Preventing Electrostatic Discharge

[hindsight]

We have all heard that ESD can be reduced by using antistatic spray, wearing cotton clothing, grounding ourselves and work-surfaces to a common earth ground (preferably with a resistor in series with the ground in case the ground is current-carrying), and by working when the relative humidity is high.

 

But my first question is whether it is the relative humidity which is important or is it the absolute humidity? This is more than just a theoretical question. When working outside in the winter after a snow, the relative humidity is quite high, but the absolute humidity is very low.

 

The relative humidity is, of course, how much water is in the air relative to how much water the air can hold at the ambient temperature. Substances are likely to dry quickly when the relative humidity is low because the air can take water from them. The more dry things are, the more static forms on their surfaces.

 

On the other hand if the absolute humidity is low, then the amount of water in the air is low. If the amount of water in the air is low, would there not be a gradient for water in substances to leave, resulting in dry substances with a tendency to form static on their surfaces?

 

I need some help here from the electrical engineers, physicists, manufacturing process engineers or anybody with a bit of smarts.

 

 

The second question is about the likelihood of ESD causing a problem when mixing comps which are wet with polar solvents like ethanol or acetone.

 

Disregarding the flammability of these solvents for the moment, does wetting comps with something other than water decrease the likelihood of spark formation ( either from ESD, piezoelectric, friction, triboelectric or other mechanism) like wetting with water does? I don't know exactly what is responsible for water's antistatic properties--scavenging electrons or whatever--but wonder if other solvents have some of the same properties. Does evaporative cooling play a role?

 

Any assistance in relieving me of my ignorance would be appreciated. --hindsight

Edited February 24, 2014 by hindsight

[BurritoBandito]

I would hardly call you ignorant. I think those are very good questions and look forward to hearing some of the more informed responses, but here's my *assumptions*

 

I think that absolute humidity is just the amount of moisture that is in the air regardless of what the saturation point is at a given temperature, and relative humidity is the ratio of absolute humidity to saturation. A half assed analogy would be "there is 6 oz water in a glass (absolute humidity), the glass can hold 8 oz. Therefor the glass is 75% full (relative humidity).". So with that being said, the ratio of moisture in the air vs substance is less of a determining factor to the desiccation than the saturation level of the air. Given 100% relative humidity water will not evaporate. You just can't fit any more into the glass.

 

To your second question I'd assume that the more polar a solvent is the more likely it is to help homogenize the charges across a surface that has regions of net charge segregations. Where the solvent carries electrons from one area with a greater negative net charge to an area with a higher positive net charge. The reduction in charge pockets is then responsible for the reduced probability for an ESD to occur.

 

Again, these are assumptions, and are likely to be incorrect. I'm eager to hear from some of the more intelligent members of this forum to see if I'm even in the right ballpark.

[hindsight]

BurritoBandito-- Thank you for your thoughtful response.

 

So is it safer to work with ESD-sensitive comps outside on a winter's day after a snow (100% relative humidity) than on a 20C day with 60% relative humidity?

 

Your explanation of a polar solvent carrying off charges seems reasonable.

Does that mean it is better to mix the ESD-senitive comp in a well-ventilated area wetted with a polar solvent than it is to mix them dry?

 

--hindsight

[BurritoBandito]

Those both seem to be reasonable assertions to me. Assuming flammability isn't a factor, because I can not stress enough that I don't know for sure. Edited February 24, 2014 by BurritoBandito

[Mumbles]

You might want to ask this on Passfire or Fireworking.com. I have my inclinations, but really don't have any proof or evidence to support it either way. You'd probably want someone like Lloyd Sponnenburgh or Mike Swisher to weigh in on this topic. They'd be the most likely to know.

[BurritoBandito]

@Mumbles: I'd love to hear your guess as to what's happening?

[hindsight]

Thankfully, Kevin (Nessalco) just posted a brief question about the effect of relative vs absolute humidity on ESD on Fireworking.com. I am eager to see a response.

[Mumbles]

My inclination is that absolute humidity is more important than relative humidity. If you take 60% humidity at 70F to be the safe level, then 55F at 100% humidity and 35% humidity at 90F are equally safe due to the same absolute humidity. Thus I wouldn't feel safe working with static sensitive materials at any temperature below 55F.

[Nessalco]

Here's a link to a spreadsheet that shows the absolute moisture content at various temperatures and relative humidity.

 

www.back2bed.com/files/humidity.xls

 

Kevin

Edited March 2, 2014 by Nessalco

[Nessalco]

No one on Fireworking has replied. I came across this which seems to speak to the uncertainty about this question.

 

 

Relative humidity vs. absolute humidity

Further muddling the issue is the fact that many people in the industry -- including some in the ASHRAE technical committee itself -- think that absolute humidity should be measured instead of relative humidity. Absolute humidity, also expressed as dew point, is a measure of the amount of water in the air independent of temperature. So while relative humidity drops when temperature goes up in a data center, absolute humidity stays the same. ASHRAE now recommends that data centers measure humidity by dew point and fall within 5.5 to 15 degrees Celsius (41.9-59 degrees Fahrenheit).

 

Degree Controls' Stine gave an example as to why dew point, or absolute humidity, is a better measure. He said that if the air entering a server was 60 degrees and had a relative humidity level of 40%, its absolute humidity would be about 0.0045 pounds of water per pound of dry air. That can then be set as your low humidity level in the data center.

 

But here's where the problem with relative humidity comes in, Stine said. As air goes through a server, it heats up. That causes the relative humidity to drop, possibly down to as low as 20%, even though absolute humidity stays the same.

 

"You can have perfectly good air going into a server and what seems to be bad air (according to the relative humidity level) coming out, but the absolute humidity hasn't changed," Stine said.

 

He added that a data center manager will then increase the relative humidity control on the air-conditioning unit to get it within the 40% to 55% range. That then causes condensation to form on the cooling coils, which causes the unit to work harder to evaporate that moisture.

 

Uptime Institute's Sullivan agrees that controlling absolute humidity or dew point is best.

 

"What you want to do is control the moisture content of the air in the room rather than trying to control the relative humidity," he said.

 

Beaty, meanwhile, said there probably needs to be more research on absolute humidity and the dew point temperature, especially as liquid cooling becomes more popular in the data center.

 

"From a practical standpoint, the dew point temperature limit is needed to avoid potential water condensation in datacom equipment as the industry introduces more liquid-cooled equipment. As more liquid cooling products hit the market, it may be necessary to tweak the currently allowable dew point limits … based on industry experience."

 

That's snipped from a larger article at http://searchdatacenter.techtarget.com/news/1261265/Data-center-humidity-levels-source-of-debate

 

Kevin

[Nessalco]

Then there's this.

 

http://dom.semi.org/web/wstandards.nsf/02AA40715664389A88257207000AD980/$file/Julian_Montoya.pdf

 

Check Slide #15.

 

Kevin

[Nessalco]

OK. I took all that reading, and created a PDF that shows the 'ideal' humidity range to avoid both ESD and excessive condensation.

 

Kevin

dew point.pdf

Edited March 2, 2014 by Nessalco

[hindsight]

Mumbles,

 

So that precludes working with ESD-sensitive comps outdoors throughout all of the northern winter.

The two references cited by Kevin seem to support the notion that absolute humidity is more important.

And his table supports your contention that at any temperature below 55F the amount of water in the air is less than the amount at 60% relative humidity at room temp.

 

Lloyd has stated that 60% relative humidity was the target for preventing ESD problems. But he didn't specify room temperature.

 

 

Kevin,

Thanks for doing the search and posting the results.

[Mumbles]

Thanks for doing all the leg work on this Kevin. I was sort of curious what the color coding meant on your table? I'd assume that red means danger, and green is sort of the sweet spot. Is yellow the area that would be safe, but has excessive condensation? It looks like we're shooting for a dew point (aka absolute humidity) generally between 42F and 60F?

[Nessalco]

Mum,

 

Exactly. The red areas are DP<49.1, green is 49.1 to 59, and yellow DP>59. That range is recommended in the article about servers, and the upper limit is as you suggested - to minimize acquiring atmospheric moisture. In a lot of cases that wouldn't matter much.

 

I got interested enough in all this that I built an electroscope, and have been playing around with generating and detecting static fields. I was able to generate enough of a static charge on my body/hair/clothing it was detected a foot away - while wearing cotton clothing.

 

After all this, I come away remembering ESD is only one factor. Most compounds take a significant amount of energy to initiate, and a simple static discharge won't do it. An example would be AP compounds. I like to make lances when it is cool out, and the humidity low. Sure, the ESD risk is higher, but the lance compound won't initiate from static, and the hygroscopic nature of AP leaves me wanting to minimize ambient moisture.

 

Kevin

[hindsight]

I have been assuming in the above that an absolute humidity below about 10.4 gm/m^3 ( 60% RH at 20C)* would be more likely to result in ESD regardless of temperature. But I wonder whether a drop in temp could have effects which counteract some of the effects of decreased humidity. One of the slides in the dew-point.pdf Kevin linked for us mentions that increasing barometric pressure decreases the likelihood of ESD.
Cold air is denser and increases barometric pressure, no? I don't know the potential magnitude of such an effect. Antagonizing this potential benefit, cold temperatures decrease conductivity of water, which (in Burrito Banditos' explanation) might enhance the development of a charge on a surface.

Kevin, is your electroscope sensitive enough to detect the static charge build-up in cold weather with snow on the ground, vs 60% relative humidity at room temperature. If so, I'll build one if you provide the plans. The only one I was familiar with was a crude bottle-with-gold-leaves-inside, that I vaguely recall from secondary school.




* (http://gapyx.com/cmt/2008/01/dsm_absolute_humidity_relative_humidity.jpg)

Edited March 3, 2014 by hindsight

[Nessalco]

Nah - crude little device. Detects, not quantifies. From what I've read, static charges are very difficult to quantify with an instrument.

 

Kevin

[BurritoBandito]

Damn, I was wrong. It's not the first time, and I'm sure it won't be the last. Nessalco, thank you for providing us with these resources... greatly appreciated.

[Arthur]

IMO if there is sight or sign of frost or snow then the humidity must be close to zero as any moisture is already freezing out of the air.

[hindsight]

Very recently, I stumbled across a thread in Fireworking where Mike Swisher and Lloyd S were debating the importance of relative versus absolute humidity. So, if the giants clash over this topic, it must be confusing. Here's the thread: http://www.fireworking.com/content/simple-whistles.

Edited June 30, 2014 by hindsight

[papasmurf]

http://www.amazon.com/Extech-RH300-Humidity-Meter-Point/dp/B000BF5400

 

Looks like I'll be adding one of these to my safety belt!

 

Another thought, I wouldn't put much weight on what your local weather man has recorded online. Their instruments are accurate, but accurate at the location and time the reading was recorded. My father in law was a meteorologist for the Air Force and he has often pointed out to me how quickly localized atmospheric high and low pressure can change, resulting in a quick thunderstorm building out of no where. And how a lot of today's forecasters rely to much on computer modeling to predict the weather, he's a bit old school. It is also very dependent on the terrain you live in as well. For example I live in the bottom between two hills, my yard and vehicles routinely have due on them in the morning when I leave for work, but once I drive up the hill a ways it's bone dry.

 

It would be better to have your own weather station!

http://www.amazon.com/Davis-Instruments-6250-Vantage-Wireless/dp/B002LLVFK0/ref=pd_sbs_hg_3?ie=UTF8&refRID=101A1PWSQQPPSA811GXD

[Bobosan]

Hi papasmurf.

My old Davis Weather Wizard III has about reached it's useful lifespan. It has a data link for direct download of stored data to a computer and did not notice any method of downloading on the model you linked to but surely Davis still has this feature.

[papasmurf]

Hi papasmurf.

My old Davis Weather Wizard III has about reached it's useful lifespan. It has a data link for direct download of stored data to a computer and did not notice any method of downloading on the model you linked to but surely Davis still has this feature.

 

The provided console has some data storage. I think the vantage pro 2 has the capability with an additional software purchase to upload the data to your PC. But your talking big bucks then, really all I'm interested in is current conditions.