Hydrometer Temperature Correction

Er, because that is the temperature at which the hydrometer reads correctly, perhaps?

It's got something to do with the difference in expansion rates between the materials-of-construction of the hydrometer and the material in which it is immersed...

Check for the below link, which tells that "Your hydrometer usually states what temperature to use it at (normally 15-20 C). If you have a liquid with another temperature there are formulas to calculate the correct value. But then of course you need an accurate thermometer as well."
http://www.hambletonbard.com/how-to-make-wine-beer-moonshine-alcohol/what-is-a-hydrometer/how-to-calibrate-hydrometer.html
But even i could not find why correction is required for the measured value of SP. Gravity.
If i've fuel temperature and corresponding Sp. gravity measured from the hydrometer, why the correction is required, unless i want to compare with the Sp. gravity at standard Temperature (Ex. 15 Deg C)?
But why the temperature is mentioned on the paper scale on the Hydrometer?
Please somebody tell me, how the readings are marked on the Hydrometer along its length and how the mentioned temperature plays a role?

Reread post # 1.

Read about coefficient of expansion.

Repeat as required until understanding is achieved.

Spose it has anything to do with the density change all fluids go through with temperature changes?

Dear Friend,

It is not hard and fast rule and necessary that calibration is to be done at 15 Deg.C. In Sugar Industry, the Hydrometers are calibrated at 20 Deg.C

DHAYANANDHAN.S