# [Guide] Anticipating Condensation

If you’ve ever been worried about water suddenly appearing in your PC, this guide is for you.  I’ll start off with some critical background information.

(Note: since this is a scientific discussion I will be using metric.)

What is Humidity?

Humidity is water in the air.  Think of the air like a sponge.  It can hold some amount of water, ranging from none whatsoever (bone dry), to as full as it can be without leaking/dripping (raining/condensing).  Where we are between these two extremes is specified as “Relative Humidity”, where 0% is bone dry and 100% is as full of water as it can be without any condensing.  This 100% full situation is called being “saturated”.  Humidity can never be more than 100%.  Take the situation of raining, for example.  In all likelihood, the air itself is saturated, but there are also physical drops of water in the air that you can feel and see.  These don’t count, however.  We just look at the air itself, and not a total of a given volume, which would include the saturated air as well as liquid water.  There is almost always some amount of water in the air – some humidity – and this can be easily checked on any good weather site.  Depending on where you live, and the recent weather conditions, it could be anywhere from a very dry 15%, up to a comfortable 30 – 60%, or even near or at 100%, especially if it is raining

Where this gets interesting is when we realize that the amount of water the air can hold – the actual mass of water that can be present in a given mass of air without it becoming saturated – varies with temperature.  Consider saturated air at 10°.  If we trap the air in a magic container so it cannot change its composition (but volume is allowed to fluctuate so pressure does not change), and then heat it up to say 30°, the mass of pure air, and the mass of water will be the same, but the relative humidity will have gone down from 100% to some other value, because it can now hold more water, and is thus no longer saturated.  If we consider the opposite scenario of taking 30° saturated air and cooling it to 10°, it will have started off at 100% and will finish at 100%, but “some water will have been squeezed out of the sponge” – it will have condensed.  This is critical to understand before moving on to the next part.

Anticipating Condensation

Now that we understand the basics, we come to the fun part.  The psychrometric chart (that is not a typo - I checked twice ) I’ve provided below will be instrumental in predicting condensation.  It can do a wide variety of interesting things, hence the great number of lines, but fear not – it is actually quite simple to use.  We will be focusing on 3 aspects:

• The x-axis (Temperature)
• The y-axis (Mass of water per mass of air – the actual amount of water in the air)
• The red lines (Humidity curves) *

I’ve drawn on this copy of the chart shown below to illustrate the next example.

Spoiler

Consider that we have 30° air at 20% humidity, and we plan to cool it down to 10°.  Will there be condensation?

1. Start at the point on the chart that represents 30° and 20% humidity.  This is marked as point “1” on the diagram above.
2. Now temperature is going down.  That means we move left on the diagram without changing our position on the y axis.  This makes sense since the y axis represents the actual mass of water in the air, and this is not changing.
3. Keep moving until you are at a point that represents 10°.  This is the only condition you are looking for.  This point is marked as point “2” above.
4. Notice the humidity line that you now lie on.  According to the chart, the 10° air will be at 70% humidity.
5. This means no moisture will condense, since we have not reached saturation (100% humidity).  Another way to look at it is that the mass of moisture in air (our position on the y axis) has not changed, so we must still have all the water we started with.  If that is true, we must not have lost any to condensation.

Note that the actual amount of water (the mass of water) in the air has not changed.  Just, the amount of water that is there is now closer to the limit of what the air can hold, since that limit has gone down by cooling it.

Consider another example.  What if we took 30° at 40% humidity down to 10°?

1. You should be able to find the starting point on your own by now.  It is marked as point “3” above.
2. Move straight across t – WAIT!  Remember, humidity cannot be more than 100%! – Move straight across until you hit the saturation curve, then follow it down until you hit 10°.  This is marked as point “4” above.
3. You will notice that we have now moved on the y axis!  This means there is less water per unit of air than we started with.  Well, the amount of air hasn’t changed, so that means we just have less water.  But mass cannot be deleted, so it is not “gone”: it is just no longer in the air… this is the definition of condensation.  Moisture that was in the air as humidity has condensed into liquid water as a result of attempting to cool beyond saturation.

Why do I care?

Consider you just got back from a LAN party with your PC in the truck bed, or perhaps more realistically, you just got a new shipment and the package has been outside in the mailbox in the winter for a while, or perhaps you were using your device while outside (think phone) and it is very cold, or consider a project like whole room water cooling.  In these situations, you are (potentially) bringing a cold object into a warm room.  If there is a cold object, like a pipe, or a computer, or anything else in a warm room, the air immediately surrounding and touching the surface of that object will cool down, potentially to the same temperature as the object itself, as that object warms up.  If that object is cool enough, and the room is humid enough, it will cause condensation.  “But how do I know if that will happen?” you ask.  Well, that is why you should care about this guide, and that is when you use the skills you just learned.  Find the point on the chart above that represents the temperature and humidity of your room.  Move across to the temperature of the object you are concerned about.  If the process reveals that condensation will occur, take precautions!  Try to dehumidify your room, or leave time for the object to warm up so condensation will cease and existing moisture will evaporate from it.  Air conditioning is well known as a way to cool things, but it was actually invented for the purpose of dehumidification, and as such it does an excellent job thereof.  Get some if you can!

I hope this was useful to someone!

*Edit: If you go looking for other charts, be careful to check that they specifically mention "Relative Humidity".  There are other ways of rating humidity, such as "Percent Humidity", and these are not the same thing!

Edited by Ryan_Vickers

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A well thought out and written, informative post, but I have to ask.....

11 minutes ago, Ryan_Vickers said:

Consider you just got back from a LAN party with your PC in the truck bed

WHY ARE YOU TRANSPORTING YOUR PC IN THE FRICKIN BED OF A PICK-UP TRUCK!?!?!?

END OF LINE

Quote me so that I always know when you reply, feel free to snip if the quote is long. May your FPS be high and your temperatures low.

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Just now, DevilishBooster said:

A well thought out and written, informative post, but I have to ask.....

WHY ARE YOU TRANSPORTING YOUR PC IN THE FRICKIN BED OF A PICK-UP TRUCK!?!?!?

I'm not, but someone might

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7 minutes ago, Ryan_Vickers said:

I'm not, but someone might

That someone would need to be smacked upside the head repeatedly.....

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Quote me so that I always know when you reply, feel free to snip if the quote is long. May your FPS be high and your temperatures low.

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On 5/5/2016 at 11:42 AM, DevilishBooster said:

That someone would need to be smacked upside the head repeatedly.....

It's just an example of how your computer might get cold.  Another example is if you ordered a part through the mail and it sat in the mailbox outside in cold temperatures for a while before you brought it inside.

Edited by Ryan_Vickers

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