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 Post subject: Face Shield
PostPosted: Thu Feb 20, 2014 5:46 pm 
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Does anyone know where I can purchase a clear faceshield with no tint at all to it?


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PostPosted: Fri Feb 21, 2014 1:28 am 
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Location: the Netherlands
Do not know where you are from but if you are from Europe then check out

[url='http://www.conrad.com/ce/en/Search.html;jsessionid=19E7DE39DEF5F0332D915EDDB601F282.ASTPCEN26?search=faceshield']http://www.conrad.com/ce/en/Search.html;jsessionid=19E7DE39DEF5F0332D915EDDB601F282.ASTPCEN26?search=faceshield[/url]

Or did you mean arc rated?

Because I do not think that they exist. The face shield has to protect your eyes from the bright intensity off the arc flash.


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PostPosted: Fri Feb 21, 2014 6:05 am 
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Luc wrote:
Do not know where you are from but if you are from Europe then check out

[url='http://www.conrad.com/ce/en/Search.html;jsessionid=19E7DE39DEF5F0332D915EDDB601F282.ASTPCEN26?search=faceshield']http://www.conrad.com/ce/en/Search.html;jsessionid=19E7DE39DEF5F0332D915EDDB601F282.ASTPCEN26?search=faceshield[/url]

Or did you mean arc rated?

Because I do not think that they exist. The face shield has to protect your eyes from the bright intensity off the arc flash.

Yes, I would like to have an arc rated faceshield, with no tint at all.


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PostPosted: Fri Feb 21, 2014 9:17 am 
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If there were arc rated face shields without tint, than we wouldn't all be using the tinted ones that we use. Everyone complains about the tint. That's why Salisbury's newer face shield includes integrated flashlights.


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PostPosted: Fri Feb 21, 2014 12:12 pm 
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The tint is similar to welding and cutting lenses and for the same reason. It is to protect against damage to the eyes from UV if an arc flash occurs. If you had a clear one, you'd still have to wear welding goggles or something similar underneath the face shield.


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PostPosted: Fri Feb 21, 2014 1:28 pm 
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Vica1ME wrote:
Does anyone know where I can purchase a clear faceshield with no tint at all to it?


http://www.bsd-dresden.de/en/, article 7406201 (I have one besides me).
Not totally tint-free , but pretty light grey (which doesn't alter the colors as much as the green ones).

ATPV 10.7 cal/cm^2 with chin cup.

Not easy to obtain from North America (don't know any distributors).


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PostPosted: Mon Feb 24, 2014 7:45 am 
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A molded lexsan yellow tint is very workable, the key is molded.


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PostPosted: Mon Feb 24, 2014 1:52 pm 
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BSD (listed above) and Oberon have arc rated gray shields. Check newer ones of the tinted types, the tint is getting less and less. Many now have 70% VLT which is an EU requirement for most work. Even the gray's have some tint and may not be better for wearing than the tinted. The tint has nothing to do with UV. It blocks visible light but the visible light is "seen" by the sensors as heat (they are gray bodies) and indicates burn. There is no way the sensors can differentiate visible light from other waveforms. It is possible that the visible portion of the spectrum does not cause burns BUT it must be blocked to pass the standard test. We would love to have research money to develop new sensors but this is huge and there has been no funding to date.
Use the flashlights and/or additional light for work which requires color recognition. This is something every faceshield company is working on. The newer shields often make some type of claim for "true color" etc. Check those out and don't buy the cheapest you can find.


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PostPosted: Tue Feb 25, 2014 10:32 am 
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OK, now it makes sense. But that would be a huge problem to overcome. Finding emissivity/absorptivity (or inverse, reflectance) values for human skin took very little effort:
http://repository.upenn.edu/cgi/viewcontent.cgi?article=1616&context=cis_reports
https://www.imt.liu.se/edu/courses/TBMT36/pdf/Text4.pdf

Both studies clearly show that absorptivity is actually extremely good but specifically tuned for infrared spectra and not visible spectra, which probably makes intuitive sense. Arcs are so hot that they are just going to have an emissivity spectrum of a good white body radiator so we only need to know or at least estimate temperature.

Still, the measurement data tells us something if you glance through the above reports. Human skin for whatever reason is pretty close to a good black body. It looks like absorption in the visible spectrum is about half of what it is in the near infrared but that's not really all that significant. I'll bet that the result would be nearly the same for a face shield irrespective of whether color spectrum is taken into account or not. Over the range of wavelengths of interest, copper calorimeters are not very discriminating either.

My very unscientific look here seems to suggest that even if we can more closely mimic radiant heat absorption by human skin, it won't change the result significantly.


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PostPosted: Tue Feb 25, 2014 10:48 am 
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Agreed that it is pretty easy to find theoretical data on human skin absorption but to correlate that to skin burn isn't easy. All the skin burn studies have been done with IR lamps so we don't know how that will work and in order to have a standard change takes real data or a strong theoretical model which we do not have to date. We have to stick with the Stoll curve and clear can't pass when we use a black painted calorimeter. I don't see a lot of burns in the old accidents when clear shields were used but they couldn't pass 3 cal/cm2 in most cases so they aren't used today. Hopefully some day there will be research money to correlate the Stoll curve with shields to see if this is the same as it is with fabric but at this point we must assume that it is.


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