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http://www.cnt.com.hr/proizvod/noris-sight-flow-okno-dn65/

Sight Flow Indicators
PresSure Products Company's sight flow indicators are used in piping to provide 360-degree observation of material flow through a line. A typical configuration consists of two circular sight glasses positioned perpendicular to the direction of the flow and opposed to one another. Our sight flow indicators reflect the advantages of our sight glass designs, namely the ease of sealing, maintenance, and availability in higher pressure/temperature combinations in larger sizes than any other manufacturer offers in the industry. Since fluid flow is always driven by pressure, ease and robustness of sealing is even more important than in plain sight-window applications. PPC's unique radial sealing of the glass decouples the sealing stresses from the process pressure, and enables higher sealing stresses to be applied without endangering the glass--which in turn enables higher pressures to be held.
http://pressureproducts.com/sight-flow-indicators/?gclid=CK3...