I came across this thread looking for a way to have an alarm to let me know my ATO reservoir is low by triggering an alarm only when it's been dry for 2 hours.
I figured anyone else looking to do something similar might also end up here, so I figured I would put the solution here.
Create a generic device with no output, for input set the the water level sensor in your return chamber that you use for ATO. Make the device on if dry, turn on advanced and give a max on time of your desired time, in my case 2 hours.
Set the appropriate alert level.
This will alert me when the ato reservoir is empty, or if the ato is malfunctioning.
I did it this way because I currently have no sensor in my reservior, and want a more noticeable alert than my ato max on time, which never triggers do to logic I have on that drive port.