Clear Sight Glass

Optically clear sight glass compared to a non-lapped sight glass.

Looking inside a reactor chamber while the process is running can be a rich experience, literally. This type of experience goes on a resume of a progressive scientist and opens more career opportunities. This is one way of looking at the benefits of an optically clear sight glass. Another way to look at it is to recognize that the reactor under observation actually needs understanding of the process going on inside it. A better understanding of the process results in better control. Better control results in a better product, higher yields and lower production costs.

The benefits of a clear sight glass unarguably are very strong. In the picture we compare two sight glasses for optical clarity. Both sight glasses are NPT – threaded housing having glass fused to the metal. These are what is known as fused sight glasses, where the glass is melted inside the metal housing, then the glass cools down at the same time as the metal housing. The glass eventually solidifies, meaning it is no longer a liquid. The metal at that temperature is still hot as the glass continues to relax its stress post solidification. The glass is “annealing” at this temperature range. When the metal and the glass cool down to room temperature, the result is an inseparable assembly of glass and the metal. The product is mechanically very strong. The glass is literally fused to the metal and cannot be separated from it by force, only by chemically dissolving it. Mechanically, a fused sight glass is the safest type of sight glasses on the planet. They do not catastrophically break, they hold high pressures, can tolerate strong vibrational forces and large temperature swings. In every way, except one, fused sight glasses are superior to composite sight glasses, that is, assembled from discrete components.

The only potential disadvantage of a fused sight glass over a sight glass assembled from several components is the optical clarity. Potential that is, not actual. If a fused sight glass is lapped post fusing, this disadvantage goes away. By nature of melting the glass, simultaneous fusing it to metal and subsequent cooling, the glass becomes concave in the process. In other words, the glass surfaces are bowed, non-flat, lens-like, where the thickness of the glass in the center is thinner than at the walls. The glass during fusing “wets” the metal walls by capillary action and tends to creep up the wall surface. This is what makes fused glasses so strong in the first place and this “bowing” is inherent in all fused glasses if they are not post-lapped.

Post-lapped fused sight glasses are typically used in imaging systems requiring good optical clarity. Applications include in-tank camera systems, viewports for in- reactor cameras, as well as windows for a laser sensor.

Leave a Reply

Your email address will not be published. Required fields are marked *