I would like to use a 9V battery to power the probe. The circuit is actually going to drink quite a bit of current, however. Particularly when it's in 100X mode:

This results in:

5-10 hours operating time should be fine though. Maybe in the future an option for an auxiliary power supply will be added so the probe can be used for very long time scale logging.

There is also a low battery detection circuit:
This circuit triggers a low battery indicator when the 9V battery reaches ~6.5V.