To calculate the change in temperature, subtract the initial temperature from the final temperature. Example: 84.2 degree Fahrenheit - 41 degrees Fahrenheit = 43.2 degrees Fahrenheit. The temperature from morning to afternoon changed by 43.2 degrees Fahrenheit.
It needs 12 volts so it will be the standard modern power supply provides 12 volts of power.
Traditional current transformers provide a current output to the meter. The CT is rated for both its primary current and its secondary current, the primary being the level of current flowing in the circuit it is to measure, the secondary being the current flowing in the output circuit (the one connected to the meter) An alternative 21st century approach is the 'voltage output' CT.