Dew Point vs. RH

[naboz]

NEWS FLASH;
IT IS HOT!!!
I am hoping to get Matthew L. to chime in here about this, as I sit in this Heat Wave and sweat while not even moving!
There is so little to do that won't bring on heat illness, and even trying to bring out a guitar and play just doesn't cut it; sweat down the arm, on the body between me and the guit', etc.
And when I look at the dewpoint at *77, yet the RH is 54%, I wonder if that level of DP is also not good for a guitar's parts.  It seems to me that it is a major factor in sweating (and hence, moisture condensation)--whether on a glass on the table, my body, or a guitar body.  I have just left my guitars in their cases and havent even been thinking of keeping them out. P.S. We don't have central air.
Any knowledgeable opinions?

[unclrob]

  Bernie GET AN A/C.....

Only kidding.I wish I could help living in Florida we're use to the heat and the humidity.I'm playing mostly outdoors these days.I can say that I'm not having any problems with my guitars though I'm not my satin finish guitar out anymore just because I'm afraid that wiping it down my cause it to semi-gloss up.

[hatofthecat]

naboz don't worry the dew point is the temperature at which the air carrying the amount of water it currently is would reach 100% RH and the water condense out.  That is exactly what you see with condensation round a cold glass.  Simply put the warmer the air can the more water it can hold.  So if you were suddenly to cool your house down to dew point temperature you'd get instant indoor fog     Air pressure also works the same... but unless you are planning to suck all the air out of your home suddenly this should not be a problem... please don't try that experiment either 

Aircon can up the RH a lot since by reducing the temperature it reduces how much water the air can hold.... so if the air isn't thirsty for it the water will go into your guitar's wood causing it to warp in extreme cases.  This is nothing like the risk from a too dry guitar though.

Pete

[jeremy3220]

Aircon can up the RH a lot since by reducing the temperature it reduces how much water the air can hold....

AC units remove water from the air and every AC unit I've experienced removes enough to make up for the temperature drop (thus lowering the RH). I live in TN and never have a problem indoors with the humidity being too high during the summer. It actually stays in the low forties most of the time.

Oh and hat is right, don't worry about dew point. All the guitar cares about is RH.

[hatofthecat]

AC units remove water from the air and every AC unit I've experienced removes enough to make up for the temperature drop (thus lowering the RH).

I was thinking of those crappy "evaporative" air coolers    Yup, with a proper aircon the water drops out of the air as it passes through the cooling elements drying it out though you can get temporary droplets of water in suspension coming out.

Pete

[Danny]

  Buy a sling psycrometer, take a few mechanical engineering courses and you will be qualified to evaluate the dew-point. Or just get a cheap humidistat and pay attention to the action on the neck.
  btw I have a Cooper meter that reads dew-point and I never use that function. It is redundant.

[Zohn]

Dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.

Relationship to human comfort:

When the air temperature is high, the body's thermoregulation uses evaporation of its perspiration (sweat) to cool down, with the cooling effect directly related to how fast the perspiration evaporates. The rate at which perspiration can evaporate depends on how much moisture is in the air and how much moisture the air (at the same pressure and temperature) can hold. If the air is already saturated with moisture, perspiration will not evaporate. The body's cooling system will produce perspiration in an effort to keep the body at its normal temperature even after the rate it is producing sweat is less than the evaporation rate. So even a body at rest (i.e., not generating additional body heat by exercising or manual labor) can become coated with sweat on humid days. (Heat stroke occurs when the body's perspiration system gives up, so the skin of a heat stroke victim is typically dry.) It is the unevaporated sweat that tends to make one feel uncomfortable in humid weather.

But the air that affects comfort is not the air where the thermometer and humidity meters are located. It is the air that is touching one's body. As that portion of air is warmed by body heat, it will rise and be replaced with other air. By moving the air away from one's body faster, as with a natural breeze or a fan, the sweat will evaporate faster, making perspiration more effective, i.e., you feel cooler. The more unevaporated perspiration, the greater the discomfort.

http://en.wikipedia.org/wiki/Dew_point