When taking hydrometer readings, is it acceptable to return liquid to the fermenting vessel after taking a reading in a vial? Must the vial/measuring cylinder and hydrometer have been sterilised to make this an acceptable practice?
I don't re-introduce from the hydrometer vial, the four books I've read on homebrewing (Papazian, Miller etc...) suggest that you don't because this is a place where you can contaminate your beer.
I doubt that there is a large risk because you've sanitized it all, but, I prefer to not take risks I only take a couple hydrometer readings so I don't lose that much beer.
I also take this time to enjoy the flavour of the product.
I put the liquid back in, just because I don't want to lose a pint or more across all the readings. To do this I sanitize the hydrometer and the thief, inside and out, before taking the sample.