Do yourself a favor, get a refractormeter to replace your hydrometer

[Randar]

I got mine from Northern Brewer long before I knew about the ones on eBay. Not sure about any differences in quality, but it's a real possibility.

Also, I seem to be able to get reproducible results from samples taken from the same source (within 0.2 Brix), so that gives me the requisite confidence.

 

[BendBrewer]

I only use mine on brew day but a lot of times that is the only day I take readings. If I do take a reading later on, I use the hydrometer.

Go visit a local brewery. Chances are you'll see a couple of hydrometers before seeing anyone use a refractometer.