Here is a neo techy explaniation.
Guage Glasses / water glasses / sight glasses and any other name it goes by all work on the same principle, that being that water will always seek its level.In the case of a Water Glass, if the openings in the steam and water admission points of the glass are unrestricted water will flow into the glass and balance itself against the steam pushing from the opposite direction and mimic , exactly, the level and movement of the water in the boiler as it seeks to rise to the same level outside the boiler.
Guage Cocks: These predate Water Glasses as the primary means of determining water level in boiler. They work by allowing the Firemen or Engineer, to see if they have all water , a water steam combination or steam with no water as a rough estimate of the water level in the boiler depending on what comes out of each valve.
They are the most accurate when the engine is not working. When under load however they lose accuracy becsaue of a rise of water near the backhead called a Water Surge. This is an upsurge of water casued by convection currents and evaporation near the back of the boiler . This hump of water can make the Guage Cocks lie to you showing a level of water that exists only a few inches from the backhead but not within the entire boiler itself.
Since the Water Glasses draw their water from well below this hump and their steam well above it, they allow the operators to see the true level of water in the boiler reguardless of any surge taking place.
There is a certain ammount of lift that occures in the glass when the engine is under power. This rise is caused by the steam being drawn off from the boiler causing the water level to appear to rise slightly when the throttle is open and settle lower when it closes. Usually this lifting action is not dramatic but there are instances when it can be a problem.
When a boiler is Foaming badly the water level in the glass will rise and fall far more dramatically due to Priming that happen with a foaming boiler. On locomotives with smaller water spaces when foaming occured the nominal water level under power could drop to at or below the visible range of the glass when the throttle is closed. Drop plugs helped with the conciquences of an unfortunate event such as this , but an experienced Fireman could recognize the signs of foaming and take the proper measures to control it as best as possible.
so thus ends this little disertation , hope that all didnt come out a clear as a Mud Ring.
K. S. Wilcomb
Engineer - Ret.
