This is a Flapper type sight glass that can monitor flow of the liquid, presence of foreign matters etc..
The product used of sight glass is SLA-SERIES and the product suitable for high pressure, temperature
and opaque fluids is SLB-SEREIS and there are also SLBE,SLBM-SERIES that attached contact signal
to SLB-SEREIS. SLBE-SERIES are registered in design and patent.
  Flow measurement is applied from the principle of the flow meter. The differencial pressure in front and behind the flapper generates force moving the position of the flapper.
Accordingly, this movement has an affection on the area and force that fluid passes and the flapper
stops in the position that the difference in pressure and force is in equilibrium. Here, the size and shape
are adjusted so the flow and the degree of flapper are in proportion.