Pausible values for UV sensor?!

[jmc...@gmail.com]

Hi!

I would like to know if these values are plausible for UV sensor here, in Brazil.

I'm located in Ribeirao Preto, SP, Brazil (-21.1767, -47.8208) and the sun is very 'hard' here. We are almost in tropical line and the sun pass almost 90º degree above us at 12h00 pm......also, we are just in the summer. So my question is: UV Index of 18~19 is a plausible value? peak of 20.

I'm asking because i'm testing a new sensor in my station, VEML6075....but I'm first trying it standalone using arduino and this library ( https://github.com/adafruit/Adafruit_VEML6075 ). Acording to this library,m the default values (calibration coefficient) are for the sensor direct in the sunlight, without any protection. That's how I tested today, sensor direct in the sunlight....but I'm thinking that 18~19 are too high, even with our 'brazilian sun'. It's very hot here, about 38~40 in the shade.

If I use the sensor indoor at night (just using my LED home lights), I got negative values in sensor (UVA and UVB, not index), but I think that's right, since LEd doesn't emmite any UV light and sensor has calibration values, right?

Here are the values that I got (just some samples). The lower value is during the shade of a cloud, but still very bright.

[Greg Troxel]

10000 mW/cm^2 would be 10 W/cm^2 which is 100 000 W/m^2 of UVB. Total
solar radiation should be on the order of 1300 W/m^2 (it's nominally
1000 max here at 42N) at overhead sun, so 10x the power just
in UVB makes zero sense.

I am at 42ish North (Massachusetts, US), and with a Davis UV sensor, the
highest values I see are about 7.

The US index talks about mid teens, but we are a less tropical place
than you. The web page inexplicably presents numbers without units as
if that is a reasonable thing to do.

https://www.epa.gov/sunsafety/calculating-uv-index-0

Reading wikipedia, 18-19 seems higher than plausible

https://en.wikipedia.org/wiki/Ultraviolet_index

This also seems to argue against 19:

https://www.who.int/uv/publications/en/UVIGuide.pdf

but this seems to say it is plausible:

https://niwa.co.nz/sites/niwa.co.nz/files/import/attachments/Liley_2.pdf


I would look at values published by your national weather or health
authorities and compare.

[Graham Eddy]

https://wiki.cancer.org.au/skincancerstats/UV_radiation:

The UV Index is a standardised measure used to express UV radiation intensity. It is equal to 40 times the erythemally effective power of the sun in W/m2. The UV Index at solar noon is generally in the range 0-12 and values above 11 are considered extreme.^[1] In Australia, peak daily values in summer are regularly in excess of 12-14, and can reach 16-17 at more northern latitudes.^[2] When the forecast UV Index is ≥3, sun protection is required.^[14]

Table 1: Australian capital city average daily maximum UV levels by month.^[15]

Location	Jan	Feb	Mar	Apr	May	June	July	Aug	Sept	Oct	Nov	Dec
Darwin	12	13	12	11	9	8	9	10	12	13	12	12
Brisbane	12	11	9	7	5	4	4	5	7	9	11	11
Perth	12	11	9	6	4	3	3	4	6	8	10	11
Sydney	11	10	8	5	3	2	3	4	5	7	9	10
Canberra	11	8	7	5	3	2	2	3	5	7	9	11
Adelaide	11	10	8	5	3	2	2	3	5	7	9	11
Melbourne	10	9	7	4	2	2	2	3	4	6	8	10
Hobart	8	7	4	3	1	1	1	2	3	4	6	7

Values are rounded to the nearest whole number. Highlighted months have an average daily maximum UV less than 3 even where rounding has made it appear otherwise.

[Anton vanNwnhzn@GMail]

An open source for getting UV-values calculated for your location is at https://www.openuv.io/
Be aware that the outputdata is for 'clear-sky':
for a cloudy or rainy day the value must be recalculated by yourself .......
But at least you get good reference/ceiling-values for the bright days.

Anton

Op 8-2-2021 om 1:10 schreef jmc...@gmail.com:

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[Anton vanNwnhzn@GMail]

For clear-sky reference values you could look at https://www.openuv.io/
That site provides info dedicated for the location you ask:
info includes much more than UVI only.
For automatic extraction of info a script will be needed.

If you want automatic info on attenuation by clouds, then https://www.ogimet.com/home.phtml.en is your friend:
look for it's octa-values for your location [=WMOID] through the url "http://www.ogimet.com/cgi-bin/getsynop?block='..WMOID..'&begin='..UTC..'"
Or look for a weathersite in your area providing values for cloud-cover.
With that info [= 'clear-sky'+ 'octa'] you are able yourself to calculate the expected practical UVI.

Op 8-2-2021 om 1:10 schreef jmc...@gmail.com:

Hi!

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[Jonis Maurin Ceará]

Ok, thank you guys!

Here, in Ribeirão Preto, I'm getting almost everyday warnings on TV about extreme UV levels, so I'm pretty sure it's more than 11 almost all day. But in my opinion, 20 it's too high.

I have tried both openuv.io and ogimet....

openuv.io shows me:

"uv": 0.041,

        "uv_time": "2021-02-08T14:54:53.953Z",

        "uv_max": 13.664,

while my sensor is reading 19. Ok, it's a little cloudy now and maybe this measure from openui is during a shade.....still, max UV is a way low than my real reading.

ogimet I could not find any station here in Ribeirao Preto with UV value. but looking at openweather map, I could find a value of 14 for UV. It's almost the same of openuv.

Now is the complicated part.....

1) What do you recommend to use as protection for the sensor? I read about teflon diffuser (with 0.4mm thickness), but It's hard to find this. I was thinking about printing a case and use a mobile phone glass creen protection (very thick) as protection. What do you think?

2) I'm getting values from UVA and UVB from sensor in mW/cm². if I'm right, weewx expect data in W/m², right? If I convert my current UVA value (8,809mW/cm²), I'll get 8,8W/cm².....and converting to meters, it's 0,00088W/m². Is this right? I'm thinking its too low.

3) Finally, I have sensor coefficients to play and calibrate values. These coefficients are 4 values, a,b,c, and d. B and D are for UV light itself, if I remember the tech docs that I read. My question is: since UVI uses UVA and UVB to be calculated, I'll have to play with both. Do you guys have any idea or type how to start calibrating this without any expensive equipment? 

Of course I don't expect perfect values, since this is just a hobby. But I would like to get as close as possible to real value, using accessible things to calibrate this sensor.

I know that this is a hard and complicated task to do, especially without adequate equipment. But I would like to have some ideas from you, more experienced users.

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[Anton vanNwnhzn@GMail]

Jonis,

My first suggestions are for 2 very pragmatic solutions.
Calculation based on reported values for UVA and UVB incl. effects from reported cloudcover is a solution in completely different league, for which perhaps other group-members may help.

1) Perhaps for testing of feasability start simple with "correction by calculation"

Be aware that openuv-values are always for 'clear-sky = without any attenuation from cloudcover!

Based on your reported rough orders of magnitude

• sensor-output reads UVI=19
  
• openuv_value reads UVI=14 for 'clearsky', with OWM-value at same level
  
• TV tells UVI=11

the order is TV < openuv_value < sensor_output

With that knowledge you might apply a very simple & very rude  scaling-function like

Step 1, to check if sensor_output already below openuv_value (because if seeming 'practical', why rescale?)
if sensor_output < openuv_value then
real_UVI1 = sensor_output

Step2, checking & reducing the value to get below openuv and fitting 'reality'
if sensor_output > openuv_value then
real_UVI0 = sensor_output * 14 /19
real_UVI1 = real_UVI0 * 11 / 14

Step1 just checks that sensor-output is plausible,
while Step2 is the equivalent of  taking 50% of the sensor_output
Because you report that at night UVI from the sensor already is 0, no need to check on negative UVI, nor check on range of values.

Result: the calculation produces a UVI which resembles the value from TV
;-) Absolutely not scientifically accurate, but a practical solution .......

2) Empirical attenuation by cap

For a similar problem with measuring light with a LDR/photodiode, I used a deodorant-rollercap as attenuation cover over the sensor setup.
A very empirical solution, but very easy to try, and ;-) no cost: see the pictures.
Negative aspect is that the colour of the cap might affect the reading for UVI.
With cap fitted, compare the reported UVI with the value reported by TV.
If the reported UVI is lower than the value from TV, you might upscale the reported UVI by ratio.
;-) Also not very scientific, but a practical solution.....

Op 8-2-2021 om 16:06 schreef Jonis Maurin Ceará:

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[Jonis Maurin Ceará]

Thank you very much Anton! I'll try all of these ideas :)

Also, I just talked to a friend who works for a company that calibrates medical instruments. They don't have a calibrated 'UV emitter', but they do have a calibrated UVA and UVB receivers (for babys that just born and need UV light). So my idea is to use any abstract source of UV (even sunlight), measure with a calibrated sensor and with mine.....and change my sensor coefficient calibration until I get the same result as the calibrated device. I hope this works!

About protection, you gave me a great idea..... I'll try to use one of these PIR sensors.

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