according to the datasheet, VMA328’s analog output is in range 0V-1V which corresponds approximatelly to UV index 0 to 10.
This summer, while official measurement stations during a clear sunny day report expectedly UV index levels of 6 or 7, my measurements under the open sky hardly go up to 1.
I finally identified the source of the problem: sensor low voltage output levels. My “boilerplate” test consists only of the sensor supplied with +5V and measuring the output voltage with a digital voltmeter (so, open circuit conditions). No matter if I point the photodiode toward the sun or toward zenith, I measure in a clear sunny day up to about 100+ mV only (according to the datasheet, for UV index 6 I should read about 700 mV).
I have three VMA328 sensors (I thought the first one was actually faulty ), but their low output voltage is quite consistent.
So, my questions are: do measure UV index correctly by positioning the sensor horizontally (so the photodiode points toward zenith? Or, do I need some kind of half-spherical “concentator” that would collect/integrate UV radiation arriving from all directions? How to explain these low output voltages (assuming the built-in op-amp amplification is correct)?
This UV sensor is for educational purposes. Also referred as “experiment” on our product page:https://www.vellemanformakers.com/product/uv-light-sensor-guva-s12sd-module-vma328/
The sensitivity of this sensor is very high, and difficult to measure with a multimeter with a low response speed (counts). This is because the output is between 0 - 1V.
To obtain better measurement results, it is better to measure the output of the sensor with a scope.
A second fact, to obtain more reliable measurement results, is to place the sensor under a UV-A lamp (or LED light). Since the sensor is most sensitive to UVA light according to the accompanying datasheet of sensor module GUVA-S12SD. https://www.velleman.eu/downloads/29/infosheets/guvas12sd_datasheet.pdf
But if you still have doubts about your UV sensor (possibly a defect), You can open a ticket here for a replacement here: https://customercare.velleman.eu/.
), but their low output voltage is quite consistent.