The heat preservation effect of the vacuum flask is related to how much water in the bottle
You can look at the two extremes, only a little water, and filled with water to make sharp contrast.
Thermal insulation is the best water level to try to hold more water, but water not touching the bottle cap, the heat preservation effect is the best. Helping to say (don't buy that kind of plastic bottle cap, or use of the good. (hope I made of glass vacuum sealer will catch on and solve the problem of fragile)
Have relationship, if special full, heat preservation effect is bad, the stopper from the water a few centimeters of distance is better, particularly less should cold water also is faster
