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)?