If the upper connection to a gauge glass is closed, what will the gauge glass show if the boiler has a normal water level?

When the upper connection to a gauge glass is closed, the behavior of the water column in the gauge glass is affected significantly. In a functioning gauge glass, water levels are indicated by the pressure exerted by the water in the boiler. If the upper connection is sealed, the pressure that normally contributes to the equilibrium in the gauge glass is removed from the equation.

When the boiler has a normal water level and the upper connection is closed, the water level will rise in the gauge glass until it reaches the same level that is maintained in the boiler. This occurs because the pressure at the lower connection still pushes the water up due to the hydrostatic principle. Thus, if the boiler holds a normal water level, and with the upper connection effectively creating a vacuum or closed system in the gauge glass, the water will measure as full.

It's worth noting that the gauge glass may not reflect the actual boiler pressure accurately, and could lead to a misleading indication if there were any fluctuations, but under static conditions where the upper connection is closed, it shows as full.