Sight glasses are available in a variety of stainless steel grades plus specialty alloys. It’s important to select a sight glass constructed of the right material for the application. Get it wrong, and the metal may experience pitting and corrosion. On the other hand, overspecifying material drives up cost unnecessarily.
Technically not a stainless steel, Hastelloy is a nickel alloy built for the most punishing chemistry a plant can throw at it. For most process observation work, it is far more material than the application requires, and you pay for that unused capability. In contrast, a sight glass made of Duplex 2205 stainless steel handles the corrosive conditions the vast majority of sight glass installations see, at a fraction of the cost. Knowing where the real dividing line sits keeps money in your budget without putting your process at risk.
Process engineers face a practical question when specifying a sight glass: what do you need to see? That question should dictate the shape question, not a default preference for round or obround windows.
Visibility is the primary requirement. Cleanability matters too, but it sometimes flows from the sight glass shape you choose rather than the other way around.
Once you understand what your process needs to reveal, selecting the right sight glass shape and the cleaning approach fall into place.