How accurate are glass hydrometers

[johnnyboy1965]

Ive been using the same hydrometer for the last 6 years and have absoltley (sp) no idea if its accurate.
The key is to use only one for your readings (not that i take readings, I gave that up years ago)

Take one hydrometer...it might not be accurate. In water @ 20oC it reads 1.003, your put this into your wort and it reads 1.043, you have a actual OG reading of 1.040...post ferment you have a reading of 1.013, when its actually 1.010

 

[Kronos]

I ordered myself a new Stevenson Reeves hydrometer and today it arrived, having just done the same test it reads spot on. My other SR hydrometer still reads at 1.007 and has gone into the bin. Although they are both SR hydrometers the new one looks completely different and feels slightly heavier even though the are the same height.