Sight glass

[1] 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.

The gauge glass on a boiler needs to be inspected periodically and replaced if it is seen to have worn thin in the vicinity of the gland nuts, but a failure in service can still occur.

[1] Familiarity with this disquieting occurrence was considered so important that a glass would often be smashed deliberately while a trainee driver was on the footplate, to give him practice in fitting a new tube.

[3] Although automatic ball valves are fitted in the mounts to limit the release of steam and scalding water, these can fail through accumulation of limescale.

In the region that is contact with the gas, most of the light is reflected from the surface of one groove to the next and back towards the operator, appearing silvery white.

Well-known makes of reflex gauge are Clark-Reliance, IGEMA, TGI Ilmadur, Penberthy, Jerguson, Klinger, Cesare-Bonetti and Kenco.

The gauge consists of a vertically oriented slotted metal body with a strong plain glass to the front and the rear.

Unlike the reflex gauge, the glass has a plane surface which it does not need to be in direct contact with the media and can be protected with a layer of a caustic-resistant transparent material such as silica.

The level indicator consists of a number of pivoting magnetic vanes arranged one above the other and placed close to the chamber containing the float.

[5] Sight glasses enable operators to visually observe processes inside tanks, pipes, reactors and vessels.