Understanding Specific gravity for home-distilling

Taking the time to learn how to use specific gravity will improve your alcohol distilling skills as much as anything else you can learn and more than a lot of things you can learn. It is possible to make moonshine at home without taking any specific gravity readings, but you'll not only have no idea how much alcohol is in a wash, you'll have no idea if you can do better on the next batch.


Once you've learned how to take SG readings, when to take them, and what they mean you will be able to know or approximate a few different things. You'll be able to know if your fermentation is finished or if it got “stuck”. You'll be able to calculate the abv% of your wash and with that knowledge you can approximate the amount of alcohol you can extract during distillation. Armed with these data points and by keeping notes of each batch you will be able to improve on your subsequent batches and hone your craft.

What is Specific Gravity?

Specific gravity is the ratio of the density of a substance to the density of a standard. In the case of home distilling we are measuring the density of a solution of sugar in water to the density of plain water.

How to measure Specific Gravity

The most common method of taking an SG reading is using a saccharometer, more commonly known as a triple scale hydrometer. It's called a triple scale hydrometer because it has three scale on it's spine. Specific gravity, Potential alcohol, the brix standard. 

Triple scale hydrometer

It is a common misconception that the triple scale hydrometer measures alcohol in the wash. That isn't accurate. It is actually measuring the presence of a solution in water. The triple scale hydrometer is carefully calibrated to measure the amount of sugar in the solution. For the remainder of this article, when I use the term hydrometer I am talking about this type of hydrometer.

You can also use a refractometer, but only before fermentation has begun because the presence of alcohol skews the readings of the refractometer. For this reason, I recommend getting a glass hydrometer and don.t worry about the refractometer. I think this is definitely one of those situations where analog is better than digital. I'm sure there is a situation where the refractometer is a better choice, but I haven't run across it.

Measuring SG with a triple scale hydrometer is as simple as putting the hydrometer in the wash/wort. It will float like a fishing bobber and the scale on the hydrometer can be read below the meniscus where the surface of your wash touches the hydrometer. The meniscus is the part of the liquid that curves up to meet the hydrometer. Your measurement is at the level of the surface.

Why Check Specific Gravity

Taking gravity readings is the only sure way to know that a wash/mash has stopped fermenting. This is the simplest function of the hydrometer and on it's own makes this a very useful tool and worth the money, but there is more you can do with the hydrometer.

If you take a SG reading before you pitch your yeast and take another reading when the ferment is complete you can calculate the ABV% of or mash/wash.

There is more still. Once you have calculated the ABV% of your wash you can get an idea of how much total volume of distillate you can end up with. There are many calculators on line that are just plug and play. Enter the data it asks for and that's it. I will say that many of these calculators assume certain data points because there are so many variables that affect final output. I would use these calculators only as a ballpark guess.

Determine if the Ferment is Finished

Since the hydrometer is measuring the sugar solution in the wash. You can easily check for completion by checking SG. If the SG is at or below 0 on the SG scale then then the yeast have consumed all the sugar. If the SG is still several points then the yeast have either not finished or they have stalled. If you get the same reading above 0 for a few days in a row, then the yeast have stopped consuming sugar but there is still a solution of sugar in the wash. There could be a number of reasons for this and we can cover those in another article.

What is the potential ABV%

With the hydrometer you can easily check what the potential ABV% will be before you start the fermentation. The potential ABV% scale is marked in percentages along the spine. Typically, hydrometers range from 0% to 20%. This only means that there is enough sugar in solution to produce that much alcohol if the yeasties do there job completely.

Simply drop your hydrometer in a sample of the wash before you pitch the yeast. Easy peezy!

How to calculate final ABV%

To calculate your final ABV% you will have to take 2 different readings then do some simple math.

The first reading is the same as the one to check your potential ABV%. This reading is called the Original Gravity or OG. The second is the gravity reading when the fermentation is done. This is called the Final Gravity or FG.

Now all you have to do is subtract the OG from the FG and multiply 131.25.

(OG-FG)*131.25

It's really that easy. All this done by a single tool that costs less than $20.

Conclusion

For all the things you can do with this simple analog tool and because without it you are stabbing at the dark when you are trying to improve from one batch to the next, I think this is a must have tool and understanding specific gravity is a must learn skill.

Triple Scale Hydrometers start at around $15. Click here to check current prices on Amazon