Solar radiation Belchertown

[Mark Fraser]

According to MQTT my weatherstation has "maxSolarRad_Wpm2":
"340.60620184000464", but the Belchertown skin is showing it as 40 W/m².
Have I got to change the units to something else?

[Mark Fraser]

Looking at this again, I was confusing maxSolarRad_Wpm2 and radiation_Wpm2, but the reading is still different to what is shown on the weather station's own display where it is showing 700 W/m² and I'm getting  129.946 W/m²  from weewx_sdr.

Running RTL_433 directly is showing Watts/m   : 129.95 and Lux       : 88753.0

Something is wrong somewhere.

[Greg Troxel]

Mark Fraser <mfr...@gmail.com> writes:

> Looking at this again, I was confusing maxSolarRad_Wpm2 and radiation_Wpm2,
> but the reading is still different to what is shown on the weather
> station's own display where it is showing 700 W/m² and I'm getting 129.946
> W/m² from weewx_sdr.
>
> Running RTL_433 directly is showing Watts/m : 129.95 and Lux :
> 88753.0

That should be W/m^2.

You should understand whether your station hardware (which you did not
specify) is measuring irradiance in W/m^2 or illuminance in lux. They
are not the same thing and one cannot convert between them without
making incorrect assumptions. However people do.

https://github.com/weewx/weewx/wiki/Watts-and-lux

https://www.extrica.com/article/21667/pdf
(likely hard to follow if you are not already familiar with the issues)


So:

Does the station have an illuminance sensor or an irradiance sensor?
(Or both?)

If it has only an illuminance sensor, common in low-end stations that
have any kind of light sensor, and it reports irradiance, then it is
applying some kind of conversion. See what the manual says and if
unsatisfactory, ask tech support. (I'll wait :-)

(Davis VP2 can optionally have an irradiance sensor, but people do not
use rtl-sdr. It also only reports irradiance and does not report
illuminance. So it seems pretty clear that's not what you have.)

Note that rtl-sdr may also have a conversion, and be populating one
field for the other. That you can read the sources of.

Note that there is no reason to expect that the conversion in the
device and in rtl-sdr is the same, and in reality both are wrong,
because strictly there is no conversion.

> Something is wrong somewhere.

Always true! In this case, much, but I hope the above will enable you
to understand better what's going on with your system.

[Mark Fraser]

On 22/06/2023 12:57, Greg Troxel wrote:
> Mark Fraser <mfr...@gmail.com> writes:
>
>> Looking at this again, I was confusing maxSolarRad_Wpm2 and radiation_Wpm2,
>> but the reading is still different to what is shown on the weather
>> station's own display where it is showing 700 W/m² and I'm getting 129.946
>> W/m² from weewx_sdr.
>>
>> Running RTL_433 directly is showing Watts/m : 129.95 and Lux :
>> 88753.0
>
> That should be W/m^2.
>
> You should understand whether your station hardware (which you did not
> specify) is measuring irradiance in W/m^2 or illuminance in lux. They
> are not the same thing and one cannot convert between them without
> making incorrect assumptions. However people do. >
> https://github.com/weewx/weewx/wiki/Watts-and-lux
>
> https://www.extrica.com/article/21667/pdf
> (likely hard to follow if you are not already familiar with the issues)
>
>
> So:
>
> Does the station have an illuminance sensor or an irradiance sensor?
> (Or both?)

It is an Aercus WS3085 and it doesn't say what sensor it is using, the
display allows you to change the units from Lux, w/m2 or fc.

>
> If it has only an illuminance sensor, common in low-end stations that
> have any kind of light sensor, and it reports irradiance, then it is
> applying some kind of conversion. See what the manual says and if
> unsatisfactory, ask tech support. (I'll wait :-)
>
> (Davis VP2 can optionally have an irradiance sensor, but people do not
> use rtl-sdr. It also only reports irradiance and does not report
> illuminance. So it seems pretty clear that's not what you have.)
>
> Note that rtl-sdr may also have a conversion, and be populating one
> field for the other. That you can read the sources of.

The source code of RTL_433 shows this is how it is receiving light data:
// GETTING UV DATA
int uv_sensor_id = (br[1] << 4 & 0xf0) | (br[2] >> 4);
int uv_status_ok = br[3] == 85;
int uv_index = br[2] & 0x0F;

// GETTING LIGHT DATA
int light = (br[4] << 16) | (br[5] << 8) | br[6];
double lux = light * 0.1;
float wm;
if (preamble == SPB)
wm = (light / 1265.8f);
else //EPB
wm = (light / 6830.0f);

[Greg Troxel]

Mark Fraser <mfr...@gmail.com> writes:

>> So:
>> Does the station have an illuminance sensor or an irradiance
>> sensor?
>> (Or both?)
>
> It is an Aercus WS3085 and it doesn't say what sensor it is using, the
> display allows you to change the units from Lux, w/m2 or fc.

If there is only one sensor, that is simply wrong on the part of the
station. I suggest asking customer support to fix their documentation.

> The source code of RTL_433 shows this is how it is receiving light data:
> // GETTING UV DATA
> int uv_sensor_id = (br[1] << 4 & 0xf0) | (br[2] >> 4);
> int uv_status_ok = br[3] == 85;
> int uv_index = br[2] & 0x0F;
>
> // GETTING LIGHT DATA
> int light = (br[4] << 16) | (br[5] << 8) | br[6];
> double lux = light * 0.1;
> float wm;
> if (preamble == SPB)
> wm = (light / 1265.8f);
> else //EPB
> wm = (light / 6830.0f);

That's indeed a clue that it has a lux sensor only. The "SPB/EPB" bit
and the two values are interesting. The first one looks like sunlight
at 1 Sun, but the comments that explain the rationale are missing :-(


You'll also need to ask support to document their conversion from lux to
W/m^2. Either that or ignore the display.