Originally Posted by unionrdr
The reading should be 1.000 @66F. Purity of tap water isn't an issue with gravity,so it should read 1 flat.
I'm not going to check the instructions with my hydrometer, but I am pretty sure all of them are calibrated to 1.000 in 59 F water.
If the temperature is higher, the reading will be lower. .005 error (reading low) would be +40 degrees, or 100F.
To the OP. Tap water is not necessarily pure, and might come in somewhere between .999 and 1.001. If you do confirm the hydrometer is reading .005 low, all you need to do is add .005 to all your readings.
If the hydrometer does not read correctly, then something has shifted. I would be concerned that it could shift again sometime.