Post Tue Dec 02, 2003 6:34 pm



OK so you have a hydrometer that you bought or came with you new brewing kit. What next? What is it for? How do I use it? Is it accurate?

The hydrometer is based on Archimedes' principle: "a body immersed in a fluid is subject to an upward force proportional to the mass of the fluid it displaces". End of history lesson. What that means is the “thicker” the fluid the higher your hydrometer will float. You can use this to check how well your brew came out before fermentation, known as Original Gravity or OG. How your brew is coming along by how much it has dropped and when it has finished and is ready to bottle or keg known as you final gravity or FG. By using some math or a brewing software program you also can figure out the alcohol by weight ABW or alcohol by volume ABV that you see on commercial beers.

The first thing you will want to do is check the accuracy of your new hydrometer. Don’t use the little plastic tube it came in to test. Buy a hydrometer flask. The plastic tube is too small and a plastic flask is cheap. Specific gravity (SG) should be measured at 60degF but don’t worry. Your hydrometer should come with a conversion chart for higher and lower temps. Now get some distilled water if you want to be very accurate and cool it to 60degF. Or use tap water and be pretty darn close. Fill the flask with it and dunk the hydrometer. If it reads 1.000 perfect, it is right on. If not adjust +/- from then on when you measure. I have two hydrometers and one is good the other off just a little. When reading the hydrometer, put your eye at the level of the liquid and read the number at the bottom of the meniscus (the meniscus is the curvature of the liquid that it makes with the sides of the cylinder).

Now that you know your hydrometer is OK what to do with it. OG, after brewing and cooling but before pitching yeast take a sample. This is your OG. There are lots of ways to get that first sample. If you are doing an extract the easy way is to use a sanitized beer/wine thief. Check with your homebrew supplier. A new sanitized turkey baster might work too but the thief is a good brew tool and not too expensive. Never return the sample to your fermenter. It is not worth the risk of infection so drink it or toss it out. Make sure your wort has been mixed well with the water first. After you remove the sample it is a good idea to “degass” it. All that means is to knock the bubbles out so they don’t lift the hydrometer higher than it should be. Pour the sample between two glasses or spin the hydrometer in the sample until the bubbles are gone.

Now that you know how to take a sample and have an OG you can use your hydrometer to check when to transfer to secondary, when to package and how powerful is that stuff. Some typical original gravity ranges are 1.032 (mild ale) to 1.1xx for big beers like Barley wine. Finishing gravity readings for beer are around 1.010 up to 1.020. Wine, mead and cider can finish under 1.000 meaning they are lighter than water due to the high amount of alcohol. If you take readings for three days and there is no change your beer has either finished or if too high it is stuck, another subject. Some brewers will judge when to transfer to secondary by using 2/3 of the expected drop.

Now you want to know the alcohol content of your beer because you know everyone who doesn’t brew is more interested in that than if it tastes good. For ABW take your OG and subtract your FG then multiply by 105. An example would be OG 1.048 minus FG 1.010 =0.038 X 105=3.99% to convert to ABV multiply that by 1.25 For example using the 3.99 X 1.25=4.98… 5%
Or to go right to ABV multiply your OG-FG by 131, same thing.

If you are really lazy like me just find a brewing calculator online to figure it.

Here are a couple helpful links:
How To Brew- Hydrometers

Northern Brewer- Analytical Instruments

I hope that helps take some of the mystery out of the little glass tube with the numbers inside.