A tubular or flat reflex glass is mounted between shutoff valves on the water column, with a drain valve below. Normal operating level is usually around the middle of the glass, and a glass showing no water means the burner must be shut down, not that more water should be added at once to a hot boiler. Glasses are blown down regularly to prove the connections are clear, and they are guarded against breakage.
Gauge glass is showing about a third full and bouncing, check what the feed pump is doing before it trips the low water.
If you can't see water in the glass on a fired steam boiler, secure the burner first; feeding cold water onto hot dry steel is how boilers crack.
Also heard as
- water gauge glass
- water glass
- sight glass
- glass
Related terms
Water column
A vertical chamber piped to a steam boiler's steam and water spaces that carries the gauge glass and level controls.
Try cocks
Small valves at set heights on a water column, opened by hand to check whether water or steam sits at each level.
Low-water cutoff
A safety control that shuts the burner off when the boiler water level drops below a safe point.
Bouncing glass
A gauge glass water level jumping up and down instead of holding steady, a warning sign of surging or dirty water.