What is the Measurement Principle of Sight Glass Type Level Gauge?

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Using a sight glass is probably the easiest way to measure the liquid level. The observation mirror is installed outside the water tank, so the liquid level can be observed through the glass. The sight glass is marked with a scale to measure the liquid level. The main disadvantage of this method is that it only gives a partial indication of the liquid level of the container.

When a part of the container is made of transparent material or the liquid in the container is bypassed through a transparent tube, a visual indication of the liquid level can be obtained. The advantage of using a shut-off valve with a bypass pipe is that it is easy to disassemble for cleaning.

Sight Glass Type Level Gauge

Type of level gauge:

Transparent Level Gauge

The transparent level gauge uses two pieces of transparent glass, both sides are equipped with liquid chambers. The indicated liquid level is the result of the different transparency of the two media. For water/steam applications, the illuminator is installed on the rear side of the meter and its light is deflected upward into the water column.

When the light incident on the separation surface of water and water vapor is reflected back to the observer's eye, this allows the observer to see the illuminated surface of the water.

Electromagnetic level gauge

The magnet containing the float follows the liquid level in the liquid chamber, which corresponds to the liquid level in the tank.

The position of the float in the room is indicated by the externally embedded two-color baffle (turned by 180 degrees and set to a uniform color along the direction of the float), which is embedded by a magnet.

Reflection level gauge

The principle of the reflective level gauge is based on the difference in refractive index of liquid and vapor. The liquid column is contained in the recess of the liquid chamber behind the sight glass, which is fixed to the gauge body. The observation mirror has a prismatic right-angle groove on the side facing the liquid and vapor space. Whether the light entering from the outside of the gauge is absorbed or reflected depends on whether they enter the liquid or vapor space. When light hits the surface of a groove in the vapor space, it will be reflected the opposite surface of the groove, and then reflected from there, completely reflected back to the viewing direction.

Therefore, the vapor space appears silvery white. When light hits the surface of the groove in the liquid space, the light will be completely absorbed, making the liquid behind the glass appear black. These gauges are used to measure the liquid level in the container.

The sight glass is also not suitable for dark or dirty liquids. When measuring hazardous liquids, do not use this type because the glass tube is easily damaged or broken. In installations where the temperature of the meter is lower than the process temperature, condensation may occur on the outside of the meter, which reduces the accuracy of the reading. The simple sight glass is an older technology and is rarely used in automatic control applications.

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