If a glaze has already been mixed and gelled to give it thixotropy these things won't bob up and down to home in on the right level. If the glaze is watery enough there are other issues. The one on the right has a 1.0-1.7 scale. Since most pottery glazes need to be 1.4-1.5 specific gravity (40-50 on this scale) it is difficult to get a very accurate reading. And it is long, you will need a container tall enough to float it and enough glaze to fill it. The small hydrometer appears better, it has a scale of 1.2 to 1.45. But it really bobs up and down (so it is even more important that the slurry be runny and thin to give it the freedom to do so). It is better to weigh a measured volume of glaze slurry and calculate the SG instead. The easiest is a 100cc graduated cylinder (from Amazon.com), if 100 ccs weighs 140 grams, that is 1.4 specific gravity.
In ceramics, the specific gravity of casting slurries and glazes tells us their water-to-solids. Body slurries especially require tight control of this property for performance reasons.
The deflocculation process is the magic behind the ceramic casting process. It enables you to make a slurry of far lower water content and thus lower shrinkage.