Ambient Weather says that the WS-2902A and WS-2000 are good for RH range of 10% to 99% with +-5% accuracy. That doesn't mean that it wont register or never go below 10% RH. It could mean that if it does go below 10% RH that it will have a higher margin of error than +-5%. How much will that error be? We don't know as it isn't published.
But we know that Ambient is using the SHT30 chip and that chip has the following specifications from Sensirion (the manufacture of the chip)
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SHT30 vs. SHT31 Relative Humidity
That doesn't look as bad as what Ambient claims. It actually looks much much better than Ambient's specs at +-2% for almost the entire range like the SHT31 except for the extremes <10% and >90%. Ambient is practicing under-promise / over-deliver. I can tell you from my experience on the high end I'm getting near perfect results above 90% RH. It is true though that after some time (After more than 18 month, gathered from other discussions) that maybe the performance degrades. It is then possible that Ambient has under promised to continue to meet their sensor specifications after the unit has had some use....and this could reduce their warranty claims and the unit would still be performing over spec within the warranty period. Companies often underrate specs if the specifications are known to drift in order to reduce warranty costs.
It is also possible though that Ambient has in software limited the normal performance of the SHT30 and limited it to 10% to 99%. For example I never see anything above 99%. But that isn't bad considering brand new Davis VP2 only see 99% or 100% for a few weeks and then they drop to a max of 94% to 96% and they have SHT31. So I don't know if the Ambient station will report anything below 10% or not. This needs to be tested. Maybe we can find an existing WS-2902A in AZ and see what it reports when below 10% RH. By the way in my Meteobridge reporting to CWOP I have it set to bump 99% RH to 100%. This can be done in other weather software too.
Since the WS-2902A and the WS-2000 use the SHT30, I don't see why you couldn't upgrade it to use the SHT31 or even maybe the SHT35. This hasn't been done yet but if you feel like getting all the fame and glory and are into hacking hardware then we need someone to test it in your extreme sub 10% RH environment.
Another interesting thread to read:
https://www.wxforum.net/index.php?topic=34873.0