Sight Glass - The sight glass is the primary way to determine the water level in the boiler. Consisting of a hollow Pyrex glass tube and two angle valves, it is connected to the boiler above and below the normal waterline. As long as the valves do not become plugged, gravity will ensure that the height of the water in the glass matches the height of the water in the boiler, with allowance for the engine sitting out of level, or the water sloshing around due to movement of the engine. Always be sure that both valves are clean and open, so that the glass will read correctly.
    The sight glass should be cleaned at least once a day by alternately closing one of the valves, and then opening the drain, thus letting steam flush any lime or sediment out of the open valve and glass. Closing the first valve, and opening the other will complete the process. Blowing steam through the glass will clean the inside - the outside can be cleaned with a rag. Care should be taken to not scratch the glass, since this can cause it to fracture and break.
    The valves for water columns were often equipped with internal balls that would be sucked against the passage by the flow in the event of a glass breaking, forming a check valve, and minimizing the danger of the operator being scalded by steam. These can often be heard acting during the cleaning process. If you can open the drain fully, without the flow lessening, then your engine may not have the check balls, or they may have become stuck in encrustation.
    Glass changes properties over time, and constant blowing down can erode the ends of the glass tube. The glass should be removed every few years (depending on use) and inspected. Older glasses, or glasses that have become chipped, cracked, or eroded, should be replaced and the rubber packing rings renewed. The original glasses were completely clear, but it is very difficult to find glass tubing in the proper temperature and pressure range that does not have a red stripe inside (the stripe is intended to make it easier to see the water level from a distance, as the water has a magnifying effect).
If a glass breaks while running, you can continue for a while by shutting off the water glass valves (a shovel is handy for deflecting the escaping steam and water) and using the tri-cocks, but you should replace the glass as soon as possible. New glass can be put in while the boiler is hot, but be sure to warm the glass by running steam through it for a while before closing the drain and pressurizing it.