Hyrdometer to use???

[stephane]

I use the plastic box from seatest for about six years and it never fail

I check it twice a year whit a glass one but it always good reading

BUT IMO you need to take care of them alway rinse them and put it upside down on a towel

once a year put vinegar one and let it soak one night to make shure that no mineral deposit will
stick on the arm and foul the reading