Circular Sight Glasses
“extra-hard” borosilicate glass
For visual control of chemical and physical processes.
The quality of a sight glass depends mainly on the chemical composition and mechanical strength of the glass material. Uniform glass quality is maintained through glass analyses and acid / alkali tests . The mechanical strength is attained throught themal pre-stressing . In this procedure the sight glass is brought to a high temperature then quickly cooled – rather as in the hardening of steel . This increases the bending strength and shock resistance of the glass to the values stipulated in the standard. Optical control of the pre-stressing is carried out with the aid of a polarization filter : the stress lines are made visible at the edge of the sight glass as interference colours . Glass which has not been thermally prestressed shows no stressed shows no stress lines and has much lower mechanical strength.