I think it'll vary according to the model. Most that I've seen referred to in books have 60 degrees as the temperature they're calibrated to, but the hydrometer I bought from my LHBS says it's accurate at 68 degrees. It should say on the pack. Alternatively, put some water in the flask and see what reading each of your hydrometers gives - if they read the same, it'll probably be OK to use them interchangeably.
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Bottle conditioning: Pliny the Elder clone; Tramp's Overcoat Barley Wine
Next up: Vanilla Porter
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