The guys at Thinger.io are very generous, you can feed as much data per device as your device can provide.
You can update as fast as every second, but be aware: time series with more than 1000 points will strain your browser and their server dramatically if updated every second.
“Free” Data buckets may only be updated >= 60 seconds.
You need them, if you want to be able to recall data existing before your started the dashboard instance.
They can only store one year of data on free accounts, which can be a lot, by the way…
Yes, you can send eMails alerts on free accounts.
Regards