What exactly does a hydrometer measure?

[volvodude]

Does a hydrometer measure just materials dissolved in the wort, or would items that exist as precipitate in the wort also influence the gravity reading? Just a mindless wondering in the shower this AM.

 

[GilaMinumBeer]

Specific Gravity as a result of density of liquid. Thus they must be calibrated to the density of the liquid to be measured and is why there are a variety of Hydrometer on market to measure anything from Beer to Anti-Freeze..

It is more likely that CO2 in solution will skew the accuracy more than solids like hop debris or trub but, yes, if the solids manage to attach to the instrument they can and will skew the result.

 

[daisyflower]

Does a Brix refractometer do the same thing as a hydrometer?

 

[PorterIV]

A Brix refractometer does essentially the same thing but it is important to note that the presence of alcohol skews the result. So, a refractometer is not useful for once ferementation has started.

 

[GilaMinumBeer]

So, a refractometer is not useful for once ferementation has started.

Entirely untrue once you have determine the correction factor.

A refractomer does work to provide similar results as a hydro but relies on refraction of light based on density of a liquid at known angles.

Alcohol refracts light at a different angle than a sugar solution. A mixture of the two further convolutes the issue.

However, one may take a sample with both a hydro and a refractometer and compare the difference to determine a correction factor for post fermentation erfractometer readings and be reasonably accurate.

BeerSmith has a utility for this as well.

 

[BigEd]

Does a hydrometer measure just materials dissolved in the wort, or would items that exist as precipitate in the wort also influence the gravity reading? Just a mindless wondering in the shower this AM.

A hydrometer measures the relative density of a liquid compared to that of pure water. Only those materials actually dissolved in solution will technically influence the reading. However, as mentioned above bubbles or other buoyant stuff adhering to the hydropmeter can skew the reading. Taking a gravity reading of the wort measures the density of the sugar-rich solution. This density is greater than water and the more sugar is there the higher the gravity reading. A Brix refractometer measures the sugar content of a solution, like grape juice from a winemaker's vine or the wort for your beer. It's not the same as a gravity reading but we can use it for the same purpose.