Conductivity of air change ; RH or Grains moisture?

[datman]

Does the conductivity of air change in accordance with absolute moisture content (ie grains of moisture) or is it affected by relative moisture content (i.e. reltive humidity ratio).

It would seem logical that as air expands, so does its ability to hold vapor, hence the conductivity decreases as the percentage of moisture increases. Or is the conductivity related to absolute moisture content.

Can anyone lead me to any information explroing this topic?

The general conditions I'm interested in are for temp ranging between 50 and 100°F with 20-80%RH in relation to ESD control.

 

[saxon]

datman, The ability of any gas to hold water as vapor is pressure and temprature dependent and is know as the science of Psychrometry. In order to understand this relationship find the appropriate Psychrometric Chart. Please bear in mind that the dew points can be listed at either, Pressure dew points/humidity fractions or as Atmospheric dew points/hmidity fractions. You've got to know the pressure-temprature conditions of the gas. You should be able to find the appropriate one on the Web using the following search terms:
Psychrometry, Psychrometiric Charts, Air Psychrometric Charts, etc.

Hope this helps.
saxon

 

[datman]

Thanks Saxon,

I'm trying to understand a little better the effect a higher humidity environment has on air's conductivity as a means to protect against electrostatic discharge ESD for electronic equipment.

For theoretical purposes I am assuming in this case sea level.

I understand that as air is saturated with more moisture is becomes increasingly conductive (equally less insulating)thus allowing any static charge that may build up to dissipate more quickly.

In detail what I'm trying to get a better understanding of is whether the increased conductivity is a function of the grains of water moisture in the air or whether the conductivity is related to the percent of moisture in the air.

Example: If I have equipment in an environment at sea level with a dry bulb air temperature of 72°F and a dew point temperature of 52.4°F (which equals about 50%RH and a humidity ratio of about 58.4 grains. This environment mitigates static discharge quite effectively. If I then raise the temperature (Sensible only) to say 85°F without any change in moisture content, (the RH is now 32.6%, dew point is still a temperature of 52.4°F and a humidity ratio of about 58.4 grains) Does the conductivity of the air change as the RH drops or is it related to actual moisture content of the air i.e. grains/dewpoint?

It would seem logical that as the air's ability to hold more moisture as it is warmed increases so does the conductivity decrease, thus providing poorer ESD protection. Is that true? Thus the link for protection would be to RH%, if this is not true than the link to ESD protection would be dewpoint.

Do you have any additional insight into this?

 

[saxon]

datman, As temperature increases, the number of grains of water vapor the air can hold also increases logrithmically to a given %RH. Also, at a given temp., the number of grains of water vapor air can hold also increases with increasing %RH.

If conductivity of air is a function of water vapor, I would bet that the conductivity would increase with increasing water vapor. It's been my experince that static charges build up when the relative humidity is low (remember wool sweaters in winter time) during high relative humidity static build up doesn't seem to occur. So there must bo some form of low voltage discharge occuring during high RH. As for ESD occuring there must be some threshold level reached based on the conductivity of the air which is related to humidity factors. This sounds like something for a Masters/doctor thesis worked out in the lab.

saxon

 

[azertyuiop]

what is the practicallity of such a question ?

I mean , you can't change the water content in the surrounding air without airconditioning equipment , so what is the use of your question if you don't apply such control mechanism to keep your ESD protections in the correct range ?

Trying to stop the rain falling ?