mass difference with time?

Prakash

Dear CFD Users I am solving mass flow split in closed channels. Certain mass flow is entering into partitioned spaces and mass split and pass into different spaces. Fluid is Gas. I would like to know how to calculate mass difference in a space with time. Means, at time step n-1, the mass in the space is "m1", at current time step "n" certain mass enter into a space, then total mass of spce get increases say "m2" after current step. I want to calculate(and monitoring) mass difference "m1-m2" with time. Anybody know how to calculate, your suggestions are appreciable

Thanks in advance