I have used a refractometer since I started making hard cider. To me, it seemed like it would be much easier to use than a standard hydrometer. I have since started using a Tilt Hydrometer, but it is completely different from how you use a standard hydrometer. I will spend a little time at the end of this article to discuss the Tilt, but my main goal is to highlight how a refractometer works and how you use it. I consistently have people asking me or making comments about their refractometer not working properly. This is mostly because they don’t realize a refractometer has some unique requirements.
Like a hydrometer, a refractometer is used to measure the amount of suspended solids in your apple juice and hard cider. These suspended solids represents the amount of sugar that your juice has and ultimately, the amount of alcohol you could create in your hard cider once fermentation is complete. These suspended solids are not just fermentable sugars. There are other solids, but for home hard cider makers and even many commercial cideries, assuming they are is close enough that you don’t need to worry about the other compounds. While a hydrometer and a refractometer both measure suspended solids or sugar, they do it in completely different ways.
A hydrometer relies on density of the liquids and solids that are mixed together in your juice or cider and floats to indicate this density. The density of pure water is the reference point. Ethanol, which is the main alcohol you make when fermenting apple juice into hard cider, is not as dense as water. It would float on top of water if separated. Sugar is denser than water and would sink in water if it couldn’t dissolve. Apple juice contains mostly fructose, sucrose, and glucose sugars but has some others as well at lower levels. These sugars form the majority of solids that are dissolved and suspended in the juice. These solids are key to what turns the water into juice and ultimately, hard cider. They make the juice denser than water. During the fermentation process, the yeast convert the sugar into alcohol, the alcohol mixes with the juice and makes it less dense.
Hard cider makers usually talk about sugar using a specific gravity scale. Specific gravity is the ratio of the density of the fluid divided by the density of water. For pure water, it is defined as 1.000. For pure ethanol, it would be 0.794. A hard cider that is 6% ABV and completely dry (no dissolved sugars remaining), should have a specific gravity around 0.988 at a specific temperature. One gallon of hard cider at 6% ABV would contain .06 gallons of ethanol and 0.94 gallons of water. The 0.06 gallons of lower density ethanol reduces the total density by about 0.012. For reference, most apple juices are between 1.045-1.070, which reflects the weight of sugar suspended in the juice. That is 122 to 194.4 grams of sugar per liter of liquid. But, I have found apples that have even higher levels of sugar. I published the following table in my post on Apples of Unknown Origin. Note, you may also hear people talk about Brix, which is another way to define the amount of sugar in your juice or cider. You can easily convert back and forth between Brix and specific gravity.
Refractometers also rely on the density of the liquid it is measuring. Just like a hydrometer, it will use that density to estimate the amount of suspended solids the liquid has. However, instead it defining the suspended solids as a hydrometer does by how high it floats in the liquid, a refractometer measures them based on how the suspended solids refract or bend light passing through it. You add a few drops to the lens of the refractometer, close the cover and look through the eye piece at a good light source. This works great for estimating sugar in juice or apples because you only need a few drops of liquid. It doesn’t work as well once alcohol is present. The alcohol refracts the light differently.
This means when you check your hard cider once alcohol has started to be produced, the refractometer reading is not accurate or true. It will require an adjustment calculation. That may sound daunting but it’s not that daunting in the world of the internet. Note that beer brewers also have to make an adjustment when measuring their wort. This is because beer wort often contains significant amounts of complex malt sugars. I think this is why many people believe a refractometer isn’t a good choice. Many people often brewed beer before trying hard cider as well as many beer ideas and information is often wrongfully applied to hard cider making. Because of the multiple adjustments needed for a refractometer in making beer, I believe many people also apply that to hard cider and therefore, use hydrometers.
Again, I still prefer a refractometer because it is easy to use and with hard cider, you only need to make the adjustment calculation on the final measurements. However, with the resources available on the internet, this adjustment calculation can be completed quickly through various online calculators. I use one from Advantage Beer (Hickory Brewer) but there are several options. Here are a few. You might want to bookmark the one you like best.
Basically, knowing your starting specific gravity reading and your current reading using the refractometer, the calculator adjusts for the alcohol to give you an estimated current specific gravity reading. This allows you to take the difference between the staring and the current to calculate your alcohol by volume (ABV), just like you normally would (Starting Gravity – Finished Gravity * 131.2 = ABV).
I did promise to talk about the Tilt Hydrometer. This device is a hydrometer and it measures density by how it floats. However, instead of floating up, it’s the tilt or angle of how it is floating that defines the specific gravity. The biggest difference is that you actually place and leave it in your fermentation container and you read the specific gravity over WiFi using your phone or tablet. You can select various units and it also gives you temperature. You can log to a device that you keep open or like me, you can record the information a couple times a day and download it whenever you want. It’s easy to use. It doesn’t require you to disturb your fermentation vessel. And, it takes the guess work out of reading your device. The one drawback, it’s not cheap. In the US, they run about $130-140 per device. You can use them as a standard hydrometer or refractometer by measuring all your batches at the start and end. But, if you want to take specific actions when a hard cider reaches a specific gravity reading, you might need multiple ones.
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