How Far Can You Trust Your Hydrometer?

Hydrometers measure soluble solids density, and we use this to closely approximate soluble solids content of fruit juice. Because almost all the soluble solids in wine grapes are sugar, we use hydrometers to determine sugar content. I’m not telling you anything you don’t already know, every book on wine making I have says something like this, but did you maybe tuck the “in wine grapes” qualifier into the back of your head and just remember the “… almost all the soluble solids are sugar” part? I know I did, and it was while researching raspberries that I discovered just how much hydrometers can overstate sugar content.

Great data on raspberries upends an old rule of thumb

I was checking university extension offices, googling, and looking anywhere I had found information on fruit composition before when I came across this great paper on raspberries. It’s got excellent data on sugar and acid content, and I highly recommend it if you’re interested in making raspberry wine. I want to zero in on the Brix and Total Sugar measurements:

Brix vs Sugar Content in Raspberries
Varietal Brix Total Sugar Sugar Brix Ratio
Meeker site 1 11 2.83 25.7
Meeker site 2 8.6 1.01 11.7
Meeker site 3 9.2 2.38 25.9
Meeker site 4 9.8 3.28 33.5
Chilliwack 9.6 3.84 40.0
Tulameen 9.5 3.09 32.5
Willamette 8.7 2.31 26.6
Yellow Meeker 10.8 4.60 42.6

Average 9.7 2.92 29.8

Total Sugar is reported as grams per 100 grams, and if the soluble solids were 100% sugar, then Brix would equal Total Sugar. The last column, Sugar Brix Ratio, is my own calculation and expresses sugar as a percentage of soluble solids. I also reported average values in the last row.

We go from soluble solids being “almost all” sugar in wine grapes to less than 30% (on average) for raspberries! I asked Michael Qian, one of the authors, about this. He said that the data were good but some fruit, like raspberries and blackberries, just have a lot of pectin and other non-sugar soluble solids. I really appreciate him taking the time to help me out; I’m sure he’s a busy guy and this was a pretty basic question from someone outside his target audience (so, if you’re reading this – thank you!).

We know more than before – it just doesn’t feel like it

It’s exciting to learn something new, even if it does make things more complicated. When can we rely on our hydrometers? Wine grapes are probably a safe bet. Raspberries and blackberries are not. We need more information about other fruits, and I’ll be looking into that. If you know something about sugar content and soluble solids of other fruits, please say so in the comments.

Alright, what do we do when we know our hydrometers will read high? I don’t have a good solution yet. For raspberries, we might just adjust the reported Brix by 30% – still not accurate, but closer than the hydrometer reading. If you have an idea, I’d love to hear about it.

Jack Keller once said that a hydrometer is like a compass. Maybe it’s like a magnetic compass. It works well enough much of the time, but the question is, how do we find true north when we really need to?

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14 thoughts on “How Far Can You Trust Your Hydrometer?

  1. Aaron

    Fascinating. As primarily a beer brewer, this even further complicates trying to track how much sugar/alcohol is added by some fruit additions to a beer (or a mead).

    It seems like information on this should be out there somewhere, fruit juice manufacturers, for example, would need to know it.

  2. John H

    Erroll, you always come up with great chunks of data like this which I would never have thought to even investigate. Your scientific approach to my most favorite hobby is very much appreciated. Thanks for yet another informative and interesting article!

  3. Erroll Post author

    Hi Aaron,

    Yeah, if you bought 100% fruit juice, then the sugar content would be on the label. Also, the USDA makes average nutrient data, including sugar, available for most foods. I’d like to find a way to measure sugar content that’s simple and economical enough for home wine (and beer!) makers.


  4. Erroll Post author

    Hi John,

    Funny thing is, the idea that hydrometers might not measure sugar content for some fruit didn’t occur to me either. I thought I was just gathering information for an article on raspberries. Getting pulled in an unexpected direction can be frustrating, interesting, and fun all at the same time!


  5. Chris

    Hi Errol,

    Hydrometers don’t measure soluble solids OR sugar content directly. What they do measure directly is density of a liquid and you need a chart for any mixture of fruit juice and water (or ethanol or any other mixture) to calibrate that mixture to the density. In the juice industry there are plenty of these tables for various juices and their concentrates. Apple, pear, concord grape, raspberry, etc… all have similar profiles that’s why short-hand they measure Brix with these or refractometers.

    But for critical, precise measures you need a hydrometer density chart curve, typically developed in a lab by measuring volumes and weights at various temperature for the juice or mixture of interest.

    Here’s an article about hydrometers in hard apple cider.

    The key is that any scale on a hydrometer (other than density) has to be calibrated to the liquid you are measuring.

  6. Erroll Post author

    Thanks Chris,

    It sounds to me like you’re saying that the Brix scale on our hydrometers is approximate, and we can’t get an accurate measure of soluble solids without a chart for the particular liquid being measured. In the case of raspberry juice, a reading of 9 Brix would be close but there would be an error term because the hydrometer’s Brix scale was not calibrated for raspberry juice. Is that right?

    How far off are we talking about? Also, are these charts available online somewhere?

    The link you provided gave a good introduction to using a hydrometer, but assumed that soluble solids were almost all sugar. I’m not sure that you and I are talking about the same thing because I was trying to point out that sometimes sugar will be a small portion of soluble solids – about a third in the case of raspberries. That means that even an extremely accurate measure of soluble solids (Brix) – before fermentation or any additions of sugar, water, etc – would not be a good indication of sugar content. The paper I referenced reported Brix readings of raspberries taken with a refractometer of about 9 Brix, while the sugar content of those raspberries were only about 3 g/100 g.


  7. John H

    Erroll, does this mean that the only reliable method of discovering the actual sugar content of a given liquid medium is to use a refractometer?

  8. Erroll Post author

    Hi John,

    I don’t know of a general way to measure sugar content (yet). Hydrometers and refractometers can help, but they have their limits. Refractometers measure how a particular sample bends light (it’s refractive index), and as Chris pointed out, hydrometers measure relative density. Most wine makers believe two thing about these measurements:

    1) You can closely approximate the fraction of soluble solids (get a Brix value) from the relative density or refractive index of a sample.

    2) You can closely approximate the fraction of sugar from the soluble solids in a sample.

    I was thinking about #2. It works well for wine grapes, and possibly other fruit. It does not work well for raspberries, and it may not work well for some other fruit.

    I believe Chris was talking about #1. The way I read his comment is that it does work pretty well in most or all cases, but it is still an approximation. Have I got that right, Chris?

    So, hydrometers and refractometers both do a good job doing what they do (measuring relative density and refractive index). These measurements can be used to get a good idea of the soluble solids content in a sample (Chris’ comment makes me want to look into this a little more), but in neither device will help you with sugar in raspberries (and possibly other fruit with the same issue as raspberries).


  9. Chris

    I sent Erroll a more detailed answer on some of this, but the crux of it to me is that hydrometers and refractomters are both rather crude instruments that do only approximate the sugars and other chemical makeup of wines and musts and fruit juices.

    As Erroll found in his raspberry examples, all of teh non-sugar materials matter to how these things work. Even in one type of fruit, grapes, raspberries, whatever, it’s hard to account for variation in varietals that may have different non-sugar compounds that make these tables off (even if by tiny fractions), as well as seeds and skins that are typically ignored in most of the time these charts are based on clarified juice. As you know, raw must from any fruit is often far from clarified. Your raspberry example is a good one where the “other stuff”, proteins, fibers, rocks, make these naturally derived substances hard to define by rather crude measures such as density=sugar content, or refracted light= sugar content.

    I don’t know exactly what wine labs do any of this, but I’d assume there are more accurate labs test used to directly measure sugars. A food chemist can even identify sucrose, vs. fructose, vs. malactose, etc…

  10. John H

    I’d be very interested to know – if only to satisfy my intellectual curiosity – what those methods are and whether one can readily adopt them in a home environment without excessive expense. To me, though, it is merely the end result of a decent batch of wine for which I labor. If I get FAIRLY consistent results from one batch to another (and the result is a good one) then it matters little to me how accurate the tools really are. The same would be true if I were building a house: using old tools which don’t do as efficient a job as new ones perhaps would, the end result will still be a well-built home that keeps the weather and critters out…given, of course, that I tended to utilize my skills in carpentry to the best of my ability. Same thing for wine making, in my opinion. I know some folks might look at that attitude as wine sacrilege, but those folks don’t have to drink my wines – my wife and I do. And if the lady of the house is happy everyone is happy!

  11. Chris

    “And if the lady of the house is happy everyone is happy!”

    John I agree!

    I also agree the tools available to a home winemaker don’t have to be fancy or expensive. After all, long before refractometers and hydrometers, men and women had figured out how to ferment grapes into wine relying mostly on the tools attached to their face, the nose and the tongue.

  12. John H

    Indeed, Chris. Indeed. I like having the tools available because it makes getting fairly consistent results more readily obtainable, but the first wines were made without even the use of yeast packets bought at the local winemaking supply store. Trillions of yeast flying around the atmosphere can settle on the fruit and trigger the magical alchemy of alcohol production, but many strains give funky flavors to the resultant wine. I prefer having control over how it comes out tasting…even if most of that control is illusion.

  13. Valerie

    I really appreciate your scientific approach to winemaking. I started with a few kits: following the instructions without knowing much about why I was doing what I was doing. Once I got hold of “First Steps in Winemaking”, it explained the basic science of it, so now I understand the cause and effect of the whole process.

  14. Erroll Post author

    When I was first thinking about blogging, I kept asking myself “what could I possibly add to the great information that people like Jack Keller have given us?” But as my wine making progressed I found I was asking questions that I couldn’t find answers to in the forums and web sites that I knew about. I started finding out that answers on my own, and I decided to write about that. Thank you for the kind words,


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