What is a common cause of a false level reading in a sight glass?

A common cause of a false level reading in a sight glass is closed sight glass valves. When these valves are closed, it can lead to a situation where the fluid level in the sight glass does not accurately represent the actual level in the tank or vessel. This occurs because the fluid cannot flow freely between the sight glass and the tank, potentially leading to a discrepancy between the level indicated on the sight glass and the actual fluid level.

In contrast, water temperature fluctuations, excessive steam pressure, and normal water column operation do not directly cause a false reading in sight glasses. While water temperature can affect fluid density and may slightly influence readings, it does not typically result in a significant distortion like closed valves. Similarly, excessive steam pressure might create turbulence but wouldn't lead to a misrepresentation of level in the same way that closed valves would. Normal water column operation indicates that the system is functioning as intended and should not lead to inaccurate readings.