humidistat question

Hello all, this is my first post.

I am a humidor builder looking to design and build an active humidification
device for my larger humidor models. I am looking for plans or help in
doing this, any advice or links are much appreciated.

The device would simply turn on a 40mm fan when relative humidity drops
below a fixed point (probably 66%). Problem is, outside of some game
console rewiring (wink!) I don't have much experience in electronics.

Thanks in advance for any help you can offer.

Sincerely,

Greg

 

Sorry left out one detail. The accuracy should be within 3%, and the device
should switch off at around 70%RH.

Thanks!

 

How does running a fan change the humidity content of the air?

The simple way to hold a fixed relative humidity in a small, enclosed
volume is to have a glob of hydroscopic salt exposed ot the air in the
volume. Various salts have different equilibrium relative humidity
values with air.

Wet table salt has about a 75% relative humidity equilibrium with air
over the normal room temperature range. Just put a perforated false
bottom under whatever you want to keep moist in your humidor. If the
humidity goes higher, the salt gets wetter by absorbing moisture. If
the humidity goes lower, the salt dries out, releasing water.

 

John,

Thanks for the reply. Activating a fan passes air across a wicking fiber
that sits on top of a distilled water reservoir. This puts moisture into
the air and equalizes the RH throughout the cabinet. What you are
describing with salt is a passive system, in which humidor builders
typically use a 50/50 propelyne glycol/distilled water solution to maintain
70% RH. Problem with passive systems is that they dry out very quickly in a
large cabinet humidor. The humidors I build can be as large as 18 cubic
feet.

Greg

 

The most common relative humidity sensor used, today, is a variable
capacitor that is sensitive to humidity:
http://www.digikey.com/scripts/us/dksus.dll?Criteria?Ref=141335&Cat=32375562

Some of these are just a sensor that needs some AC capacitance
measuring circuit to convert the capacitance ot a humidity signal, and
some have this circuit included. Then you need a comparator to make
the on off decision for the fan as the humidity rises and falls
through the decision value.