Because cylindrical gauge glasses frequently jump, nowadays Reflex or Klinger Glass is increasingly being used for low vapor pressures. The glass is about 10 to 15 mm thick and rectangular in cross-section, which makes exploding glass to be a rarity. On the inner side of the glass are triangular-shaped grooves, arranged in a longitudinal direction. As a result, the light is reflected, at the location of the steam. Whereas this is not the case with water. The result is a clear separation between steam and water. The steam portion is bright, the water part dark. In connection with the reflective effect, this glass is also known as reflex sight glass. Sight glass protectors are not necessary.
For high steam pressures the reflex glass cannot be used. Because of the softening agents used in the boiler water and high temperatures, the glass is affected. As well as the condensation of the steam on the ribbed surface, the reading of the actual level is less accurate.
For high pressures, transparent sight glasses are used.