[time-nuts] Temp/Humidity control systems?

Poul-Henning Kamp phk at phk.freebsd.dk
Thu Oct 27 11:06:27 EDT 2016

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 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:


Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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