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What is a Demister?

Mary McMahon
Updated May 17, 2024
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The term “demister” can be used in reference to several different types of devices which are used to remove mist or fog. The device being discussed is generally clear from the context of the conversation, as they are utilized in very different applications, from cars to scuba diving equipment. These devices are available from a number of different manufacturers, some of whom also develop custom products for specialty uses.

In the scuba sense, a demister is designed to remove fog from inside diving goggles. There are a number of tactics used by divers, snorklers, and other people who wear goggles to remove fog and to prevent the buildup of fog. In this case, the demister is actually a chemical compound which is swilled through the goggles to remove buildup and to prevent future buildup of fog. Demisting solutions are available from stores which sell dive supplies, as well as through diving catalogs.

Many cars have a feature known as a demister which is used to deal with condensation on the window glass. Since condensation can impair visibility, drivers want to be able to remove it as quickly as possible. The demister heats the glass, encouraging the condensation to dissipate and clearing the window. These devices are generally not designed to melt away frost, with frost requiring a scraper for successful removal to clear the window unless the frost is very light.

In the industrial manufacturing sense, a demister is a filter which is fitted into vapor processing units. As vapor passes through the demister, droplets adhere to the filter, pulling large droplets out of the vapor. The droplets can be filtered out to the bottom, allowing the vapor to rise and exit the device from the top, while the fluid collects at the bottom. Once passed through a vapor separator, the respective vapor and water can be utilized in a variety of ways.

Classically, the demister used in a vapor processing unit is a demister pad made from knitted metal mesh. The device is usually designed to be cleanable and replaceable so that it can be serviced as it wears down with use. One issue with mesh pads is that the demister tends to trap particulates, as droplets often form around particulates, and as a result it can become fouled over time. Thus, it is important that it be serviceable by technicians and users so that the vapor processing unit is kept in good working order.

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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 aviva — On Jul 11, 2011

I used to have a terrible problem with condensation building up on my vanity mirror on our boat until my husband installed a heated mirror.

It's the only mist eliminator that truly works in my opinion. It's a lot more expensive than those defogger sprays we've been using, but in the long run we feel we've gotten our money's worth.

By Markus — On Jul 11, 2011

@ladyjane - I use the same demisters on my bathroom mirrors that I use on my vehicle's windshield. You can purchase them from any auto parts store or in the auto section at most all discount stores.

By ladyjane — On Jul 09, 2011

I can't think of anything more frustrating than stepping out of a hot shower to find the mirror is covered with steam. This is especially frustrating in the morning when I'm trying to get ready for work and I have to wait nearly fifteen minutes for the steam to evaporate.

It seems like wiping it down with a towel only makes matters worse because it leaves a big smear. I find myself peeking through a tiny area that is clear enough just to put my make-up on.

Isn't there any solution to the problem? Can you buy a mirror demister or something that quickly removes the steam?

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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