large sample size - for the results you need to fill up the cylinder with a fairly large amount of your beer What is a Refractometer? A refractometer is another tool used to measure sugar content of your beer, but in a different way than the hydrometer. Refractometers use the refraction of light to determine the amount of sugar. A refractometer is a little more expensive than a hydrometer but it still won’t break the bank. I personally use this Brix Refractometer from Northern Brewer and I highly recommend it! It even comes with automatic temperature correction for values between 50 – 86 to make sure you are getting the most accurate reading possible. To use a refractometer, you place a small drop of your beer on the prism inside. Then hold it up to the light and look through the eye piece. The light passes through the prism it will reflect at a certain angle depending on the sugar content. Inside there is a scale and you can read off the specific gravity. Some refractometers give the results not in terms of specific gravity but instead they use the Brix scale. The brix scale is the measure of the amount of sugar dissolved into a sample of water at a certain temperature. Measuring the sugar content with a refractometer Calculating Specific Gravity To calculate the specific gravity from the Brix value, you can use the following equation: For those whose math skills are a little rusty you can just use this refractometer calculator. All you need to do is plug in your Brix value and it will automatically convert it to specific gravity. Calibrating a Refractometer Similar to the hydrometers, you first want to calibrate your refractometer so you get accurate results. The calibrating process for refractometers varies for each brand however the general procedure and concept is the same. First you add some distilled water to the refractometer. Then adjust it so the reading is zero. This is usually done by turning a small screw somewhere on the meter. For the exact procedure you need to follow, you should consult the instructions that came with the refractometer. Pros and Cons of Refractometers Pros: Easy to use - place a drop on the prism and hold it up to the light for your results Small sample size - you only need a drop or two of your beer to determine the specific gravity Can use with hot wort - since you only need a drop the liquid cools fast enough so no need to worry about temperature corrections Cons: Higher cost - compared to a hydrometer, a good refractometer can be a little more expensive Doesn't work post fermentation - can’t use it for accurate results unless you have a conversion factor (which in many cases aren't reliable) What to do with Specific Gravity? Both devices measure specific gravity which is great, but what can you do with that information? Well, specific gravity can be used to calculate the alcohol content, or ABV, or your beer. To determine what the ABV is you need two specific gravity measurements. The original gravity measurement should be taken before you start fermentation. The second reading is the final gravity taken after the fermentation is complete. Then use the formula below to calculate your ABV: ABV = (OG - FG) * 131.25 And again, for those who don't want to do the math, you can use this specific gravity to ABV calculator. Conclusion Ideally, you should own both a hydrometer and a refractometer, but if you can only choose one, I would recommend starting out with a hydrometer. They are a little more affordable and give you accurate results for both the original and final specific gravities. The only downside is you will use up a little more beer during testing, which means less beer for you. While refractometers only require a small sample size, the fact that they cannot give accurate final specific gravities is a huge con. In the end it is up to you to weigh the pros and cons of each tool and decide which is best for your brewing setup.