The Rino 530/520 series contains a sensor in the lithium ion battery that allows the outside temperature to be displayed when the temperature data field is chosen to show.

Two reasons the temperature may not display in the temperature data field are:

When using the alkaline battery pack instead of the lithium ion battery pack that comes with the device. The alkaline battery pack does not contain a temperature sensor, therefore, the device will not have the necessary information to display this reading.
When connected to external power (device is connected to a computer or a cigarette lighter adapter outlet) the device is no longer using the lithium battery pack. Therefore, bypassing any information it once gathered from the sensor contained in the battery.

If using the lithium ion battery pack supplied with the device or purchased as an additional accessory and the device is not connected to external power, the temperature value will display.