Hydrometer accuracy?

A thought occurred to me recently so I thought I would voice it for discussion.
I have been using mostly my refractometer for all my readings and when double checked with a hydrometer I have found the readings to be very accurate, UNTIL I start reading real dry, near and below 1.000, I find the hydrometer to ready lower than I get from my compensated brix reading.
Then I started thinking, how are we compensating for alcohol with hydrometer readings?
Take this as an example just for argument. I start at 1.020 and finish at 0.990,
take that at face value I would have fermented .030 sugar? how when water is 1.000, I only had 0.020 of sugar to begin with. Alcohol being lighter than water accounts for the other 0.010.
Just saying I am not real sure ABV is being calculated properly just because of the effect of alcohol itself on the hydrometer.
Any thoughts?

The height of the hydrometer is based on the density of the liquid. It doesn't know how much sugar you put in originally so that is immaterial. The reason it goes below 1.000 is that the overall density is decreased to less than that of water when alcohol is introduced to the solution. It's not just taking that 0.020 of sugar and making it disappear; it is converting it into alcohol and other byproducts.

As far as deciding which instrument to measure final gravity, I always pick the one that requires fewer conversions, adjustments, or corrections. Each correction you have to make is another vector of error. A hydrometer that is properly calibrated to indicate gravity readings at the correct temperature...is by design the best instrument of the two to use.

menerdari said:

how are we compensating for alcohol with hydrometer readings?

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There's nothing to "compensate" for in the hydrometer reading. All you're reading is the specific gravity of the liquid, irrespective of what the liquid itself is composed of. The presence of alcohol does not throw this off; it's in fact part of what you are reading.

The only "compensation" that comes into play is when using beginning and ending hydrometer readings to approximate the ABV, which is built into your ABV formulas (ex. ABV=[og–fg]*131.25)

The only reason you have to "compensate" using a refractometer is because the refractometer is not actually measuring specific gravity; rather, you are measuring light refraction, which is being used to determine specific gravity...which is in turn used to determine ABV. But since alcohol does throw off the refraction, it must be compensated for.

menerdari said:

Just saying I am not real sure ABV is being calculated properly just because of the effect of alcohol itself on the hydrometer.
Any thoughts?

Click to expand...

If your hydrometer is properly calibrated, it is reading correctly. Remember, the hydrometer is actually measuring specific gravity of the mixture as a whole, not specifically alcohol or sugar content. The affect of the alcohol is part of what you are measuring in the FG reading. You're only using these readings to calculate approximate ABV, based on the change in gravity.

hunter_la5 said:

There's nothing to "compensate" for in the hydrometer reading. All you're reading is the specific gravity of the liquid, irrespective of what the liquid itself is composed of. The presence of alcohol does not throw this off; it's in fact part of what you are reading.

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To be complete, it would be a good idea to mention here that hydrometers must be corrected for temperature. When manufactured, they are calibrated with water at 60F (typical, but RTFM). Density of water changes enough with temperature to produce some pretty impressive errors, and we see this in the Beginner's forum all the time.

passedpawn said:

To be complete, it would be a good idea to mention here that hydrometers must be corrected for temperature. They are calibrated when manufactured with water at 60F (typical, but RTFM). Density of water changes enough with temperature to produce some pretty impressive errors, and we see this in the Beginner's forum all the time.

Click to expand...

True, I was talking about only with respects to corrections for the presence of alcohol, but I see in retrospect that my phrasing was misleading. Yes, hydrometers must be corrected for temperature. Good catch.

A properly calibrated and graduated hydrometer, when the reading is corrected for temperature, is the most accurate of the two. The hydrometers that we use in homebrewing, the $10 models, do not have the quality control during manufacture to be consistently accurate from one hydrometer to the next. But cheap refractometers imho, are not to be trusted. And no corrections are necessary with the hydro as mentioned previously.

To check if your hydrometer is reading correctly, use distilled water at the calibration temperature (usually 60F, but my latest one is calibrated at 68F per the instructions that came with it). The reading should be 1.000. Your tap water may give a different reading.

The calibration temperature will be written on the hydrometer.

60F is more common but I have one cheapo calibrated at 68F

Had a similar problem when using a brix. Brixometers are not good for final gravity readings and are always way off due to alcohol as you mentioned. Use a brixometer for OG and measurements during the hot side of brewing. Use a hydrometer for final gravity readings and cold side fermentation.