Rayotek’s 316 stainless steel/fused glass NPT Sight Windows are rugged, thermal and corrosion resistant Sight Windows. Rayotek’s NPT Sight Glasses are the right choice for high vacuum to ultra high pressure in applications such as pumps, engines, furnaces, compressors, hydraulic systems and boilers. Rayotek NPT fused glass Sight Windows are so robust that even when cracked from mechanical damage they continue to seal and protect against release of pressure.