time derivative of average temperature
 

Hello,
I'm still rather new to OpenFOAM and still far from really understanding it. I ran my case and it works fine. Now I want to calculate the time derivative of the average temperature of my volume.
I can calculated the average temperature using foamCalcEx average T, but I haven't found out how to get the time derivative of the average. Can anyone please point me in the right direction?
Thank you