Hydrometer or Refractometer?
Both refractometers and hydrometers are used in the brewing process to measure the gravity of your wort whether it be before boiling, after boiling, before pitching yeast or measuring your final gravity to derive alcohol content.
Which is best?
The truth is, you probably need both to be able to measure your different stages of brewing accurately.
Read on to compare both instruments.
Hydrometers are a basic tool for measuring specific gravity of a liquid. A hydrometer works by displacing liquid based on its density and measuring the amount of liquid it displaces.
You will see a small amount of metal or beads at the bottom of a hydrometer which functions as ballast (to keep the hydrometer upright and to weigh it down properly).
A scale is visible up the side of a hydrometer that has a prominently marked 1.000 on it. This is the specific gravity of pure water and all gravity measurements are taken in relation to this number.
Hydrometers are the most basic and least expensive tool for measuring the specific gravity of a liquid.
When using a hydrometer, you’ll want to buy a 100ml borosilicate glass cylinder, is the best size to place a hydrometer as it has the right amount of space for the wort and the hydrometer.
You can pick up a hydrometer for as low as $5, but they do have a few draw backs.
- Hydrometers break…. easily…. a lot….
Speak to any brewer and they’ll tell you that they’ve broken a fair few hydrometers.
The most common way I’ve broken my hydrometer is by dropping it into the 100ml cylinder. It would hit the bottom and shatter.
I’ve also broken them while cleaning / sanitising and just being clumsy.
Most retailers know this is going to happen and sell them in packs of 5. I’d highly recommend purchasing a 5 pack, the last thing you want to be doing it breaking a hydrometer during your brew day.
- Hydrometers are temperature based
If you look closely at your hydrometer you will notice that it says “calibrated to 20 degrees c” or similar.
This means that if your sample isn’t 20 degrees Celsius, the reading it provides will be inaccurate
Here is a great temperature correction calculator you can use.
- Hydrometers require larger sample sizes
While 100ml of beer isn’t a large amount in the scheme of things, other methods can use as low as 5ml.
When you take samples during brewing process all of this adds up.
The typical brew process involves taking samples before the boil, after the boil, before pitching yeast and after pitching yeast. If you’re using a 100ml cylinder to measure samples, that’s almost a pint of beer wasted!
- Using hydrometers risks contamination
If you’re removing the lid of your fermenter to take a sample, your wort is at risk of being exposed to oxygen.
Some brewers get around this by leaving a hydrometer in their fermenter the whole time. This is also risky and isn’t recommended unless you’re extremely confident in your sanitising regime. Its not worth losing a whole batch of beer.
Refractometers work by measuring the refraction of light through the sample liquid and calculate the density based on that measurement.
They generally come with a kit that includes a small medicine dropper (or thief) to obtain samples of beer. While the dropper still has to be cleaned and sanitised, its much less of a task than cleaning a cylinder and hydrometer. The sample is placed on the glass and the cover is placed on top, this spreads the sample uniformly across the surface. You then hold the refractometer up to the light and take your measurement.
Refractometers are a heavy rugged type build and are much less likely to break when in use and when storing / cleaning.
So refractometers sounds great compared to hydrometers now right?
Have a read of a few of the caveats below and you might change your mind
- Refractometers must be calibrated before every use.
Due to the nature of how a refractometer works, you must calibrate it with water before using it. You’ll find instructions on how to do this with the refractometer you purchased.
Further more, they must be calibrated for the same temperature of the sample. So if you calibrate yours to 70°f and your sample is 90°f, you must re-calibrate or wait for your sample to cool down to 70.
- Refractometers that have ATC
Some refractometers have automatic temperature compensation. If you have a cheap $30 refractometer it may advertise this feature however it would be wise to avoid it.
I’d only trust this feature if it was on a more expensive type digital refractometer around the $300 mark.
- Refractometers that display SG… ignore them
Some refractometers display BRIX% and SG. The problem with displaying both next to each other is that they are not related in a linear fashion. The SG reading can be ignored as we need to do some additional calculations which you’ll see below.
- Refractometers need a LOT of samples
If you’re using an el cheapo refractometer you’ll want to take 5-10 samples and average them out.
Some times you’ll notice variations in your readings by as much at 10%. This is a must for any brewer using a cheap refractometer.
- Refractometers don’t work with alcohol
While there is not a lot research in this field directly, one brewer has come up with a formula to calculate final gravity but it only works if the original gravity is known.
FG = 1.0000 – 0.0044993*RIi + 0.011774*RIf + 0.00027581*RIi² – 0.0012717*RIf² – 0.0000072800*RIi³ + 0.000063293*RIf³
- Wort Correction Factor
So you’ve taken a few samples and averaged them out to get your average Brix reading of wort. Is it correct? how do you know? If I measure your wort with my refractometer will I get a different reading? To answer above questions we need to calculate something called the Wort Correction Factor. The reason we do this is that our wort doesn’t just contain water and sugar – It contains mant complex sugars, carbohydrates and molecules such as monosaccharides, sucrose, maltose, maltotriose, etc.
So how do we account for these in our calculations?Head over to the Brewers Friend website and download their spreadsheet.
You’ll see that the process to correct readings of your wort using your refractometer is fairly simple but repetitive.Multiple measurements of multiple batches of wort will eventually give your wort correction factor of your refractometer and the more samples you take, the more accurate you will be.
So what should you use? and when?
Given the above factors we’ve compiled a list of whats best in each situation.
When brewing with extract cans and dextrose, you can get away with using a hydrometer 99% of the time. You probably don’t need to break the bank with a fancy hydrometer because you’ll be taking simple measurements at low temperatures.
All Grain Brewing
A refractometer will be your weapon of choice to measure gravity both before and after the boil. This is because of the small sample size required.
Before the boil, temperatures will be around 167F (or 75C). After the boil, your wort will be just below boiling point. You only need to sample a few drops with a refractometer which means they will cool to room temperature relatively quickly enabling you to take an accurate gravity reading.
Before pitching yeast, either a hydrometer or refractometer may be used. If you leave a sanitised hydrometer in your fermenter, it will be easy to see the gravity value right before pitching your yeast but you may also wish to sample it using a refractometer.
At the end of fermentation your wort will contain alcohol. Because of this it will be easier to use a hydrometer to measure gravity – otherwise you will have to calculate the wort correction factor as described above.
Thanks for reading, if you have any additional questions or need clarification on any of the above steps, please contact us.