On the very bottom of the picture is the Encole sight glass where the window is bonded into the housing. In the middle and the top of the picture there are fused sight glasses. The two fused sight glasses have drastically different pressure, temperature and corrosion resistance ratings. The shiny one – the top of the picture is rated to 1450 psi, and 280 degrees C. It’s also chemically resistant to most oxidizing environments. The sight glass in the middle is weaker, rated to 125 psi and 150 degrees C. It’s also made from zinc plated carbon steel, less corrosion resistant than the one on the top.
The sight glass on the bottom has all the strength and corrosion resistance of the best fused glass plus optical clarity bar none, as shown with this NPT thread page in the background. Courtesy Machinist Handbook, circa 2010 edition.
All sight glasses shown are standard NPT 1/2″ thread. Usable aperture size differ significantly, as seen in the readable text behind the glass.
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