In many cases, fluid levels may be observed directly and consequently measured to obtain trends or magnitudes in volume.
Graduated scale:
Level is measured directly from a vertical graduated scale partially immersed in the liquid.
Glass window:
Level is observed through a transparent window in the side of a tank. The window may be graduated with a vertical scale.
Gauge glass:
Level is observed in a transparent vertical tube attached to a closed tank. The bottom of the tube is connected to the liquid space and the top of the tube to the gaseous space. The liquid level in the tube corresponds with the level in the tank and may be observed or measured against a graduated scale. Isolating valves usually are fitted in the upper and lower connecting pipes to allow for replacement of the transparent tube (which is usually of glass) without draining the tank (Fig. 1a).
Fig. 1 Direct visual and float-type measurements. (a) Gauge glass. (b) Float with electrical sensor. (c) Float with magnetic switches. (d) Float with buoyancy effect.
Closed tanks are often under some pressure, hence the use of external tubes able to withstand pressure rather than windows. In pressurized systems the upper and lower connecting tubes are fitted with ball check valves to avoid a dangerous discharge of fluid in the event of a tube rupture. For very high pressure systems, metallic ducts with thick glass windows are used. These windows are sometimes made refractive and artificially illuminated to show more clearly the difference between the two fluids (such as water and steam).