Measurements with hydrometers all have a temperature at which the were standardized. Unless you are measuring a liquid at the standardized temp, a correction is necessary. The larger the temp differential, the larger the correction.
To take his further, high quality hydrometers need to be calibrated. The paper scale can move or the weighting change. Even durther, you should even calibrate a thermometer.
In the end it's all about understanding fluid dynamics and the error you are willing to accept in your measurements.