More is better if one is worried about preparation for the winter.
The number today: 112. At the link, scroll down to see the graph.
See "NG_Fill_Rate" at bottom of blog for past posts on this subject.
This link probably explains the most.
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Pending Patents
I believe Apple has a patent pending on a little application for the new AppleWatch that would measure CO2 PPM instantaneously. And how much CO2 the runner wearing the watch is contributing to the atmosphere.
