Sight Flow Indicators are identical in Constructions to FSG 210 except that these are fitted with the flappers to indicate flow. These flow indicators are recommended for horizontal pipe lines are vertical pipe lines with upward flow. These units are best suited to transparent or slightly opaque solutions and are also used for gases. Position and flappers indicates flow. One important use of these flow indicator is the line where a downstream valve may remain closed and hence even through the flow glass remain full there may not be any flow. In such cases, the flapper position alone can indicate whether there is flow or not Example :Cooling water flow to bearing may be stopped by a human by a human error of closing a downstream value and only the flapper can indicate the real situation.