When determining the level of a potable water hydro pneumatic header tank fitted with a tubular sight glass, what must be taken into consideration?

Correct Answer: Option D — The actual water level in the hydro pneumatic header tank is identical to the level shown in the sight glass.

In a hydro-pneumatic tank, the tubular sight glass is connected to the tank at the same pressure levels. Because the liquid column in the glass is subject to the same pressure as the tank interior, the level in the sight glass accurately reflects the actual water level within the tank.