Sodium Hydroxide in high concentration tends to "frost up" glass rendering a sight glass unusable. Fifty percent caustic will freeze or crystalize at about 50 degrees F.

This is not only a problem for the sight glass that is exposed to cold ambient air but will also tend to freeze up inside the chamber of the magnetic level gauge. Fortunately, the stainless steel chamber can be heat traced with low pressure steam or other hot medium.

Caustic soda is a very common and very nasty material found in many plants. It tends to form white crystals on everything in sight. Since the level gauge is all stainless steel and glass, it can be hosed down occasionally which solves the problem of corrosion and readability.