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I was wondering if anyone leaves a glass hydrometer in their carboy during primary fermentation so that you don't have to take samples. I figured it would be an easy way to tell that the SG has leveled off.

Update: I tried adding the hydrometer to the carboy and it worked out well. I used my phone to zoom in on the reading. I used a hose with a stainless steal rod inside to get the hydrometer in without dropping it. It was easy to get out. I left it in while transferring the beer to a secondary fermentor. When I went to clean it out I filled the carboy with water and it floated up to where I could grab it.

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  • I was actually thinking about this last night. I wonder if you could reasonably see through the carboy and get a decent reading. Also, if you drop a hydrometer in through the neck of a carboy, would it potentially hit the bottom of the carboy and shatter? Would someone other than me mind giving it a shot :) ?
    – Matthew Moisen
    Apr 6, 2015 at 2:50
  • in theory you could, but i think trying to see through the krausen that sticks to the side of the carboy would make it hard to read.
    – jsolarski
    Apr 6, 2015 at 2:57
  • This is an interesting. I might try this soon. I guess it really depends on the style and how much krausen it creates - I've got one in a carboy right now where primary fermentation just settled down. There wasn't much krausen on this one, and I would very easily be able to see it I think.. I'll let you know if I end up dropping it in there
    – Ben Strombeck
    Apr 6, 2015 at 3:48
  • I need to take some readings tomorrow so I think I am going to try it. I have a tool with three prongs on it that will allow the hydrometer to be placed without letting it drop. A sanitized brush might allow you to brush off enough to get a good view.
    – Brian Kalski
    Apr 6, 2015 at 6:07
  • While you might be able to tell that the fermentation has largely completed, I'd expect the SG reading to be way off. The hydrometer will have tons of yeast, etc, stuck to it which will affect its buoyancy.
    – djs
    Apr 16, 2015 at 22:48

5 Answers 5

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For the benefit you'd gain from leaving your hydro in there (maybe saving some volume as you won't take samples) I think it wouldn't really be worth your time as I imagine it would be pretty difficult to read without having to clean it off. Also having to open up your fermentor each time to take a reading exposes the wort to possible infection.

I usually only take 3-4 samples max so the volume loss is pretty negligible.

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  • This is the best answer, if you're concerned about volume loss, then a hydrometer is your best bet (you need a couple of drops of the beer/wort to test the Brix, don't read the gravity). Then you can also combine this with a wine thief to further reduce/simplify the amount you draw.
    – Doug Edey
    Apr 9, 2015 at 12:28
  • @DougEdey Do you mean refractometer?
    – Matt
    Sep 17, 2020 at 4:13
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The biggest issue in doing that is that krausen will get stuck to both your hydrometer and your carboy walls.

Even if you wait for the krausen to die off before dumping your hydrometer in, you will still have a bit of a hard time reading it through the krausened carboy walls...

But hey, go ahead and try! That is the essence of homebrewing.

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I dropped mine into the glass carboy and left it in (wasn't an easy way to get it out). It was a bit difficult to read but I managed. Removed it carefully after bottling. No disasters.

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I've tried several variations, including putting the hydrometer into the fermenting vessel and also a mini fermentation in a sample tube kept alongside the fermenting vessel. The former is hard to read; the latter may not ferment at the same rate as the main brew. In both cases, you get sediment settling on the hydrometer potentially weighing it down and giving a false reading.

My best solution so far is to wait until the krausen starts to break up and draw a sample. That goes into a sanitised sample tube with the hydrometer and gives me a first reading. I then cover the tube and check the next day. Given my fermenting conditions, this seems to give an accurate reading (testing the main wort again once I'm ready to commit to bottling if I want to double check).

YMMV - to be honest, my observations over the four or so years I've been experimenting with homebrewing suggest that fermentation has almost always stopped by this point so the reading is more to put a number on where the gravity has fallen to than to check that it is safe to bottle. However, that is based on the yeasts, temperature control and other factors I use.

Wulf

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I see stories all the time about broken hydrometers. They are fairly fragile devices. Why risk contaminating a batch of beer you worked so hard on (and is worth more than the hydrometer even) with thin shards of broken glass that would be the result of breaking the hydrometer inside your carboy? Especially in a narrow-necked glass carboy, I could see this being a disaster. Even on a stainless steel "bucket" fermentor, I could see it breaking on the unseen thermowells or pickup tubes.

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