Flapper Type Sight Flow Indicators are recommended for horizontal and vertical pipeline with upward flow. These units are best suited transparent or slightly opaque solutions and are used for gases also. Position of the flapper indicates flow. One important use of these flow indicators is, in pipe lines where a downstream value may remain closed and even through the flow indicator remains full, there may not be any flow. In such cases, the flapper position alone can indicate where there is flow or not.