23-04-2022, 12:24
Hi,
The graph in post #14 is very interesting and confirms my suggestion in #9 that Perspex (Acrylic sheet) is not suitable as a cover/protector for (at least) the UV sensor. But we can learn much more from the graphs: First, it appears that the "2mm plastic" is NOT "Perspex" (because considerably more UV is reported); Google indicates that there are about 8 different "Clear" Plastics, of which about half are transparent also to UV. However, some of these plastics are not even "resistant" to UV, so it would be very unwise to choose a material simply because it looks transparent!
Then, note that the Ratio of w/m2 against UV is different for the two "clear" (uncovered) periods, and also that it changes during the time after 14:00. This suggests that one of the sensors (probably the UV) has a different angular response to the other. That the 3mm and 2mm glass (and the 2mm "plastic") all have the same transmission is not surprising, because they are presumably all "100% transparent", but about 8% of the light is reflected by their two outside surfaces. A problem is that the proportion of this reflection increases as the sun moves close to the horizon, probably up to about 50% (lost) at around 10 degrees above the horizon (Google: Fresnel Equations). Thus the sensor is unlikely to give a good "Global" Sine Response, which might lead to an incorrect indication of whether the Sun is "shining", when it has a low elevation above the horizon:
This becomes almost a "philosophical" question: Do we want to measure if "The Sun is Shining" within an hour or two of Sunrise/Sunset? If you are interested in "Sunbathing" or perhaps the energy output from your PV Solar Roof Panels, then perhaps not. But if you wish to use it as a (meteorological) indicator of Daily "Cloud Cover" then it needs to be recorded accurately.
Note that "Anti-Reflection Coatings" (even if available) are not very appropriate here, because their operation is dependent on the wavelength of the light. That's why camera lenses look "Pink" or "Purple", because it's mainly the Green light which is NOT reflected and Red and Violet are still reflected (to some extent). This is where PTFE comes of interest; it is NOT "transparent", but it is an excellent diffuser of light, i.e. it reflects (some) light, but independently of the angle of incidence. Thus it can be arranged to indicate a genuine Global (Sine Response) Solar light level, and perhaps even correct for a poor angular response of the sensor(s). Of course the sensitivity (or amplifier gain) of the circuit would need to be increased (i.e. a change in the circuit design), but the PV panel in particular should easily be able to deliver sufficient current (measured across a somewhat higher value resistor).
An interesting aside: The original Fine Offset "Solar Pods" had their light sensor underneath a "white dome", but they appear to have "forgotten" this when they re-launched their new (generally improved) stations under the Ecowitt (and other) Brand names. However, it appears that the latest Ecowitt sensors have been "improved" by adding a "white filter" above their Lux sensor ! But sadly, it's also been reported (and appears to be true) that their "UVI" measurement is simply the w/m2 value divided by ~100 !
_____
IMHO the graph in post #11 is quite "confusing": It has a partially Linear/Logarithmic wavelength scale and also the middle "Yellow" band does NOT correspond to the Visible Spectrum! The "Violet" region on the left has a linear scale which corresponds basically to the UVC wavelengths which do NOT pass through the Earth's atmosphere, so are irrelevant here (the UVA and UVB wavelengths which do reach the Earth's surface are in the Yellow region). Similarly the "Near InfraRed" wavelengths from 700nm to about 1000nm are not shown in the "Pink" region. So personally, I'm not convinced that the high cost of the Fused Silica Quartz Glass is justified in this application (and the "ripple" in the JGS-1 response at 1000nm might be more significant than what happens below 270 nm ! ).
In passing, Google found This PV Spectral Response Graph , but that whole web document appears to contain a wealth of useful Educational Information (all of which I haven't actually read yet).
Cheers, Alan.
(20-04-2022, 20:48)SR123 Wrote: ... Today I decided to try different materials to cover my solar & uv sensor. ....... then 2mm clear plastic - not sure what it is, it was from a box lid,
The graph in post #14 is very interesting and confirms my suggestion in #9 that Perspex (Acrylic sheet) is not suitable as a cover/protector for (at least) the UV sensor. But we can learn much more from the graphs: First, it appears that the "2mm plastic" is NOT "Perspex" (because considerably more UV is reported); Google indicates that there are about 8 different "Clear" Plastics, of which about half are transparent also to UV. However, some of these plastics are not even "resistant" to UV, so it would be very unwise to choose a material simply because it looks transparent!
Then, note that the Ratio of w/m2 against UV is different for the two "clear" (uncovered) periods, and also that it changes during the time after 14:00. This suggests that one of the sensors (probably the UV) has a different angular response to the other. That the 3mm and 2mm glass (and the 2mm "plastic") all have the same transmission is not surprising, because they are presumably all "100% transparent", but about 8% of the light is reflected by their two outside surfaces. A problem is that the proportion of this reflection increases as the sun moves close to the horizon, probably up to about 50% (lost) at around 10 degrees above the horizon (Google: Fresnel Equations). Thus the sensor is unlikely to give a good "Global" Sine Response, which might lead to an incorrect indication of whether the Sun is "shining", when it has a low elevation above the horizon:
This becomes almost a "philosophical" question: Do we want to measure if "The Sun is Shining" within an hour or two of Sunrise/Sunset? If you are interested in "Sunbathing" or perhaps the energy output from your PV Solar Roof Panels, then perhaps not. But if you wish to use it as a (meteorological) indicator of Daily "Cloud Cover" then it needs to be recorded accurately.
Note that "Anti-Reflection Coatings" (even if available) are not very appropriate here, because their operation is dependent on the wavelength of the light. That's why camera lenses look "Pink" or "Purple", because it's mainly the Green light which is NOT reflected and Red and Violet are still reflected (to some extent). This is where PTFE comes of interest; it is NOT "transparent", but it is an excellent diffuser of light, i.e. it reflects (some) light, but independently of the angle of incidence. Thus it can be arranged to indicate a genuine Global (Sine Response) Solar light level, and perhaps even correct for a poor angular response of the sensor(s). Of course the sensitivity (or amplifier gain) of the circuit would need to be increased (i.e. a change in the circuit design), but the PV panel in particular should easily be able to deliver sufficient current (measured across a somewhat higher value resistor).
An interesting aside: The original Fine Offset "Solar Pods" had their light sensor underneath a "white dome", but they appear to have "forgotten" this when they re-launched their new (generally improved) stations under the Ecowitt (and other) Brand names. However, it appears that the latest Ecowitt sensors have been "improved" by adding a "white filter" above their Lux sensor ! But sadly, it's also been reported (and appears to be true) that their "UVI" measurement is simply the w/m2 value divided by ~100 !
_____
IMHO the graph in post #11 is quite "confusing": It has a partially Linear/Logarithmic wavelength scale and also the middle "Yellow" band does NOT correspond to the Visible Spectrum! The "Violet" region on the left has a linear scale which corresponds basically to the UVC wavelengths which do NOT pass through the Earth's atmosphere, so are irrelevant here (the UVA and UVB wavelengths which do reach the Earth's surface are in the Yellow region). Similarly the "Near InfraRed" wavelengths from 700nm to about 1000nm are not shown in the "Pink" region. So personally, I'm not convinced that the high cost of the Fused Silica Quartz Glass is justified in this application (and the "ripple" in the JGS-1 response at 1000nm might be more significant than what happens below 270 nm ! ).
In passing, Google found This PV Spectral Response Graph , but that whole web document appears to contain a wealth of useful Educational Information (all of which I haven't actually read yet).
Cheers, Alan.