The Kestrel 4000 series can log data while the unit is turned off.  This is a powerful feature for those needing data logged over time while preserving battery life.  

Setting it up:

Turn the unit on, hit the power button again, scroll down 'memory options', and all the functions are in the easy menu.  Be sure the auto log is set to 'on'...

Choosing store rate:

Interval store rate options: 2s, 5s, 10s, 20s, 30s, 1m, 2m, 5m, 10m, 20m, 30m, 1hr, 2hr, 6hr, & 12hr.
With the Auto Log on, and the meter off, the only interval it won't work in is 2-5 second intervals.  The reason being, the time the unit takes to turn the functions on, take a reading and turn off, is about 5 seconds, so it won't work.  All other interval data reading options will function. 
To see the options, hit power button to menu, then memory options...

How does it work/How long will it continue to log data?

Basically, the unit will go to sleep, it wakes up... takes reading then goes back to sleep.  The screen will NOT power up, its all done within the unit itself.  The intentional reason behind not having the screen turn on is to save battery life.  While it hasn't 'officially' been tested for the life a battery, power out calculations estimate that a Kestrel meter with a full battery could take a log at 1 hour intervals for up to 1 year!  

How many data logs can be stored?

It varies from meter to meter, but here's some stats.
Kestrel 4000: 4000 Data points
Kestrel 4500: 2900 Data points
Kestrel 4200 & 4250:  3200 Data points
Kestrel 4300: 3600 Data points
Kestrel 4400:  roughly... 2000+ Data points (has not been fully tested as of writing this)

The system as an over ride feature that will loop around and over ride the oldest data log.

How do you view it?

When the data has been entered, you can pick up your meter and view within the meter, see the graph for trends, or see exact time of each logged data point... simply press center button see when each was taken.  The other option is to upload the log to your computer for viewing.