How to Calculate the SAFE minimum Thickness of an Viewing Pane Crystal?

Fragility increases proportionally to the ratio of the diameter to thickness. If thickness stays the same, as the diameter increases so does the fragility factor.

Minimum thickness of a window required to with-stand a pressure difference may be calculated by the following:

Th = thickness, inches
DIA = unsupported diameter, inches
P = pressure difference, psi
MR = modulus of rupture, psi

Pressure at 1 atm = 14.7 psi = 101.324 kPa
Modulus of rupture (MR, psi) of commonly used IR Crystals:

BaF2 3,900
CaF2 5,300
Ge 10,500
Sapphire 65,000

NOTE:
The minimum thickness of a CaF2 crystal to resist 1 atmosphere (14.7 psi) to be:
2 inch window = 2.8 mm 3 inch window = 4.2 mm 4 inch window = 5.6 mm

25 psi is the safety point where arc protected switchgear pressure relief devices are designed to work.

The minimum thickness of a CaF2 crystal at this point needs to be:
2 inch window = 3.7 mm 3 inch window = 5.5 mm 4 inch window = 7.3 mm

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