The datalogger measures the supply voltage and amperage. The voltage is calibrated during manufacturing.
If the displayed value is different from your voltmeter it can be recalibrated on the PDA by touching on the number on the screen to present a dialog to enter the correct value.
The calibration can also be entered using the command line interface by typing "AV" followed by the correct value in millivolts. For example, if the voltmeter says 12.5 volts, enter the command "AV 12500".