Reynolds and Branson autopermeameter

This is another variation of the constant volume apparatus in which air is pumped into the iidet side to unbalance a mercury manometer. The taps are then closed and air flows through the packed bed to atmosphere. On rebalancing, the mercury contacts start-stop probes attached to as timing device. The pressure difference (Ap) between these probes and the mean pressure p are instrument constants. The flowrate is given by  

Substituting this in the Caiman-Kozeny equation yields a similar equation to the Rigden equation. 

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