What happens to the reading on a sight glass when temperature changes?

The reading on a sight glass can become inaccurate when the temperature changes primarily due to the thermal expansion or contraction of the fluid within the sight glass. As the temperature rises, most liquids expand, potentially leading to a higher level reading than what is actually present in the container. Conversely, if the temperature decreases, the liquid contracts, which could show a lower level reading than the true amount.

This phenomenon illustrates that the physical properties of the fluid—such as density and volume—are influenced by temperature. Therefore, while the sight glass itself may provide a visual reading, it does not account for these temperature-induced changes in liquid behavior, resulting in potentially misleading measurements. Recalibration may be necessary only in specific systems where precise measurements are critical, but generally, thermal effects are what primarily impact the accuracy of the reading in a sight glass.