Hydrometer differences

[Centar]

I have two beer and wine hydrometers, one indicates 20% potential alcohol at a S.P. of 1.150 and the other says its 21% on the paper scale inside...how can this be? the only diff between them I can find is one is calbrated at 60 F and the other at 20 C which is about 5 degrees C between them but that still shouldn't change the scales with reference to one another....what gives? (the bailing scales do this too)

[Angel_Kefka]

if tehy are calibrated for use at different temps and you are testing the liquid at one temp then it would read different, because the temp is wrong for one and temp affects density, which is whaqt they actually read.

[goose eye]

Them cheap ones go on abouts. They got some hi doller testers calybrated for differently ranges if you worryed that much about what is is

So im tole

[F6Hawk]

Calibrate them both by putting them in the correct temperature plain tap water. Each should read 0.00. If not, gently tap the end of the hydrometer to get the paper inside to move slightly with your finger tip until it reads correctly.

[oliver90owner]

Hydrometers are not a measure of absolute density; it's close enough for use for fermentation, but not absolute.

SG is simply a comparison of the densities of two liquids (one being water), so has a numerical value with no units (density is mass/volume).

So if you compare at two different temperatures the different volumetric expansions of the two liquids will always lead to differences in the readings. For brewing/distilling the small differences are ignored (and likely immeasurable), as long as the standard temperature, at which the hydrometer is calibrated, is adhered to. So if you see results quoted to 3 or 4 significant figures, ignore the 'claimed' precision and go for about 2 significant figures at best. It is a rough and ready indication.

Remember the density of water is a maximum at 4 degrees (that is why ice always forms on the surface of water - which is lucky for life, as we know it) and only closely approaches 1 000kg/cubic metre. At 20 degrees the water has already expanded in volume by about 0.2%. Close enough to ignore by lowly fermenters, but not by scientists doing precision work.