In-line sight flow indicators, also known as a sight glass and Visi-Flo, offer a simple, inexpensive means to qualitatively gage flow rate, direction, color and clarity. Typical applications are on filter, lubrication and cooling lines, to positively verify fluid flow or as backups for meters, switches, process indicators and other control devices.

Modular units allow the installation of various indicators within a standard body, permitting easy access for servicing or interchanging indicators without removing the body from the process line.

A number of different indicators have been developed to serve specific needs:

A propeller indicator is the best way to show opaque liquids and is also ideal for observation at a distance. But if flow rate is too low, the propeller will not turn.

A bi-directional flapper not only indicates flow, but points in the flow direction.

A plain bi-directional sight flow indicator without a flapper is used where observing color and clarity are more important than verifying flow.

A drip tube is used in vertical lines. The drip tube ensures constant visibility by keeping the fluid from dripping onto the glass.