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[Reply #15 on:]

Please tell what you wanted to add.

Thanks.

[Reply #16 on:]

You should still get "5V" even using USB. 
At least, I do with my "2009".
But, +V_usb has a wide range, too, so that could throw your readings off.  Is that it?

[Reply #17 on:]

Here I add some results:

Only USB - Vcc = 5V

Code:

Celsius: 41.79
Kelvin: 314.94

Celsius: 41.79
Kelvin: 314.94

Celsius: 41.79
Kelvin: 314.94

Only USB - Vcc = 3.3V

Code:

Celsius: 23.24
Kelvin: 296.39

Celsius: 23.24
Kelvin: 296.39

Celsius: 23.24
Kelvin: 296.39

With adopter - Vcc = 5V

Code:

Celsius: 23.24
Kelvin: 296.39

Celsius: 23.24
Kelvin: 296.39

Celsius: 23.24
Kelvin: 296.39

With Adopter - Vcc = 3.3V

Code:

Celsius: 3.22
Kelvin: 276.37

Celsius: 3.22
Kelvin: 276.37

Celsius: 3.22
Kelvin: 276.37

Temp reading in case when Arduino was connected through USB only and Vcc = 3.3V is exactly equal to Arduino plugged in with adopter and Vcc = 5 V.
Strange, isn't it?