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What is a Sight Glass?

Mary McMahon
Updated May 17, 2024
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A sight glass is a device which allows people to see the level of fluids or gases in a system, or to look inside a closed system to see what is happening inside. Glass panels which allow for internal views are sometimes referred to as viewing glasses. Sight glasses are used on many different types of equipment and they add to the safety and ease of operation. Like other equipment parts, a sight glass is replaceable if it becomes damaged or compromised in some way.

A very simple sight glass can consist of a glass tube which is linked with the contents of something such as a tank. The level of fluid in the tube indicates the level of fluid in the tank. The back of the sight glass may be patterned or striped to make it easier to see the fluid level. Larger tubes may also offer people an opportunity to identify contaminants in the fluid which might interfere with the function of the device.

These devices can also be used to monitor pressure, especially with equipment like steam boilers. In this case, the sight glass contains both fluid and vapor and the varying level indicates how pressurized the environment is. Equipment like espresso machines can be equipped with a glass gauge which allows people to determine the level of pressure in the system so that adjustments can be made as necessary. For safety, the glass in such gauges is usually tempered and reinforced to reduce the risk of explosion.

Viewing glass can also be highly useful. For example, many heaters which run on gas, kerosene, and other fuels have a viewing glass which people can use to see if the pilot light is on or not, and to make sure that all of the jets in the heater are working. Likewise, viewing glass allows people to see into kilns, fermentation tanks, and so forth. The glass provides a port for viewing without altering the environment inside, ensuring that the device will function properly while offering people an opportunity to monitor the contents safely.

Companies which make sight glass gauges produce a variety of sizes and styles to meet the needs of equipment manufacturers and operators. It is important to make sure that glass of appropriate strength is used to reduce the risk of breakage or cracking. The sight glass can also have a testing mechanism which people can use to confirm that the glass is measuring levels properly.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a About Mechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By lluviaporos — On Aug 21, 2011

@KoiwiGal - I think most of the time, rather than a digital display a float switch is used now instead of a pressure sight glass. Usually they only need it to do one thing, switch off when it reaches a certain level, and a float switch is better at that task without using a sight glass.

That said, both a traditional tank sight glass and a digital readout are not uncommon in certain kinds of machinery.

As long as the sight glass is maintained properly, it works just as well as any alternative means of seeing how much liquid is in a tank.

By KoiwiGal — On Aug 20, 2011

I've noticed it's become trendy to include sight glasses on things like toothpaste tubes recently.

I think it can get a bit silly actually. I mean, it is usually fairly obvious when you need to change your toothpaste tube!

But, I do appreciate the electronic version of a sight glass in my MP3 player. The little icon that shows how much juice you have left in the battery.

I think a lot of the time now, sight glasses are electronic and are linked to a digital display rather than an actual liquid sight glass that shows how much is left.

Here's hoping they never come up with a digital version of the toothpaste sight glass.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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