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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

Safe distance to watch arc welding



 
 
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  #11  
Old October 28th 05, 10:46 PM
Leo Lichtman
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Default Safe distance to watch arc welding


wrote: A lens can't collect and focus more energy
than is intercepted by it's total area. (clip)
^^^^^^^^^^^^^^^^^^
That is true. However, a given light source at a greater distance produces
a proportionately smaller image. so the brightness of the image remains the
same. I'm not making this up. Lay out a diagram of a lens and do the ray
tracing, and you'll see.

Or, as experimental evidence, think about what i said about taking a
photograph. Let's say you take a picture of two billboards. One is 100
feet from you, and the other is 1000 feet. They are in the same picture,
and they are both correctly exposed. The more distant billboard has the
same illumination falling on it, but, being 10x farther away, the lens will
receive 1/100 as much light. Inside the camera, the image of the more
distant billboard will be 1/10 as large, so it will have 1/100 as much area.
So the brightness of the image will come out the SAME.


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  #12  
Old October 29th 05, 12:40 AM
[email protected]
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Default Safe distance to watch arc welding

Leo Lichtman wrote:
wrote:


A lens can't collect and focus more energy
than is intercepted by it's total area. (clip)
^^^^^^^^^^^^^^^^^^
That is true. However, a given light source at a greater distance produces
a proportionately smaller image. so the brightness of the image remains the
same. I'm not making this up. Lay out a diagram of a lens and do the ray
tracing, and you'll see.


Yes, and I allowed for the possibility of that in the part of the post
you clipped (though without the certainty).

However the power of the lens is limited. At some point the number of
incident photons becomes small enough and the ability of the lens to
focus them to a point limited enough, that the power density of the
image on the retina can no longer exceed the threshold of damage. This
is clearly the case when looking at stars other than our sun; at what
point it becomes true for a welding arc I don't know.

It's also going to depend on the size of the pupil, which is going to
depend on what impression of light levels the eye has adjusted to -
being suddenly flashed at night is presumably worse than the same arc
at the same distance on a sunny day.

  #13  
Old October 29th 05, 12:41 AM
Howard Eisenhauer
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Default Safe distance to watch arc welding

Les. here's my free advice(& you know what that's worth )-

The problem ain't how bright the visible light is. If the visible
light is too bright you'll naturally move back to a distance where it
doesn't bother you. The problem is the UV that's generated, your eye
has no way of detecting dangerous levels so you can get a "burn level"
dose & not know about it until the pain starts up hours later. IIRC
the UV is actuually burning the cornea, not the retina, which is why
people get the "gritty eye" pain instead of just going blind. Also
IIRC prolonged exposure leads to cataracts.

UV does , above & beyond cubed root losses, attenuate with distance
through air but I have no idea what the factor is .

My recommendations a

a. If your going to watch anyhow wear a pair of good quality UV
blocker sunglasses. If you get a sunburn you're too close (don't ask
me how I know )

b. Ask this question on sci.engr.joining.welding, I can absoulutely
guarentee you'll get the real skinny there.

H.



On 28 Oct 2005 08:18:04 -0700, "Les"
wrote:

Is there any rule of thumb on how close I can be and still safely watch
(not a casual glance) arc welding without any eye protection? Does
this distance vary by amperage or type (stick, wire, TIG, etc.)?

- les


  #14  
Old October 29th 05, 12:49 AM
scruttocks
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Default Safe distance to watch arc welding

Leo Lichtman wrote:

wrote: A lens can't collect and focus more energy
than is intercepted by it's total area. (clip)
^^^^^^^^^^^^^^^^^^
That is true. However, a given light source at a greater distance produces
a proportionately smaller image. so the brightness of the image remains the
same. I'm not making this up. Lay out a diagram of a lens and do the ray
tracing, and you'll see.

Or, as experimental evidence, think about what i said about taking a
photograph. Let's say you take a picture of two billboards. One is 100
feet from you, and the other is 1000 feet. They are in the same picture,
and they are both correctly exposed. The more distant billboard has the
same illumination falling on it, but, being 10x farther away, the lens will
receive 1/100 as much light. Inside the camera, the image of the more
distant billboard will be 1/10 as large, so it will have 1/100 as much area.
So the brightness of the image will come out the SAME.



Brightness is a slightly misleading term here, what we need to worry
about is energy absorbed by tissue.

Consider a match and a nice big wood bonfire, both will burn at the same
temperature and be emitting the same infra red wavelengths, if you look
at them they will have the same brightness, but one will be much bigger
than the other.

Now hold your hand an inch from the burning match, then try the same
thing with the bonfire. Obviously the match won't be a problem and the
bonfire will burn you - to put it another way, the energy absorbed from
the match isn't enough to damage the skin, the energy absorbed from the
bonfire is enough to blister it, keep it there long enough and the
tissue will be destroyed.

It all comes down to how much energy, how long ?.



--
scruttocks
k12rs r80g/s
  #15  
Old October 29th 05, 01:28 AM
Leo Lichtman
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Default Safe distance to watch arc welding


"Howard Eisenhauer" (clip) the UV is actuually burning the cornea, not the
retina, which is why people get the "gritty eye" pain instead of just going
blind. (clip)
^^^^^^^^^^^^^^^^^^
Howard, thanks for flushing the toilet on all my fancy optical reasoning. I
believe you are correct, and it doesn't matter about the focused image on
the retina. So, since the inverse square law is in effect, the OP's
original question is valid, and unanswered.


  #16  
Old October 29th 05, 02:12 AM
R. Zimmerman
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Default Safe distance to watch arc welding

Training texts for welders in British Columbia Canada say 40 feet. That was
when I took welder training and wire feed was some high tech thing.
There is more radiation from wire feed because there is so little smoke
to diffuse the light.
Randy


"Les" wrote in message
oups.com...
Is there any rule of thumb on how close I can be and still safely watch
(not a casual glance) arc welding without any eye protection? Does
this distance vary by amperage or type (stick, wire, TIG, etc.)?

- les



  #17  
Old October 29th 05, 02:56 AM
SteveB
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Posts: n/a
Default Safe distance to watch arc welding

After watching a mob of people flog a dead equine, let me toss this in:

I have welded since 1974.

I DO NOT

REPEAT

DO NOT

watch arc welding FROM ANY DISTANCE unless I have a hood on.

I have seen it at night from 1/2 mile away, and it sure is pretty, but
there's always that tinge that comes from being flashed or flash burned.

This stuff is dangerous, and there is very very little margin for error.

Intellectual masturbation aside on the inverse square therories versus
statistical correlaries, if you are watching arc welding, it is always a
good idea to have a hood on.

Do as you like.

Steve


  #18  
Old October 29th 05, 03:39 AM
Martin H. Eastburn
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Default Safe distance to watch arc welding

Brightness at the source is the same - but light falls off by the square of the
distance. It is fainter and fainter.
So there is a safe distance - but it is dependent on so many variables.

Martin
Martin Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder



Leo Lichtman wrote:
"Les" wrote : Is there any rule of thumb on how close I can be and still
safely watch (not a casual glance) arc welding without any eye protection?
(clip)
^^^^^^^^^^^^^^^^
The eye is just like a camera--the farther away the smaller the image, but
the brightness stays the same. By standing farther away, you are damaging a
smaller area on the retina. How large an image are you willing to damage?



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  #19  
Old October 29th 05, 03:51 AM
Martin H. Eastburn
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Default Safe distance to watch arc welding

Doing a tracing of lenses doesn't show you much but direction of the image
and compression or expansion.

You say you can read a news paper in pitch darkness with a candle 200 yards
away just as well as having it over the shoulder. We don't buy the concept
or the fact. It isn't rocket science.

Martin
Martin Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder



Leo Lichtman wrote:
wrote: A lens can't collect and focus more energy
than is intercepted by it's total area. (clip)
^^^^^^^^^^^^^^^^^^
That is true. However, a given light source at a greater distance produces
a proportionately smaller image.**I DON'T See the SO** so the brightness of the image remains the
same. I'm not making this up. Lay out a diagram of a lens and do the ray
tracing, and you'll see.

Or, as experimental evidence, think about what i said about taking a
photograph. Let's say you take a picture of two billboards. One is 100
feet from you, and the other is 1000 feet. They are in the same picture,
and they are both correctly exposed. The more distant billboard has the
same illumination falling on it, but, being 10x farther away, the lens will
receive 1/100 as much light. Inside the camera, the image of the more
distant billboard will be 1/10 as large, so it will have 1/100 as much area.
So the brightness of the image will come out the SAME.



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http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
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  #20  
Old October 29th 05, 09:31 AM
granpaw
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Default Safe distance to watch arc welding and further.

"SteveB" wrote in
news:0FA8f.30901$fE5.30430@fed1read06:

After watching a mob of people flog a dead equine, let me toss this in:

I have welded since 1974.

I DO NOT

REPEAT

DO NOT

watch arc welding FROM ANY DISTANCE unless I have a hood on.

I have seen it at night from 1/2 mile away, and it sure is pretty, but
there's always that tinge that comes from being flashed or flash burned.

This stuff is dangerous, and there is very very little margin for error.

Intellectual masturbation aside on the inverse square therories versus
statistical correlaries, if you are watching arc welding, it is always a
good idea to have a hood on.

Do as you like.

Steve




Agree fully here and would add, You need not look directly at the arc to
get yourself a bad flash burn. Shop walls have been the cause of quite a
few incidents of flash burn inasmuch as almost any surface will reflect the
harmful rays..Including the inner surface of your hood, many burns in the
shipyard back in the 70s this way for me.

Related question for comments;
My wife is blind and often sits on our back porch (outside) while I am
welding in the shop about 50 yards away with the doors open. I wonder if
the arc is dangerous to her at that distance, I know it would be up close
but at that distance??

Just wonderin'
granpaw
 




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