What is mean by PBM in mass transfer?

It is the log mean value of the inert B in counterdiffusion usually used to find a flux.

P = pA1 + pB1 = pA2 + pB2

thus,

pB1 = P - pA1, and pB2 = P - pA2, so:

pBM = [pB2 - pB1]/ln(pB2/pB1) = [pA1 - pA2]/ln[(P-pA2)/(P-pA1)]

happy engineering :)