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Why is the Hydrometer so under used?


I thought I’d take a shot at this and see if I can make using the hydrometer a little less daunting. As I said in my equipment blog, the hydrometer seems to be the most under used piece that comes in the equipment kits. Keeping good notes and using your hydrometer, properly, is the only real way someone can help you if you think a brew has gone bad. The very first question Robb or I will ask you, when you come to us, is; “What are your hydrometer readings?”. If you respond, “I didn’t take any.”, we can only guess at possible issues. Truth is, we won’t have a clue without added information like hydrometer readings.

These readings can tell so much about a brew. Original gravity readings can tell us the possible potential alcohol. Subsequent readings will tell us if your brew has stalled, finished, reached your yeasts tolerance, or not started at all.

Keeping good brew notes will help you repeat your successful brew and allow you to see if your current brew is on track for what you are striving for. If, for example, you’re making a mead and have successfully made one you enjoy and wish to repeat it. All you need to do is match the numbers and you will come very close to repeating your successful brew. I say very close because there are so many variables that can affect the outcome of a beer or wine. Your ingredient’s yield will change from batch to batch, yeast will work different from packet to packet, your choice of water will affect outcome too. If you keep good notes, you will have a shot at reproducing the brew you enjoyed so much. I use a worksheet Randy Mosher put out in his book Mastering Homebrew. Instructions for using the worksheet can be found here.

Okay; we have a hydrometer, worksheet, and a test jar. So how and when should we use it? What, exactly, does a hydrometer read? Let’s start with that, the hydrometer is just an instrument for measuring the density of a liquid. In our case we are measuring the density of the sugars, in a wort or must, for the yeast to convert to ethanol. There are other compounds produced by yeast, but this blog is only going to touch on the alcohol. So, the denser the wort or must, the higher your hydrometer will rise. The opposite will also be true; the less sugar density the lower your hydrometer will float.

When should you take those reading? I take readings pre-boil, just before I pitch my yeast and of course a final reading after the fermentation has stopped. Why pre-boil? Because I can look at my past brew notes and see if I’m on track for my post boil sugar levels. Let’s say, for example, that I’m shooting for an Original Gravity reading of 1.050. Because the boil will cause water to evaporate and concentrate your wort’s sugars, a pre-boil reading will tell you if it’s okay to proceed to the boil. Let’s say your pre-boil reading is 1.055 and we know that number will go higher after the boil. A 1.055 tells you that you need to add more water because you’re over your target of 1.050. I’ve found that a pre-boil hydrometer reading 5- 8 points below target is good to proceed to the boil. What happens if you’re lower than that? You need more sugars. You need to add an extract (liquid or dry) or increase your boil time to concentrate your sugars to the desired level. DO NOT ADD LIQUID OVER 100 DEGREES TO A GLASS TEST JAR! DO NOT ADD YOUR GLASS HYDROMETER TO LIQUID OVER 100 DEGREES!!!! Once my wort has cooled to pitching temperature, I will take my Original Gravity. After 2 weeks I will take another gravity reading, wait 2 days take another, if the 2 reading are the same I’m ready for conditioning. If not, wait 2 more days and take another until you get 2 stable reading in a row. With my mead I wait 2 months to do this step.

How do we use the hydrometer? Place your hydrometer into your empty test jar and add enough wort or must to make it float, read the number that is represented by the liquids level. AGAIN, DO NOT DO THIS WITH ANY LIQUID OVER 100 DEGREES! Now because hydrometers are calibrated for either 60 or 68 degrees, to be accurate you need to use a hydrometer calculator. I use the one from “Brewer’s Friend”. Just punch in the appropriate number and record your numbers on your worksheet.

You can then use your numbers to determine the ABV (Alcohol By Volume) of your brew using the following formula:

(Original Gravity – Final Gravity) x 131.25 = ABV
Let’s say Final Gravity is: 1.015
(1.050 – 1.015) x 131.25 = ABV
.035 x 131.25 = 4.6 % ABV

Is there a way to take the high temperature readings without having to cool the hot wort? Yes, with a refractometer. A refractometer has a built-in temperature compensator so all you need to do is add a couple of drops of your hot wort and read the numbers. I keep disposable straws on hand that I dip into sanitizer to extract my hot wort to do readings. Follow the directions that came with the refractometer to calibrate and use properly. Here’s the thing; pre-fermentation reading can be used as indicated but post-fermentation reading require a calculator because the ethanol skews the numbers. Again, I use the one from “Brewer’s Friend”. Make sure you write down both numbers as most brewers will talk Specific Gravity over Brix.

That’s it! You now have the ability to understand and control your brews progress. As always, let me know what you think. If you would like to see some of my older blogs, just click here.


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  • John Thompson
Comments 4
  • John

    Howdy Austin,

    Thanks for leaving a comment! because I use a refractometer for my pre-boil reading I don’t have to worry about cooling down my wort. If you do, on the other hand, need to cool for your hydrometer reading; I would either stick it in the fridge or a container of water. I totally agree with your assessment of the hydrometer! Your borosilicate glass should take the temps ok, but the hydrometer will not. You also stumbled upon the magic formula for never breaking a hydrometer; having multiples on hand. I once broke 2 in one day (had to make a run to see Robb to get another for my final gravity reading and then broke that). Once I bought multiples, I never broke another. (Your mileage may very though).

  • Austin Knies
    Austin Knies

    Thanks for posting, John – I realized the same thing shortly into my homebrewing career that nothing beats the hydrometer – and I still use it over my refractometer almost every time.

    Do you wait for your samples to cool (or stick them in the fridge to do it fast)? I’ve been using the hydrometer temperature conversion tool in beersmith, I’ve found it’s usually within about 0.001 from when the sample has actually cooled. I have a borosilicate test cylinder (safe at very high temps) and have been able to reliably get readings over 140F handling with gloves or something else to avoid burning my hand. Haven’t cracked a hydrometer yet, but that’s also why I have more than one. I’ve found the ability to get a fast read encourages me to do it more often, even a couple times during mash just to see when conversion is complete because… science, I guess.

    The only thing I make sure NOT to do is immediately rinse the hydrometer with cold water – borosilicate glass can handle the temperature delta, but I’m pretty sure the hydrometer won’t.

  • John

    Hey Richard,

    Thanks for commenting. Actually, using the hydrometer isn’t about the next level, it’s about the basics. In my humble opinion; the hydrometer is king. It can be used to determine your target figures (say you want a sweet mead. You need to know approximately how much sugar to achieve that……Specific Gravity…….Hydrometer). Without it you are just assuming your fermentables are correct and that a still airlock means it’s completed fermentation. Again, as far as I’m concerned, assumptions in brewing is a good way to make a bottle bomb. Hope you feel better!

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