Originally Posted by HomebrewJeff
Personally, I would not recommend dumping anything back into the fermenter after it comes out. If you are using a plastic hydrometer tube (as most are), it can have tiny scratches on the surface that can be very difficult to clean / sanitize, even with star san. For the 1/2 cup or so of beer you lose, it's a big risk IMO to possibly infecting the entire batch.
I don't believe that yeast will make a significant impact to the FG, but I wasn't really able to find any information to back that up.
If your using the tube that the hydrometer came in, I'd agree. The tube is made of very flimsy plastic which can't stand up to too many proper washings. I assumed the original post was using a different kind of container because he said jar.
I use a plastic tube that looks like a graduated cylinder, but without the markings on it. I wash it out after each use. I only pour the sample back if I sanitize both the tube and the hydrometer. I agree that it is a risk, but I don't think it's a big risk. Any time you remove the lid to your fermentor you are risking infection. Transferring wort to the primary, creates a risk. Transferring beer to the secondary creates a risk. The risk isn't big, but it is a risk. I think that transferring wort from a hydrometer sample is of comparable risk to any other time the lid is removed and the wort is transferred. Whether someone feels that this is a risk worth taking is up to them.