[time-nuts] Temp/Humidity control systems?

[jimlux]

On 10/27/16 8:06 AM, Poul-Henning Kamp wrote:
> --------
> In message <837330DB-2015-4AE5-8C9C-F444F569FC1B at n1k.org>, Bob Camp writes:
>
>> Your “time cave” does not have a specific spec on temperature or on humidity.
>> You get to pick a number for either one. Anything in the “non condensing” (let’s
>> call it < 80%) range for humidity is likely ok. Temperature up to 40C is probably
>> ok for any gear that I can think of. As long as you *never* go in and out of the
>> cave, comfort in the cave is a non-issue.
>
> Danger Will Robinson!
>
> Under no circumstances should your dewpoint be above the temperature on
> the other side of the door.
>
> If you have 40C at 80% humidity your dew-point temperature is 35C, which means
> that whenever you open the door your clock cave will fog up.
>
> If you run your clock cave at 40C, humidity needs to be well below
> 30% to hold the dewpoint below room tempreatyre.  That is both hard,
> expensive and prone to electrostatic discharges.
>
>> Assuming the humidity in the basement is under control (if not, fix that), all I need
>> to do for humidity in the closet is to exchange air with the basement.
>
> "fix that" is usually non-trivial, and from very to horribly expensive.
>
> The cardinal rule is that you should only exchange air (basement/outside,
> or cave/basement) when the air outside has lower *ABSOLUTE* humidity.
>
> And I keep stressing that it is *ABSOLUTE* humidity, because people simply
> don't pay it enough attention.
>
> 25C/40%RH air holds 9.2 g/m³ water ... as does 15C/72%RH air.
>
> This is why a lot of people in costal climates who ventilate their
> basement during summer "to dry out the basement" get the exact
> opposite result:  The air outside is a lot wetter than on the inside.
>
> One would think that somebody had designed fans to measure this,
> but it is expensive: Cheap humidity sensors measure relative humidity
> and you need to correct for both temperature and pressure to get
> absolute humidity.
>

And having looked into this in some detail - there's a whole lot of "go 
look at a psychometric chart" when you try and write some code to do the 
conversion.

It's similar to the "steam tables" (in fact, it's identical to the 
problem of generating steam tables, it's all about vapor pressure of 
water vs temperature and such)
There's not some nice equation to solve - It's more done by an iterative 
solution of the equation that goes the other way.


https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4813973/pdf/sensors-16-00398.pdf
has some of the background



> And therefore, at the risk of repeating myself again:
>
> If you build your clock cave in the basement, it should be air-tight
> and you should manage the humidity in it separately from the rest
> of the basement.
>
> PS: Here is a good webcalculator:
>
> 	http://go.vaisala.com/humiditycalculator/5.0/
>
>