Borosilicate glass is the most common type of optic material used in Penberthy gages. Borosilicate glass offers good resistance to most chemicals at temperatures at or below 600°F (315°C).

 

Penberthy uses tempered borosilicate glass in its gages to improve thermal shock resistance. The tempering process involves heating the glass to a glassy transition point and rapidly cooling the glass to create compressive stress in the outer 1/6 of thickness.

Aluminosilicate glass has a higher viscosity and a lower thermal expansion coefficient compared to Borosilicateglass. Aluminosilicate glass is commonly used where process temperatures are greater than 600°F (315°C) but at or below 800°F (427°C).

Fused quartz is made from crystalline silica with few impurities (50 ppm by weight). The highest temperature rating, 1000°F (538°C), is achieved by using quartz.

Materials other than the quartz limit a gage assembly's maximum operating temperature.

Penberthy offers quartz in its model TH only. The physical properties of quartz require an extremely flat seating surface. Flatness tolerance of gasket and cushion seating surfaces on chambers and covers must be maintained to avoid torsional or point stresses on the quartz.

Shields (e.g., PCTFE, Mica) can be installed to prolong the life of transparent flat glass in corrosive or erosive environments such as hydrofluoric acid, sodium hydroxide or steam.