Re: What is the optimal relative humidity of a room containing electronics?

Good question.  As in a lot of things, you have to define the environment 
more closely, and the values are a function of the operation you're trying 
to perform.  I don't know of any absolute "optimum" humidity level in any 
circumstances.  Also, the acceptable ranges seem to be a matter of opinion 
rather than hard fact.  There are several situations to consider:
1.  IC manufacture:  in my experience, the minimum humidity is considered 
to be 50% to avoid electrostatic discharge (ESD) problems.  The humidity in 
the rooms is constantly monitored and an alarm sounds if the relative 
humidity drops below 30%.  The rooms have humidifiers in them to ensure 
that the relative humidity never drops below this value.  I believe that 
the upper acceptable range is 70%, for the worker's comfort and to avoid 
problems with the wafer handling equipment.  The humdifier's setpoint is 
usually 50%.
2.  IC packaging:  bone dry is usually used to avoid getting moisture in 
the IC package.  Extreme ESD precautions are mandatory.
3.  Electronics manufacture using packaged devices:  30%--70%;  the 30% 
figure is to minimize ESD problems; the 70% for the same reasons as above.
Please note that other ESD reduction measures (grounding all equipment, 
wrist straps for the workers, air ionizers, grounded working surfaces, 
cotten uniforms) are invariably used.
4.  Operating Electronic equipment: The 30-70 range is usually used, for 
the same reasons as stated above.  In your case I'd try for the low end of 
the range to minimize arcing.  However, I'd also implement the ESD 
precautions discussed above for people's safety.

KEY POINT:  These are the values used in industry, but I've never heard of 
any solid research into the "optimum" level.  All of these numbers were 
developed through experience rather than derivation from first principles!