Volume monitors - Mass or Volume average
 

Hi,

I make in-cylinder 3D simulation in Fluent. I want to set in program, writing of average pressure in cylinder (monitors/volume monitor). In report type there are volume average and mass average (static pressure). Which should I choose and why? What is the difference.
How does this averaging work...I mean does anybody have particular pattern how Fluent make averaging (in my case of pressure).

thanks

Matthew