Keeping An Eye On Wort Gravity And Gravity Points

Being able to achieve your target original gravity on brew day gives you a fighting chance when planning recipes.

To do it you need to take a close look at how your brewing set-up works.

wort gravity

Following on from my recent look at wort volume, this article is all about watching wort gravity on brew day.

You may want to read the previous article first, as it’s hard to understand gravity without thinking about volume at the same time. It is for me, at least.

For general information about beer gravity, I’ve also written these posts:

What Connects Degrees Plato To Specific Gravity?
A Visual Guide To Beer Gravity : Original and Final Gravities By Style

Otherwise, carry on and find out when to take measurements and what to do with them to make them useful.

When To Measure Wort Gravity

Most brewers measure gravity before and after fermentation. This tells you a bit about your beer (how strong it is, how dry it is, etc.) as well as confirming that fermentation has actually happened.

It’s also useful to take readings at a few other moments:

wort gravity

1. Removing bag; 2. Drained wort 2a. 1 + 2 = Pre-boil; 3. Post-boil; 4. Start of fermentation (Original Gravity); 5. End of fermentation (Final Gravity)

When I started checking gravity more frequently I was amazed. It really varies a lot according to the stage in the brew process and wort temperature.

The additional measurements tell you how the brew’s going, and whether you’re on track to match your recipe. If not, you’re able to make adjustments as you go along.

If you’ve been brewing for a while you may think this is basic stuff, but as a beginner I found it confusing and am in fact still getting used to it.

An Example Brew

To give you a better picture, I recently brewed a special bitter and took gravity readings at the points shown in the drawing above.

measuring wort gravity of a special bitter

Note the thermometer in the photo. For the hydrometer readings to mean anything it’s essential to correct for temperature.

At wort temperatures the difference between the measured and corrected gravities is significant.

The gravity readings for this bitter illustrate that point well:

NumberEventTemperature (°C)GravityTemperature Corrected Gravity
1Brew bag out521.0201.031
2Drained Wort451.0381.047
2aPre-boil (2 + 3 combined)471.0281.037
3Post-boil391.0401.046
4In fermenter311.0411.045
5Bottled/End of fermentation

The numbers themselves aren’t really that important as they’re different with every beer.

What is interesting is how different the gravity is at each stage. It’s clear that one single gravity reading could be misleading, and that it’s worth checking it several times in the day.

For me the most revealing thing is that the wort drained from the brew bag (2) is considerably higher gravity than the main wort (1), brew in a bag’s equivalent of first runnings.

If you think you’re having problems with efficiency, bag drainage could well be an area of your process worth looking at.

I noticed big improvements in my original gravities after refining the drainage process, and the gravity readings above explain why that extra wort is so important.

What To Do With This Information

The idea with taking multiple readings is to find out where in your system your beer goes off track, if indeed it does.

For instance, I now know how important the drainage stage is in my brewing process. Without it, it’s more than likely I’ll undershoot my target original gravity.

Another area that influences the original gravity is the boil.

After the mash, with the drained wort added back, the total quantity of sugar is fixed (assuming nothing else is added).

The amount is described in gravity points, an absolute value that unlike specific gravity is independent of volume.

Because the number of gravity points doesn’t change, with a simple calculation you can work out your post-boil gravity before starting the boil.

Calculating Gravity Points

The relationship between pre-boil and post-boil wort volume and gravity is:

{{Pre-Boil Volume}*{Pre-Boil Gravity}}={{Post-Boil Volume}*{Post-Boil Gravity}}

For example:

Pre-boil gravity: 1.050
Pre-boil volume: 18 litres
Evaporation rate: 2 litres per hour
Boil length: 1 hour

Your evaporation rate (as explained in the post about wort volumes) gives you the post-boil volume.

Post-boil volume: 16 litres

For the equation to work the gravity needs to be converted into brewer’s points, which is just a matter of taking the last part of the number and multiplying by 1000. E.g.:

{1.050}={.050}*{1000}={50}

Feeding that into the formula:

{{18}*{50}}={{16}*{Post-Boil Gravity}}

{{18}*{50}}/{16}={Post-Boil Gravity}

{900}/{16}={Post-Boil Gravity}

{56}={Post-Boil Gravity}=1.056

Depending on what you’re aiming for in your recipe, you take a view on what to do.

Adding more water, boiling longer to evaporate more or adding sugar to bulk up the gravity are all ways of tweaking the beer if necessary.

Seeing how volume and gravity change in in an actual brew has really helped me understand this idea.

If reading about it makes it seem complicated, I recommend you do a similar study of your own brew process. Seeing it in action makes much more sense.

As a word of warning, this is not about calculating the gravity points in your wort before you start. It’s the total number of gravity points after the mash, whatever that may be, and how the change in wort volume leads to your original gravity.

I now take these readings every brew day, and will continue to do so.

I want to see how consistent my evaporation rate is, whether I can rely on it to make predictions, and if the type of beer being brewed makes any difference.

I’m updating my brewing record sheet shortly to make this easier so stay tuned if you want to try it yourself.

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