Never seen this device. Definitely getting one tho!TM hydrometer is the one. I have 2 other and can use them as long as I know what the TM reads vs them.
My skimmer removes 300ml a day and I add it back manualy. My system is large enough that 300ml does virtualy nothing to the SG though.
Stability is king.
I saw hydrometer and thought of this fossil I still have somewhere lol