Level indicators and sight glasses

Direct level measurement using a ruler

Usually, these devices are simple and produce only a visual output. Direct measurement is performed by inserting a ruler inside the reservoir so that the zero of the ruler coincides with the bottom of the reservoir. The surface of the liquid will mark the reading point on the ruler, which can then be removed, and the level reading can be taken from the mark.

In the case of a translucent reservoir, the ruler can be placed against the reservoir and the reading will be taken without contact with the content.

Figure 1.1: Direct level measurement using a viewer.

Figure 1.1 shows the level measurement in a reservoir using a ruler and an external glass viewer. The measurement technique in Figure 1.1 is based on the law of communicating vessels. Attached to the reservoir there is a vase, or transparent, graduated tube, which, due to equal pressure on the surface, keeps the liquid at the same level. 

In a slightly more rustic way, civil construction workers use this same principle to align walls using a transparent hose containing water, as can be seen in Figure 1.2.

Figure 1.2: Photograph of level measurement with a transparent hose.

The main advantage of glass displays is their low cost. For this reason, these devices are widely used for local monitoring of liquid levels in reservoirs, except in places where pressure and temperature conditions prevent their use.

Tubular glass sight glass

The shapes and types of windows vary and must be analyzed according to the application. Tubular sight glasses consist of tubes connected to shut-off valves that can vary according to needs. The length and diameter of the tube depend on the conditions of use (temperature and pressure) and must be rigorously analyzed when choosing the device. 

The tubular glass sight glasses have a standardized diameter and length, so these values are related to the maximum temperatures and pressures. In general, it is not recommended to use glass displays in systems that contain toxic or flammable liquids, or that have pressure and temperature higher than those specified by the manufacturers, due to the ease of rupture and the possibility of accidents. It is also recommended that viewers do not exceed the specified maximum length. In cases where very large tracks are needed, the use of two or more viewers is recommended.

Flat glass sight glass

Another type of geometry used is flat glass windows. In fact, due to their construction characteristics, these sight glasses become more robust and have replaced many applications previously made with tubular sight glasses. Figure 1.3 shows what flat glass glazings look like, which can be made up of one or more glazing modules (sections). 

Figure 1.3: Flat Level Measurement (a) schematic; (b) photographs.

The main disadvantage of displays with modules is the lack of visibility between sections, but they can work in higher temperature and pressure ranges, in addition to offering less risk of rupture due to mechanical shock.

Wet dipstick

Another very simple method for level monitoring is the use of a wet dipstick. This method is commonly used to monitor oil in combustion engines and fuel at filling stations. It consists of inserting a dipstick into the reservoir to the bottom and observing the length of the dipstick that is wetted by the fluid. Figure 1.4 illustrates the use of this method.

Figure 1.4: (a) Wet dipstick method and (b) photograph of application in an automotive oil reservoir.