What is the recommended way to check the water level in a kettle with a sight glass?

Filling the kettle about halfway up the sight glass is recommended because it allows for an adequate water level to ensure optimal steaming while preventing any risk of boiling dry, which could lead to both equipment damage and operational inefficiencies. A sight glass provides a visual indicator of the water level; filling it to the halfway mark helps maintain a balance between having enough water for consistent steam production while avoiding the problems associated with having too much water. These problems could include potential overflow or flooding, where water may end up in areas it shouldn't, which can cause safety issues and damage to heating elements.

Filling the kettle to the top of the sight glass runs the risk of overflow if the water expands while heating, while underfilling by one-third may not provide enough water for effective steaming, potentially leading to overheating. It's crucial to ensure that the kettle operates at its optimal water level, and filling it about halfway strikes that necessary balance.