Tubular Sight Glasses:
Diameter and wall of glass tubing is expressed in millimetres and generally length is also although there is some tradition still of describing gauge glasses in inch dimensions.
The common types are:
SG-901 gauge glass, DURAN®, fire polished ends, annealed.
These are available in a range of sizes but the most common which replace the old inch sizes are:
12 x 2.2 (replacing 1/2”)
16 x 2.5 (replacing 5/8”)
19 x 2.2 (replacing 3/4”)
SG-900 sight glass (tubular), DURAN, parallel ground ends (for compression between flanges).
Again available in a wide range of diameters, typically, these are relatively short for their diameter and are made from heavy-wall tubing. For instance, SG-900, 80 X 5.0 X120 which describes a parallel-ground sight glass 80 mm in diameter with a 5 mm wall and 120 mm overall length.
Disc Sight Glasses:
Specified by diameter and thickness:
SD-920 sight glass disc, soda float glass, annealed
SD-921 sight glass disc, soda float glass, heat strengthened
SD-922 sight glass disc, SCHOTT Borofloat 33, annealed
SD-923 sight glass disc, SCHOTT Borofloat 33, heat strengthened
Discs can also be manufactured from other vitreous materials such as Schott Robax (glass ceramic) and clear fused quartz (silica).