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Default How do welding shade numbers add up? Eclipse viewing.

On Thursday, May 17, 2012 at 1:53:14 PM UTC-4, mike wrote:
I want to look at the eclipse this weekend.
Everybody says a #14 welding shade is good.
I don't have one.

So, how do the numbers stack up when you use 2?
I'm guessing it's not linear, so 7+7 doesn't = 14???

What's the math for stacking welding shades?

Thanks, mike


I found a transmittance chart at
https://www.smokstak.com/forum/attac...1&d=1503068175

The person who posted it said that you take the figure given for the percentage of transmittance from the nominal percentage column for each shade you want to use in your stack and multiply those number to arrive at a figure for the resulting transmittance and find the shade having that number in the range given for that shade between the nominal and minimum columns.

That is not exactly correct. You must divide the percentage figures by 100 before multiplying in order to the the transmittance factor, multiply those and then multiply the result by 100. Take that number and look to see what range it falls into.

The shade being given by NASA as being safe is 14.

You are therefore, looking for two shades (could do three, but will not go there just now) that work out to a combined value of between 0.00027% and 00016% Luminous Transmittance.

If you had two shades of square root (0.00027 / 100) x 100 = 0.1643 %, those together would equal 0.00027% transmittance.

A way of looking at that is that 0.1643 percent of 0.1643 percent is 0.1643 percent times 0.1643/100 = 0.0002699. (rounding affected the figures at little)

The chart shows that two #5 shades have a range including 0.1643.

If you take the nominal figures for two non-identical shades and work them out, you can see what shade number would be comparable.

If you had a #3 at 13.9% transmissivity, you would need #12 at .0019 to get into the range of a #14. That range is from 0.00027 percent up to 0.00016 percent.

0.139 x 0.000019 x 100 = 0.0002641 percent.

Using a #11 with a #3 yields 0.139 x 0.000052 x 100 = 0.0007228 percent, which falls a smidgen above the range of a # 13.

There are gaps in the transmissivity ranges between the shade numbers.

One might assume the Maximum Effective Far-Ultraviolet Average Transmittance figures are also percentages and it might do to verify those figures and see now they work out when stacking.

I followed the advice of the person whose post I saw that included the chart at the above address and ended up with very dark shading, since I did not have a #14.

I am not sure I missed a lot since the difference in the views would be some brightness but no detail would be apparent. Just the clear partial discs.

A proper pin hole viewer would have been better, I expect.

I posted this mainly because I am interested in understanding things and take notice when I see little errors in logic. You can't directly multiply percentages as was in an earlier post.

Otherwise, 5 percent times 5 percent would be 25 percent. In fact, it is 5 x .05 = 0.25 percent.

Tim.



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