Level Measurement

What is a slight glass? How can we use the slight glass for level measurement?

What are the uses of slight glass?

The slight glass is a type of direct level measuring instrument and it can be used to measure the pressurized tanks. It can be used to measure the non-corrosive or non-sticky liquid materials. The slight glass is also known as gauge glass which can do the continuous liquid level indication. The closed tank measurements can be done by the slight glass, also it can be used to do the level measurement of the toxic or flammable process. The tank could be connected to the block valves and this could provide maintenance on the slight glass and the slight glass body. We can also use slight glass for interface measurement.

How does a slight glass work ?

As shown in the above image a slight glass consists of a toughened glass and this glass would be mounted outside the vessel and connected to the vessel through pipes. The liquid level in the slight glass matches the liquid level in the process tank. So when the tank level changes the slight glass level would change in the same manner and thus level measurement can be done. The operation of the slight glass is based on the principle that the equal pressure on the same surface of the two columns could make the liquid to seek the same level. So the level of the liquid in the slight glass would be equal to the level in the vessel. So if the slight glass has a scale in it then it would indicate the liquid level as the direct reading. 

How is a slight glass constructed?

The construction material would be simple glass or plastic material and it can handle good temperature and pressure. There is another type of slight glass which is the reflex type and it has a metal casting tube with thick glass material and this type can be used in high pressure and high-temperature applications.

What are the types of slight glass?

Reflex slight glasses

We can use reflex slight glass for high-pressure applications, the level measurement of the reflex type slight glass is based on the principle of optical law of total reflection of light, and in this law, the light would pass from a medium of greater reflective power to a medium of lower reflective power. So in order to make it applicable, the glass is cut at proper angles in the inner surface and it would remove the light from the back portion of the glass, and also the light will be allowed to pass through the part of the glass which is covered with the process fluid. A clear line marks the height of the liquid surface, above which the glass has a bright mirror-like appearance. The light would be reflected by the grooves in the glass above the liquid but not below the liquid surface. So because of this, the color of the liquid will be the same as the background of the chamber, mostly the black would give the greatest contrast. If the natural light is not enough then a lighted plastic strip will be placed in the backside opposite to the viewer and this would help the reflex glasses to be used in low light areas and also at night.

What are the advantages, disadvantages, and applications of reflex slight glasses?

The reflex slight glass cannot be used in applications where a liquid interface needs to be detected. This type of slight glass needs proper good gasket selection and a special torquing procedure. The weld pad type reflex slight glass can only be used in ambient temperatures.

Window type slight glass

The above image is of the window type slight glass and this device is mounted on the side of the rectangular type tank, like a gas compressor lubricated oil storage and circulation tank. The slot-shaped overlapping window is made of flat transparent heat-resisting glass and it can show the actual level of the oil and it can also show this level with abnormal conditions such as excessive foam or turbulence. This type of slight glasses is not easily damaged.

Tubular type slight glass

In the above image, we can see a slight glass connected to the vessel, and the connection is done on the outside and the connection is done by the help of small piping and also with the help of isolation valves that have safety shut-off ball check valves. In case of any tube breakage, the quick loss of pressure in the tube makes the pressure in the vessel to move the balls and reseal the lines and thus checking the liquid or vapor escape from the process vessel. We need to connect the top of the slight glass to the vapor space of the vessel if it is operated under pressure, if not the glass could blow-out. This type of slight glass is not used for high temperature and pressure applications.

What are the advantages of slight glass?

  • The simplicity and the reliability of the slight glass is really great
  • If the level transmitter fails or if it is taken for maintenance, so then we can use slight glass to measure the level manually
  • It can do the continuous indication of the tank level

What are the disadvantages of slight glass?

  • Slight glass  cannot be used for applications that have sediments or solids
  • The slight glass has a limitation that the if the level is between the glasses and in the brass area and because of this we can’t determine the level
  • This instrument needs frequent maintenance, if there is a presence of hydrocarbon liquids then it could cause staining and this would also make determining the level very difficult.

What are the applications of slight glass?

The slight glass can be used for open and closed tank applications, the closed glass slight glass can be used for pressurized process. Common applications of this device includes pressurized vessels, boiler drums, evaporators, condensers, liquid accumulators, etc. We can use slight glass for low and high pressure applications.


post-graduate in Electronics & communication.

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