[pogodzib]

Hydrometers are usually consistent as long as the salt is fully mixed and aged, you test the water at the same temp. every time, and you make sure that there isn't bubbles on the swing arm. If you are getting different readings, more than likely one of the previously mentioned reasons is usually the cause. With that said, the readings that the hydrometer is giving you probably isn't accurate. From what I have found over the years, hydrometers are generally up to 0.005 ppm off. Get a good refractometer, that has automatic temperature compensation (ATC) and you'll know that you're getting an accurate reading every time (if calibrated properly).