1. Direct fluid visualization systems

1.1 Principle and features

With this method, direct visual inspection of a liquid level is carried out through a transparent wall. In principle, this is a direct measurement only if the device is attached directly to the tank skirt. However, most glass level gauges operate bypassing the main tank, so the level behind the transparent wall corresponds to the liquid head from the plane of the lower connection, and only coincides with the level in the tank if the medium is stable, homogeneous and isothermal. In a boiler drum, for example, due to steam generation, boiling and priming caused by a sudden call for steam, the level indicated is fictitious, which does not prevent it from being very useful.

The transparent wall material is very often glass, but can also...