Sticking to his guns I see:
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14 mW/cm2 In space!?!
Sorry Mick,
We are seeing up to 18mW/cm2 of combined UVA/UVB and at the same instant 7mW/cm2 of UVA.
And yes you can subtract the A from the A/B and get a good measurement of B.
In the end we can only convey what these instruments output.
Which are wrong. It does not matter how good you think the meters are if they show more radiation than is in space. And my initial estimate of 14 mW/cm2 turns out to be wrong, it's more like 10 or 11. See below.
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How does he know how much is actually hitting the ground?
3.2mW/cm^2 doesn't look like much.
We have other meters in place now, the same exact UV A/B meter I am using, which are also displaying high readings as high and higher than my own measurements.
"doesn't look like much" is hardly a strong argument here. Let's find some references:
American Conference of Government Hygienists Guidelines for Exposure to UV light Related to Typical conditions of exposure:
http://www.dymax.com/pdf/literature/lit023_guidelines_for_uv_exposure.pdf
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1 mW/cm2 is the intensity level one would expect to read on a cloudless, spring day in in New England if the radiometer sensor/probe were aimed directly at the sun. In subtropical latitudes on sunny days, a reading of 3 mW/cm2 would be typical.
Pacific University Oregon:
http://www.pacificu.edu/optometry/ce/courses/15719/uvradiationpg2.cfm
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The sun is the direct source of UV in the natural environment. The solar constant is the total radiant energy received from the sun, approximately 1367 W/m2, based on an average earth-to-sun distance of about 150 x 106 km (93 x 106 mi). UV radiation comprises almost 8% of the solar constant
...
8% of 1367 is 109, or 10.9 mW/cm2. So you can't be getting 18.
Back to Dane:
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That 1,366 W/m2 stuff was ditched in 1982 when they found out the instrument calibration was whacked….
Other values for the solar constant are found in historical literature with the value 1,353 W/m2 appearing in many publications. It is now generally believed that most of the historical discrepancies have been due to instrument calibration error (White, 1977). Recent satellite and rocket data (Duncan et al., 1982) and (Hickey et al., 1982) have confirmed that the 1,353 W/m2 value was low
His linked text says the solar constant has been updated from 1,353 to 1,367 W/m2, an increase of less than 1%. This makes zero practical difference to the figures.
The
official figures from which these numbers are derived are the ASTM Reference Air Mass 1.5:
http://rredc.nrel.gov/solar/spectra/am1.5/
You can download the spreadsheet for them, and add up the watts for rows 3-243 (280-400 nm), you get:
- Extraterrestrial: 10.33 mW/cm2 (i.e., the value in space, just above the atmosphere)
- Global Tilt: 4.64 (standard total average for hemisphere facing the sun)
- Direct+Circumsolar: 3.07 (at an angle selected to give an average in the US)
The last two figures are for the sunlight reaching the ground. But the bottom line here is the value for UV in space, just over 10. Dane is claiming 18 on the ground.
If you break down the extraterrestrial UV in UVA and UVB, you get:
- UVB in Space = 2.07 mW/cm2
- UVA in Space = 8.18 mW/cm2
So Dane is actually claiming a UVB on earth of 4.5x that of the UV in space.
I have attached a spreadsheet that shows the working for these figures, with a detailed graph of the UV portion of the spectrum.
So it's irrefutable that the figures Dane is quoting are wrong.
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Like this?:
And this is our UV A/B meter:
Again, neither meter is old, and both are at 4% accuracy with NIST traceability
The readings are wrong. The question is why they are wrong.
Now based on what's been said, I think that my earlier suggestion for why they were wrong (A/B vs. A+B) is itself wrong. I'd still like to see a video of the actual reading though. But I'll accept that these are the numbers you are getting out of the meters.
So why is it wrong? I think either the meter is not doing what you think it is doing, or there's some other user error.
Have a look at the spec sheet for the $800 HHUV254SD UVA/UVB meter.
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UV Light Types Measured: UVA, UVC
Measurement Range: 240 to 390 nm
Measurement Accuracy: ±4% of full scale reading + 2 digits
Full-Scale Autoranges: 2 mW/cm2 and
20 mW/cm2
and also:
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The HHUV254SD has the performance and features needed to satisfy the most demanding
aspects of these applications. For example, it combines the capabilities of UVA (black light
in the long-wave 365-nm band) and UVC (short waves in the 254-nm band) measurement
in one instrument.
First a minor quibble, 4% of full scale reading means 4% of 20 mW/cm2, so it has a claimed accuracy of +/- 0.8.
But the interesting thing here is the measurement range of 240 to 390 nm. Now we have:
- UVC - 200-280
- UVB - 280-320
- UVA - 320-400
So I suspect that what is going on here is that the meters simply measure one very narrow band of radiation, and then extrapolate this out to the full range. Giving vast room for errors. The A/B meter is far cheaper, and probably even more prone to error. But again, I'd like to see it in use.
But bottom line: clearly the figures are wrong. They are greater than the figures for extraterrestrial radiation.