Sight glass

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Water gauge on a steam locomotive. Here the water is at the “top nut”, the maximum working level.

A sight glass or water gauge is a transparent tube through which the operator of a tank or boiler can observe the level of liquid contained within. In the case of a boiler, the pressure of the water below and the steam above is equal, so any change in the water level will be seen in the gauge. It is important to keep the water at the specified level, otherwise the top of the firebox will be exposed, creating an overheat hazard and causing damage and possibly catastrophic failure.

Simple sight glasses may be just a plastic or glass tube connected to the bottom of the tank at one end and the top of the tank at the other. The level of liquid in the sight glass will be the same as the level of liquid in the tank.

However if the liquid is hazardous or under pressure, more sophisticated arrangements must be made. The transparent tube (the "glass") itself may be mostly enclosed within a metal or toughened glass shroud, reinforcing the tube against the pressure of the liquid, preventing it from being damaged through scratching or impact and offering protection to the operators in the case of breakage. The first locomotive to be fitted with the device was built in 1829 by John Rastrick at his Stourbridge works.^[1] Automatic or manual valves are commonly installed at each end of the glass to prevent a catastrophic draining of the tank in case of breakage. To check that the device is offering a correct reading and the connecting pipes to the boiler a not blocked by scale, the water level needs to be “bobbed” by quickly opening the taps in turn and allowing a brief spurt of water through the drain cock.^[2]

Sight glasses were often used to allow a boiler operator to control the operation of the boiler feedwater pump or injector. Nowadays, sophisticated float switches have now replaced sight glasses in many applications.

[edit] References

^ Snell, John B (1971): Mechanical Engineering: Railways. Longman, London.
^ Unidentified author (1957): Handbook for steam locomotive enginemen. British Transport Commission, London.