Level measurement with sight glass


At this point I would like to introduce a simple solution from Jürgen with which you can implement level measurement without electronics that always works reliably. It dates back to the early days of steam shipping and is based on a liquid level indicator and a system of communicating tubes. The liquid level of a tank is connected to an open glass or Plexiglas tube via a hose system. Due to the direct connection to the tank, the same liquid level in the tank is displayed in the sight glass. In Jürgen's boat, this technique is used to determine the contents of the water tank.

Fig.: Measuring principle diagram

Fig.: Sight glass at the top of the stairs

There is a ventilation hose at the upper end of the sight glass to enable pressure equalization when the level of the liquid in the sight glass changes. Depending on how the ventilation hose is laid, it may happen that some water comes out of the ventilation hose when the boat is in position. It is also important to ensure that the tank is not filled to the point of overflowing, otherwise the excess water will flow into the boat. Alternatively, the ventilation hose could also be routed outboard. However, this requires a breakthrough in the boat's hull. A sensible solution to prevent overflow would be a non-return ball valve, which automatically closes the opening when water overflows.


  • Safe functioning
  • Simple solution without electricity
  • Material can be obtained from hardware stores
  • Works with water, diesel, oil, wastewater


  • Only applicable if tank is mounted above the floorboards
  • Safe overflow necessary
  • Only a small display is possible for flat tanks

Below you can see some detailed images of the implementation.

Fig.: Upper outlet on the sight glass

Fig.: Corner connector

Fig.: Ventilation hose at the top end

Fig .: Check valve with ball (RST)