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Thread: Sad and Embarrassing Fact I Must Face....

  1. #1
    Join Date
    Mar 2017

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    Default Sad and Embarrassing Fact I Must Face....

    Can someone PLEASE point me to a reputable youtube vid or some other site that can go in depth to why and how you read a Hydrometer and understand what it is telling you??

    I have read the newbie guide.. even printed it.. but I somehow am NOT catching on....

    Thanks... and sorry...


  2. #2


    No worries bud. Always better to ask than assume you know what you're doing haha.



    One thing these don't mention is that most hydrometers have 3 different measurement units on them (triple scale hydrometer). The 3 different units are specific gravity, Brix, and potential alcohol, but they are giving you all the same information, just in different ways.

  3. #3
    Join Date
    Sep 2013
    Saratoga Springs , NY


    Sorry, but I don't have a Youtube reference.
    The problem may not be you but the instructions that accompany your hydrometer. And the problem is how the manufacturer of some hydrometers have calibrated their device. The cheapo models do not necessarily follow standard scientific procedure (and my knowledge of that is very limited!).
    Inside your cylinder water forms a meniscus when it comes into contact with the sides of the tube. That is to say, the surface of the water rides up the wall. The liquid behaves in the same way when it comes into contact with your hydrometer when placed inside the cylinder. In other words, a meniscus forms what looks like a very broad and somewhat flattish letter "u". You can think of it a bit like a lens
    To obtain an accurate reading you need to have your cylinder on a flat and horizontal surface and you need to view the hydrometer with your eye at the same height as the surface of the liquid in the cylinder. The measure that the bottom of the meniscus cuts is the density or gravity of the liquid (assuming you are using the specific gravity etchings). So if your instructions simply say where the surface intersects with the scale on your hydrometer it is not telling you whether that intersection is at the top of the meniscus or at the bottom. My understanding is that the science community ALWAYS uses the bottom but some hydrometer instructions seem to suggest that the highest point is where your reading is to be taken from...

    Your hydrometer might have other scales too - one that is often there is potential alcohol which is simply a conversion of the SG to ABV assuming that a) this is the reading before you pitch the yeast and b) that the entire density of the liquid is from fully fermentable sugar and c) the attenuation of the yeast is 100 percent (so you cannot really use that scale when brewing beer). There may be a third scale - balling which is apparently similar to brix and brix is a measure of the percentage of the liquid which is made up of sugar (this is useful when you have a fruit and you want to know how much sugar is in the fruit before you press the fruit)

    And here is how you can check everything I wrote and test your own ability to use your hydrometer.
    Buy some distilled water - You can get that from your supermarket or local pharmacy. Distilled water is sold where they sell bottled water.

    1. Fill your cylinder and gently drop in the hydrometer.
    2. Noting that hydrometers are in fact calibrated to take readings at a specific temperature (60 F is typical, I think), check the temperature of the water. The warmer the water the less dense it will be but if the calibration was for 60F then at 60F your water sample should be accurate.
    3. Spin the hydrometer to dislodge any air bubbles and to ensure that the hydrometer floats freely.
    4. Take a reading. Your eye should be at the level of the surface of the water and you are looking for where the bottom of the meniscus intersects with the specific gravity scale on your hydrometer. Murphy's Law ALWAYS dictates that that scale is at the other side of the hydrometer! You will need to give the hydrometer a little twist.
    5. The intersection should be at 1.000. If the temperature of the water is a little higher than 60F the intersection may be a hair below 1.000 and if the water is colder the intersection may be a little higher (water (and most liquids) become less dense as the temperature increases and more dense as it drops)
    Hope that this helps.
    Last edited by bernardsmith; 03-31-2017 at 09:12 AM.

  4. #4
    Join Date
    Jun 2016
    Brookline, NH


    I always thought this guy (Craigtube) did a really good job.


  5. Default

    A hydrometer measures specific gravity (SG), which is the density of the liquid. Density is expressed in grams/liter. Water has a density of 1000 grams/liter. Wood and oil have less (800-900 grams/liter), that's why they float on water. Sugary water has a higher density than 1000.

    Adding 10 grams of sugar to one liter (=1000 grams) of water will result in a 1% sugar solution (1 Brix). This solution will now have a density of 1004 grams per liter. This scale is somewhat linear for reasonable values: adding 100 grams of sugar to one liter gives a 10 brix solution (10% sugar solution), with a density or specific gravity of 1040 grams per liter.

    If I give you a sugar/water solution without telling you how much sugar is in there, you can derive this by measuring the SG: e.g. If you measure 1040 grams per liter, you know this is a 10% sugar solution by weight. If you know how much liquid you have, you can derive the total amount of sugar as well. A hydrometer does just this: it tells you what the specific gravity is on one of the scales, and the equivalent amount of sugar/liter on another scale. If it reads 1.04 on the SG scale (a density of 1040 grams/liter), it will read 100 grams sugar per liter (or 10 brix) on another scale. The hydrometer is actually a hollow tube, and the lower the density of the liquid, the deeper it will sink. Careful calibration allows you to derive a density.

    Now, you are planning on fermenting this sugary water. Every sugar molecule gets broken down into one ethanol alcohol molecule, and one CO2 molecule. These two latter molecules weight roughly the same (ethanol slightly lighter than CO2), so this gives a ballpark figure of 1 gram sugar becoming 0.5 gram alcohol. (0.52 to be exact). So if you turn all the sugar in your solution into alcohol, you know exactly how much alcohol there will be as long as you know how much sugar there was to begin with. Your hydrometer tells you this as well: It knows the sugar density, so it also knows what the alcohol density will become when all this sugar is fermented. This explains the third scale: The potential alcohol scale. This way, the density of the liquid (SG), the amount of sugar/liter, and the possible future amount of alcohol, can all be derived by a single device.

    There are several caveats here: First of all, the potential alcohol level is only reached if all sugar is fermented. If your ferment stalls earlier, there will be remaining unfermented sugar, and hence a lower final ABV. The difference between the original and final gravity can be used to derive the ABV in these cases. It will be roughly the difference between these two potential alcohol values. Second, all this assumes that the water is at 20 degrees C, the scientific "room temperature". Colder/warmer water is denser/less dense, and the SG readings will be off. Your device usually comes with instructions, containing an equation to correct for different temperatures. Furthermore, these numbers assume that your solution consists only of water and sugar (and not just any type of sugar: sucrose). There are other sugars that behave slightly differently, and other soluble chemicals in your must that will also alter the gravity.

    The mechanics of reading a hydrometer is explained very nicely by bernardsmith above. I would only like to add that the scales can be confusing: These are small numbers with lots of significant digits, and there is not much room on the little piece of paper inside. That's why they list only the significant digits: there is a line that says 1.000. But the next numbers are 10, 20, ... These are the final significant digits: If I end up at 10, it means the SG is 1.010 (the last two digits of the 1.000 become 10). Further down the scale you find 1.100, again followed by 10, 20, 30, ... Same story here, only the final two digits get changed from 1.100. So finding a 30 below 1.100 means a SG of 1.130. Note that the fourth digit is not written explicitly, but can be derived from the scale lines. These are often in 0.002 increments, not 0.001, so watch out there.

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