The influence of humidity when creating a vacuum

In summary, the conversation discusses the difficulties in creating a vacuum and evacuating a chamber when the air inside is humid. It is explained that this is because water vapor is difficult to pump out and requires a combination of flushing with purge gas and baking to remove it. Nitrogen is commonly used as a purge gas because it is easy to pump and can help achieve faster evacuation times. The conversation also mentions the importance of understanding vacuum technology and suggests consulting a book for more information. The use of hydrogen as a purge gas is questioned, as it has a tendency to penetrate enclosure materials, and the tendency of some molecules, such as water, to get adsorbed on surfaces is discussed. The conversation ends with the clarification that the topic is related to
  • #1
Karido
4
0
Hello folks,

I have two simple questions and some ideas about it:1. Why is it harder to create a vacuum and takes more time to evacuate the chamber when the air inside is humid?

Humid air is lighter than dry air. So I have no idea why it takes longer...2. Why do you use nitrogen to vent the chamber?

Is it because nitrogen is lighter than air and so at the same gas density you have more particles of a light gas than of a heavy one and because the pressure only depends on the particle noumber density (same temperature)? So I reach the pressure compensation with nitrogen faster and I use less mass. When I evacuate my chamber I have a lighter mass inside it and the amount of time to evacuate is shorter because my pump has to work less?

I really appreciate any help and explanations. Feel free to mention details if you like! :)

Thanks in advance guys!
 
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  • #2
Because water is difficult the pump out.
The speed at which you can pump depends on the particular molecule (and the type of pump used); nitrogen is easy to pump which is why it is used to flush vacuum systems.
Water is much more difficult to pump on, and generally speaking you need to use a combination of flushing with purge gas (e.g. nitrogen) and baking of the system to get rid of it.

The difference in pump speed between molecules depends on several factors: the weight of the molecule is one factor (helium pumps slowly) also important is the tendency of some molecules -such as water- to get adsorbed on surfaces, once on the surface you can only get rid of it by raising the temperature (baking) or "hitting" it with another inert molecule (e.g. purging with nitrogen).
Any good book in vacuum technology will have tables where the pumping speed of different molecules is tabulated.
 
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  • #3
Hello Karido, welcome:
Karido said:
Feel free to mention details if you like
Same to you :smile: ! These questions come from a context, right ? A lab or a textbook story ? Or perhaps homework (for which we have a separate forum !

1. Who says that it is harder ? What chamber ?
2. What gave you the idea to use nitrogen in the first place ? Why not hydrogen or carbon diaoxide ?
 
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  • #4
Karido said:
Why is it harder to create a vacuum and takes more time to evacuate the chamber when the air inside is humid?

see
Water vapor is frequently removed by pumps that operate with water or
steam as a pump fluid,
for example, water ring pumps or steam ejector
pumps. This depends considerably on circumstances, however, because
the economy of steam ejector pumps at low pressures is generally far
inferior to that of rotary pumps.
For pumping a vapor – gas mixture in which
the vapor portion is large but the air portion is small, the vapor can be
pumped by condensers and the permanent gases, by relatively small gas
ballast pumps..

If you are actually involved in generating Vacuum
can consult...<https://www3.nd.edu/~nsl/Lectures/urls/LEYBOLD_FUNDAMENTALS.pdf> [Broken]
agree with
@BvU comments... above
 
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  • #6
f95toli said:
tendency of some molecules -such as water- to get adsorbed on surfaces
That's what I would have thought also.
 
  • #7
BvU said:
1. Who says that it is harder ? What chamber ?
2. What gave you the idea to use nitrogen in the first place ? Why not hydrogen or carbon diaoxide ?

Wow... Well you can ask questions if you start writing something about the topic... you don't have to know the surroundings. Start posting constructive stuff and stop this "separate forum" stuff! I know where to post! Thanks!

So, to mention some details, I just want some basic knowledge about this topic because I am doing some SEM research! And Nitrogen is used in this context to vent the chamber and get faster evacuation times! Not finding too much on the fly I think it is a nice topic for a discussion! Thanks for the other answers!
 
  • #8
If you frequently work with vacuum pumps etc (as you will do if you use a SEM) if might be worth getting a book about vacuum technology. I still use Chambers et al "Basic Vacuum technology" as a reference. I bought it when I took a course in vacuum technology back when I was an undergraduate in the late 90s but I think it is still available. It is quite a good "practical" book and reasonably up to date.
 
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  • #9
Karido said:
Wow... Well you can ask questions if you start writing something about the topic... you don't have to know the surroundings. Start posting constructive stuff and stop this "separate forum" stuff! I know where to post! Thanks!

So, to mention some details, I just want some basic knowledge about this topic because I am doing some SEM research! And Nitrogen is used in this context to vent the chamber and get faster evacuation times! Not finding too much on the fly I think it is a nice topic for a discussion! Thanks for the other answers!
I can't dive into the background of each and every poster for each and every thread. If you had referred to your first (!) thread, that would have helped me help you better a great deal.

In the forums you know so well, there are guidelines to make life better for all. Your original post (in this thread) looked a lot like that from a high school kid having to answer a few questions from the instructions leaflet.

For my masters I worked with a He cryostat at a few K with a pretty good vacuum and for my PhD with the best vacuum this side of the moon. The sheer fact that I answered, even without you giving any context ('the' chamber ?) should indicate that I actually try to provide constructive help. Sorry you mistook my further inquiries as being non-constructive. They were not intended as such.
 

1. How does humidity affect the creation of a vacuum?

Humidity can affect the creation of a vacuum in several ways. Firstly, the presence of water vapor in the air can reduce the overall pressure within a vacuum chamber, making it more difficult to achieve a complete vacuum. Additionally, humidity can cause condensation to form within the vacuum chamber, which can interfere with the vacuum creation process.

2. Can humidity levels impact the efficiency of a vacuum?

Yes, the humidity levels can significantly impact the efficiency of a vacuum. Higher levels of humidity can make it more challenging to achieve and maintain a vacuum, which can result in a longer and less efficient vacuum creation process. It can also impact the performance of vacuum pumps and other equipment used in creating a vacuum.

3. How does humidity affect the integrity of a vacuum?

The presence of humidity in a vacuum can compromise its integrity. Humidity can lead to the formation of water droplets within the vacuum chamber, which can cause air bubbles to form and disrupt the vacuum. This can also affect the accuracy and reliability of any experiments or processes conducted under vacuum conditions.

4. Are there any ways to control humidity when creating a vacuum?

Yes, there are several methods to control humidity when creating a vacuum. One way is to use a desiccant, such as silica gel, to absorb moisture from the air within the vacuum chamber. Another method is to preheat the chamber and materials used in the vacuum creation process to reduce the risk of condensation forming.

5. Can humidity affect the longevity of a vacuum?

Yes, humidity can affect the longevity of a vacuum. High levels of humidity can lead to the formation of rust and corrosion within the vacuum chamber and its components, which can reduce the lifespan of the vacuum. It is essential to maintain proper humidity levels to ensure the longevity and effectiveness of a vacuum.

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