Measuring Specific Gravity

Homebrewing can be as precise as you want it to be. But even if you're brewing by the seat of your pants, you still need to measure things: temperature, volume, weight/mass, pressure, and density (as specific gravity). This last one is important since it tells us where our batch started, its potential for ethanol creation  and where/when it is finished.

The Gravity of the Situation

Specific gravity is the ratio of the density of a given liquid to a reference liquid, usually water. This value is expressed as a number to three decimal places, and each 1/1000th is called a point. Water has a specific gravity of 1.000 and a solution registering 1.025 is 25 points denser.

If you've ever used brewing software like BeerSmith or Brewer's Friend, you'll see that it lists a PPG (points per gallon) value for malts and adjuncts. This means that using brown sugar or caramel 20L malt will add X points of sugars to your wort per gallon of water.

These programs calculate your starting gravity (SG) using these PPG values. This is sometimes referred to as original gravity (OG). It then extrapolates how many of those points can be consumed by the yeast and gives you an expected final gravity (FG) as well as an expected percentage of alcohol by volume (ABV).

These are calculated values based on the grist bill you input. But that might not be the gravity you get. If your mash is too warm, your SG/OG will probably come in a little low.  If you lost count and put in an extra pound of brown sugar, your SG/OG will come in high. (True story: I built a recipe with a pound of honey, but input the same PPG value as LME. The actual SG/OG came in 30 points high at 1.084. Oops!)

Displacement: Glass Hydrometers

Triple-scale (left) and
FG hydrometers

For the homebrewer, there are two methods for measuring specific gravity: refraction and displacement.

The simplest, most direct way to measure specific gravity is by displacement. Weighted glass hydrometers are calibrated to float at a certain level in water at a certain temperature. You read the gravity by looking at the bottom of the meniscus in a sample and read the value on a paper scale inside the hydrometer.

The most common hydrometer is a triple-scale, which covers a wide range of gravities (mine goes from 0.982-1.060) and has scales in points as well as brix and potential alcohol. This is the kind that often comes in beginner homebrew kits. There are also more sensitive hydrometers that have narrower scales.

PRO:

• Cheap - The triple-scale costs only $7, which is a good thing. More sensitive FG versions can run up to $30.
• Reasonably accurate - For the money, a hydrometer is hard to beat in the accuracy department. You can get much better accuracy in a lab-grade device, but these are well over $100. 
• Versatile - The triple-scale can be used for SG and FG readings of beer and wine.  

CON:

• Low sensitivity and hard to read - This is my biggest knock on hydrometers, the triple-scale in particular. The broad scale means the markings are pretty close together, so it can be hard to read. I spend a lot of time staring at the meniscus trying to figure out if it's 1.044 or 1.046. I bought a second hydrometer with a much shorter range (0.990 to 1.020) to measure FG. If nothing else, it's much easier to read. 
• Waste of wort - It takes a fair amount of liquid to float them. Not a big deal with pre-boil wort since you can just pour it back into the pot, but it can be a waste of finished beer. 
• Fragile as all get-out - They are very fragile, and there's many a homebrewer that's broken one. HomeBrewTalk has an Official Broken Hydrometer Count thread going, and the grim tally is over 2,200. I did say they were cheap, so replacing won't pinch your wallet.
• Sensitive to temperature - Hydrometers are calibrated to water at a certain temperature, usually 60F. If your sample is considerably warmer or colder, the reading will be off. There are plenty of online calculators to correct the reading, but it's another step to take and you'll need a thermometer.
• Scale can drift - I did say they were accurate, and they are pretty good considering they cost under $30. But the paper scale inside can shift, especially if it's handled roughly. A 1.040 now might look like 1.036 if the scale slides down.  

Displacement: Electronic Hydrometers

The Beer Bug uses a float to measure
specific gravity from inside
your fermenter (Photo: MoreBeer)

In recent years, some clever folks have come up with electronic hydrometers that take readings constantly inside the fermenter. The Beer Bug and the Tilt float in the wort and tilt as the gravity changes. They transmit their data to a nearby computer, so you can see minute by minute how fermentation is going. These new devices eliminate all of the cons of glass hydrometers, but they have their own drawbacks.

PRO: 

• Easy to read - These are the digital watch to the glass hydrometer's sundial.
• Continuous readings - If you just love data, you can see how fermentation is going by the minute.
• Won't disturb primary - It takes its readings inside the fermenter, and you can check on its progress remotely. 
• Saves beer - No need to draw off samples, so you won't waste any beer.

CON:

• Expensive - $120-140 new, or what you'd spend on a handful of glass hydrometers.
• Requires some technical proficiency and extra electronics - These devices need another computer/tablet/smartphone to record data, and it'll be up to you to set it up. If you have an old smartphone with Bluetooth, great! Otherwise, you'll need to shell out for a compatible device.

Refraction: Analog Refractometer

My analog refractometer

The other method is to measure how much light bends - or refracts - as it passes through a liquid sample. Refractometers only need a few drops to get a reading, so they work great for checking the gravity of your mash or wort while boiling.  

OG of 1.057

PRO: 

• Easier to read - The scales can be pretty tight, but much easier to see where the blue stops than determine where the meniscus is in a turbid wort sample.
• Automatic temperature correction - The sample doesn't need to be room temperature, though I suspect it cools down very quickly when dropped onto the prism.
• Saves beer - Just needs a few drops of wort.
• Relatively inexpensive - They can be had for $20-30, though nicer ones will cost more
• Easy to calibrate - Add two drops of distilled water, then turn the adjustment knob or screw until it reads 1.000

CON:

• Cheap ones aren't very accurate - This is simply "you get what you pay for." I wouldn't be surprised if cheapo models need  constant adjusting. For what it's worth, I bought mine off of Amazon for $25. The picture on the right is an OG reading I took from my amber ale. It matched the expected value form Brewer's Friend and was pretty close to triple-scale hydrometer.
• Can be knocked out of calibration - Dropped it? The you'll want to re-calibrate. That said, I imagine they're far more durable than glass hydrometers.
• Wider scales are hard to read - Mine goes from 1.000 all the way up to 1.130 (or 0-30+ brix). This is fine for wine making, but it's is ridiculous for home brewing. A scale topping out at 1.060 with a more sensitive prism would be much easier to read.
• Dissolved solids make for fuzzy readings - Refractometers work best with simple sugars in solution. Wort is more complicated, sugar-wise, so readings can be a little off. Samples with any bit of cold-break proteins will make the line fuzzy and hard to read. 
• Alcohol skews readings - If you're sampling after fermentation has started, the ethanol will distort the readings. You'll need to use a calculator to correct the readings. 

Refraction: Digital Refractometer

Hanna digital refractometer
(Photo: MoreBeer)

These are laboratory-grade electronic devices. Definitely the most accurate tool, but you will pay $$ for it.

PRO: 

• Easy to read - Spits out a number on a digital display.
• Precise - Readings are +/- 1 point.
• Saves beer - Just needs a few drops of wort.

CON:

• Expensive - The cheapest one I've found is $200 from MoreBeer, and it only displays brix, which means you have to convert it to points. Meters that will display points go for $350-450. IMHO, these are overkill for home brewing, but hey, it's your money!

Which is Better: Hydrometer or Refractometer? Yes!

As we've seen, both methods have their drawbacks. You can stick to using one or the other, but I've embraced the notion of using both by playing to their strengths and using the best tool for the situation.

Refractometer for Pre- and Post-Boil, OG

Since there's no alcohol yet, I don't need to adjust my mast or wort readings. I can also get a reading quickly, which is handy if you're trying to boil down to your OG target. It can take a while to cool down a sample jar's worth of wort to get a hydrometer reading.

Hydrometer for Fermentation, FG

Triple-scale (left)
FG hydrometer (right)

Once alcohol is in the solution, the refractometer is at a disadvantage. I don't want get a reading and then convert it, so the hydrometer is the way to go. The best choice, however, isn't the triple-scale but a hydrometer specifically for FG. While they are a bit more expensive ($15), they are considerably more sensitive and easier to read.

I put my FG hydrometer next to my triple-scale with 1.000 marks lined up to show you the difference in ranges. The triple-scale covers 54 points (in two-point increments) in the same space as 10 points (in one-point increments) on the FG hydrometer. Since I only need to be accurate to one point, the FG hydrometer is much easier to read.

It will take a fair amount of beer to float the hydrometer, and you can't pour it back when you're done. But you can give it a taste. And if you have a carbonation cap and an empty plastic soda bottle, you can blast carb the sample to see how it tastes when it's all nice and fizzy.