coolant reservoir

[crawdaddy]

95 diesel 1.9. I noticed two hoses, one small dia, one large. The smaller (overflow) is above max level.
My question is, with engine running, stat open, is the coolant in the reservoir constantly being turned over?
Don't want to take a reading with an antifreeze hydrometer and get the wrong reading.

[OM617]

No, the top hose only vents air if the system pressure gets too high.