A double window sight glass, often abbreviated as M.S. (which typically stands for Mild Steel), is a type of sight glass commonly used in industrial applications. It is designed with two transparent windows, typically made of glass or other durable materials, separated by a chamber. This design allows for viewing from both sides of the sight glass, providing redundancy and flexibility in monitoring processes. 

Material: The main body of the sight glass is often made of mild steel (M.S.), known for its strength and durability. The windows are typically made of materials such as borosilicate glass or toughened glass to withstand the pressures and temperatures of industrial environments.

Design: The sight glass consists of a sturdy metal housing enclosing two transparent windows, separated by a chamber that contains the process fluid. The windows are typically circular or rectangular in shape and are securely sealed to prevent leaks.

Dual Viewing: The presence of two windows allows for viewing from both sides of the sight glass. This feature provides redundancy and flexibility in monitoring processes, as operators can observe the contents from different angles or positions.

Pressure and Temperature Resistance: Built to withstand high pressures and temperatures commonly encountered in industrial applications, ensuring safety and reliability in demanding environments.