[JT118]

This calibration calculator may be of use when looking at the various Solar Radiation Sensors that are available for use in the Weatherduino System.

http://clearskycalculator.com/pyranometer.htm

The Website is set up by Apogee for use in assessing when their sensors are in need of calibration, so I assume they are confident of its output;

http://www.apogeeinstruments.co.uk/pyranometer/

[JT118]

An interesting set of comparisons;
Lattitude 55.34o N 4.69oW
Apogee Clear Skys Model: (http://clearskycalculator.com/pyranometer.htm)
24/April/2017 Day 114 @ 12.75 hours (12hours 45minutes)
@ 12:45 hours: Max solar insolation possible  750 W/m2
================================================================
Reading from Davis Groweather 1999 model; (www.ayrshireweather.org )
@ 12:45 hrs: 781 W/m2
https://www.wunderground.com/personal-we...R8#history
================================================================
Reading from Weatherduino using Didcot Pyranometer; (www.ayrshireweather.org/weatherduino )
@12:44 Hrs 1045 W/m2.
https://www.wunderground.com/personal-we...E2#history
================================================================
I always thought the Didcot read very generously and altered the op amp gain using the potentiometer to reduce this reading.  However when I did this I began to lose early morning and late evening readings altogether. The sensor does not seem to respond in a straight line to altering the op amp gain.
Any ideas??

[werk_ag]

Hi JT118

Thank you for the link to the calibration calculator. Certainly it will help me with the Apogee SP-110.

I don't know nor the specs of your Didcot Pyranometer nor the settings of your OP, but in any case just adjusting the OP gain is not enough, you also have to adjust some data on the WeatherDuino software.
Do you know what are the maximum reading expected from the Didcot Pyranometer?

[JT118]

Hi Werk,

Please see attached spec sheets for Sensor.

Didcot Weather Station.pdf Size: 763,25 KB  Downloads: 309

Didcot DRS-5 Calibration Certificate.pdf Size: 78,93 KB  Downloads: 314

Thanks Jeremy

[werk_ag]

Hi JT118,

I think you are using the WeatherDuino Solar/UV interface, do you remember the values of the fixed and variable resistors that you are currently using? I'm asking about this just to try to do some calculations based on the existing resistors. If possible, it will be easier than have to change them.
Thanks

Regards

[AllyCat]

Hi,

I began to lose early morning and late evening readings altogether.

The nominal output voltage from that sensor is rather low (but consistent with a single silicon photodiode) at only 14 mV / kW/m2, so the "input offset voltage" of the Op-Amp may be quite significant.  That could cause the Op-Amp to either "ignore" or "invent" a small voltage near to the origin (zero), which would appear as a (rather severe) "non-linearity".

You may need to use an "instrumentation" grade (very low-offset) Op-Amp, or include a "bias offset" (balance) potentiometer.   But my preference is to use a "Current to Voltage" or "Current to frequency" Op-Amp Configuration, the former as used by Davis (basically just a capacitor instead of a resistor in the Op-Amp feedback path).

With such an arrangement, I devised a simple microcontroller single-diode "Solar" sensor that was linear down to less than 10 mW/m2 (not far from moonlight ), even using the internal amplifier (having probably many mV of offset voltage).  But on the KISS (Keep It SimpleS) principle, I now use a small "5 Volt" PV panel like Werk's.

Cheers,  Alan.

[JT118]

Hi,

OK thanks Allycat
I used your post from 2015 and have used 47k fixed resistor and a 1k pot set to about 850ohms (not entirely sure, it is on the roof and have not checked it recently.)

http://www.meteocercal.info/forum/Thread...ght=DIDCOT

Allycat wrote:
"The calibration certificate says that the solar sensor delivers 12.71 mV at 1000 watts/m2, which doesn't seem too far from the nominal specification of 14mV. I don't know what input level werk_ has used for the input calibration, but I see the "full scale" is (about) 1.1 volts and his maximum expected insolation is around 1400 Watts/m2. So I guess the required input level for 1000 W/m2 is about 700 mV. If you want to use the actual calibration of your sensor (assuming its calibration is still acurate after 15 years) then you will need to know (or be able to set) the exact value.


So basically you need to amplify the signal by about 700 / 12.71 , or around 55. If you are using a circuit as shown in post #52 here, then if the pot R3 is set to the top of its track (or better replaced by a 47k fixed resistor) then pot R4 needs to be set to about 850 ohms (takiing into account the 18 ohms source resistance). So a 1k or 2k2 pot might be better, or ideally an 820 ohm fixed resistor if you are able to use a calibration factor in the WeatherDuino software.

Of course the optimum calibration range will depend on your location (very different peak light levels between UK and Australia) and I don't know how accurate/stable is the "1.1 volt" reference in the Arduino. "

[werk_ag]

Hi JT118

Despite the WeatherDuino Solar/UV interface be equipped with a "instrumentation" grade OP (AD822) which have a very low input voltage offset, as said by AllyCat the Didcot Pyranometer have a rather low output voltage, which implies amplifying the signal to around 55 times. That is not a problem for the OP-Amp itself, but it have impact on the resolution.

Just for comparison:
DidCot at 1kw/m2 - 12.71mV
Apogee SP-110 at 1Kw/m2 -  200mV

I sugest that you try to use the resistor values proposed by AllyCat,  they are absolutely correct. If you note that the readings are higher than they should be, you can try to reduce the value of the last parameter used on the formula which WeatherDuino uses to to convert the ADC readings to W/m2.

Code:

       SolarRad = map(RX_Data[2], 0, 1023, 0, 1400);

This formula is used on the RX software.
By comparing the readings with your Davis Pyranometer, you should achieve a reasonable result. Of course a clear sky day is important to do this task.

...But on the KISS (Keep It SimpleS) principle, I now use a small "5 Volt" PV panel like Werk's.

It may be interesting referring that the readings of my home made pyranometer based on small solar cell, are very consistent with the readings from your Apogee SP-110. Last week we had here some days of clear sky, and the readings of both systems don't differ more than 5 to 7 %, and are both consistent with the theoretical maximum. The differences are more notorious when the sun is high.

Here you can see graphs from both systems, installed just a few meters apart:

Solar cell pyranometer
https://www.wunderground.com/weatherstat...=ICERCALC2
Note: Up to 9 AM, the installation location always have some shade from a large tree

Apogee SP-110
https://www.wunderground.com/weatherstat...ILISBOAA11

[AllyCat]

Hi,

Measurement of low light levels is probably more significant to us here in the UK than in Portugal or Australia.  

Presumably the source resistance (18 ohms) is built into the sensor and I would be the last to suggest that you should "hack" a precision, calibrated sensor (even if 15 years old).  So the options are a little limited:

The AD822 Op Amp certainly has a "better" Input Offset Voltage than many "ordinary" Op-Amps, but even the "nominal" tolerance of 100 uV represents around 1% (or 10 W/m2) of the Didcot's maximum (UK) level and the "worst case" is 800 uV, or 80 W/m2!  The maximum (noon) level in mid-Winter down here is less than 200 W/m2 (onto a horizontal surface) and I guess may be approaching 100 W/m2 in Scotland?

There are Op-Amps with lower Input Offset tolerances (probably using bipolar rather than JFET inputs, but then you'd need to look at the input current specifications) and chips with additional "input offset balance" pins, but on the KISS principle I suggest the following:

Add a potentiometer that can "sit up" the voltage across the sensor (i.e. on the  Op-Amp input pin) by up to 1.8 mV.  That needs 0.1 mA into 18 ohms, or a 47k resistor to the wiper of a pot (any value up to say 10k) wired across the 5 volt rail to Earth.  1 mV (nominal) will then increase the Op-Amp Output Voltage (to the ADC) to around 55 mV, which also overcomes another potential issue that a "rail to rail" Op-Amp will normally not be able to pull accurately all the way down to true zero (milli-) Volts.

Then you will need to modify the software to subtract about 55 mV (I make that a value of 50 with a 10-bit ADC and 1.1 volt reference) and finally adjust this new "balance" pot so that "zero" light level is just reached when the sensor is in darkness.

Cheers,  Alan.

[JT118]

Thank you for all this, I shall have a go. I am also going to try my Apogee SQ110 PAR sensor as per comms with Werk.
http://www.apogeeinstruments.co.uk/origi...um-sensor/
To try and create a complete Agrimet station. Might need to move to Weather Display to display this.
http://www.weather-display.com/index.php