I am a tech diver. I will go through Trimix in 2016. I run the petrel on planned deco, as well as "I shot a big damn aj, unplanned deco." I use lithium double AA batteries, and they last quite some time.
You would have to read the operators manual as far as all of the "on the fly, or in dive" functions that you have control over. But to sum it up, most computers are set at the surface, and allow you to scroll through screens "in dive," and that is it. Petrel will allow certain settings to be changed "in dive."
One other great thing about it that I like is, it won't lock out on you. It will always calculate and give you answers. You may not like what it tells you, but it won't lock up and refuse to do its job for "safety" reasons.
If you've got the feelin', then jump across the ceilin'!