The Hydrometer - Your Bestest Brewing Buddy.

The hydrometer is a pretty humble looking device, but it's seriously one of the most important items you can have as a brewer. It's so simple to use, yet it's one of the most important safety items you could ever possibly own.

Basically, a hydrometer is a weighted glass or plastic quasi-cylindrical device which has a calibrated scale on the upper end. There are many kinds of hydrometers for specific purposes, but we'll just stick to talking about the ones used in brewing.

It can tell you when fermentation is in progress or if it's complete and with a little simple mathematics you can even work out the alcohol percentage of your beer. Pretty awesome for a tool with no moving parts, isn't it?

Here's what it looks like:

The one in the pic is a slightly fancy one with some additional scales on the other side of it for approximate grammes of sugar per litre and alcohol percentage, as well as approximate OG and FG for beer and wine. We'll just focus on the scale which is most important to us as brewers: the specific gravity scale, as shown in the photograph.

There are three terms you need to know:
* SG - Specific Gravity: A measure of the density of the liquid comparative to water.
* OG - Opening Gravity or Original Gravity: The specific gravity of the wort prior to pitching the yeast.
* FG - Final Gravity: The specific gravity of the competed fermentation.


Each different brand of hydrometer may have slightly differing instructions as to how to read it. In the case of this one, the reading is taken at the upper end of the meniscus. It's calibrated to 20C which is usual for Australia. Other parts of the world use different calibration temperatures. Read the documents that came with yours so that you know.


Using it is very, very easy:
* Take a sample of the brew to be measured in a suitable container
* Insert hydrometer gently
* Twirl it a little to remove any gas bubbles which might be adhering to the bulb and let go
* Take reading and use the information wisely.

Take a look at this pic:

The pic is showing a reading of about 1011. This is the FG for a rather tasty English Ale - an ordinary bitter - which is in my fermenter at the moment, having completed fermentation and now being ready to bottle later today.


When you draw the sample, do it gently so that you don't put a head on it. If you put a head on it, you will need to wait for that to go away so you can get a good reading. Also purge the tap a little at first as there will usually be yeast and trub (waste by-product of the brewing process) in the tap, which would cause a false reading.


In the case of this brew, the OG was 1049 and the FG is 1011. From this, we can work out how much alcohol by volume as a percentage (ABV%) is present.

Here's the formula according to Coopers (www.coopers.com.au): (OG - FG)/7.46 = ABV%.

In this case, it would be:
(1049 - 1011) = 38
38 / 7.46 = 5.09%

Because you will be bottling it, you need to add 0.5% to account for the secondary fermentation from the priming sugar in the bottle.

Therefore, this beer will end up at 5.59% approximately after bottle conditioning.


There are different methods of calculating ABV%. 7.46 is sometimes replaced with a lower or higher number, depending on what method the person or company recommending that method used to arrive at that number. 7.46 seems to be a good reliable number and a reasonable comparative rule of thumb.


To be certain that your fermentation is complete, take two readings 24 hours apart. If there is any difference in the readings, the fermentation is not complete.

The danger of bottling a brew that hasn't finished fermenting is that it will continue to ferment in the bottle with possibly disastrous results. This is where the hydrometer is probably the best safety item you can own as a brewer.


If you're bottling in PET plastic bottles, it's not too bad.

The way to check is to give the bottles a squeeze. If they're slightly soft, then secondary bottle fermentation is underway and there is still a while to go before your beer is carbonated. If the bottle is quite firm and it's a week or so since you bottled, then all is well and it's quite alright to chill a bottle and do a taste and aesthetic test. If the bottle is rock hard and it's only a few days in, you may be in trouble.

If there is excess pressure when you check how the aging is going, you can release some of the CO2 gas build-up by unscrewing the lid a little and then re-tightening. However, if it goes unchecked the bottle may rupture and spill your precious, precious beer on the floor and make a sticky mess in the worst case or you may just end up with gushers, where the beer gushes out of the bottle as soon as you uncap it, much like a bottle of Coke that has been dropped or shaken.



However, bottling in glass is a different matter. You can't check if there is excess pressure except by opening a bottle and pouring a serve. If it is over-pressurised, you will have a gusher.

This is a serious and dangerous situation.

Check with your local home brew shop or an experienced brewer before attempting this. Get knowledgeable, experienced advice that is relevant to your situation. I accept no responsibility for your actions under any circumstances.

The usual procedure is to put on as much heavy protective cloting as possible: full face welding mask or helmet, motorcycling leather jacket and gauntlets and go outside with the entire batch to uncap it very gently one at a time, let it breathe for thirty seconds and then recap it. If this over-pressure condition is unchecked you quite probably will end up with what is referred to as 'bottle bombs'.

Bottle bombs are quite literally that. Shards of glass being propelled at high speed. Definitely a major safety issue. Potentially quite deadly. I've seen photos on a brewing forum where a longneck bottle, or at least half of it, was stuck in the ceiling having been driven through the Gyprock by the sheer force of the explosion. The brewer had bottled too early and there had been an explosion a little under a week later. It was in his laundry, so no one was hurt - just piles of broken glass and beer everywhere.

One of the worst parts of bottle bombs is that it can often cause a chain reaction. One explodes and then the rest also explode owing to the concussion of the first one's blast. My grandfather managed to do this once during WWII in his garage and then was banned from brewing by his wife. You have been warned.


My recommendation is to start off bottling in PET bottles. Do a few batches to gain confidence and competence. In the meanwhile, drink plenty of commercial brew that comes in good quality, thick, heavy glass bottles and good strong boxes which you can re-use for bottling and storage later. At least that was my excuse.


So, to recap:
* Hydrometer good. It's certainly your bestest buddy in brewing.
* Bottling early bad. You can become your own worst enemy and perhaps kill or maim others.


As with anything in life, be sensible, read the manual and get advice from someone with experience and everything will be sweet and easy.

Cheers!!