I had a big red style of hydrometer. I found when new it was repeatable but once used and rinsed several times was inaccurate,
and imprecise and not very repeatable.
I now use an OTC 4619 glass turkey baster style. Precise and repeatable as long as no bubbles are sticking to float, jury's till out on accuracy.
5 years is pretty good on a set of 24's, but they might have some service left in them. Might not be worth the 24$ core return. I've had three chances to turn the screwy31 in for a core charge and put that cash in my pocket, but it still has enough capacity for my workshop LEDs and fans.
I'll core charge it when it shorts a cell.