Bin -Bottoms

i.e. the stagnant 10%-15% of material the never seems to discharge.

In this example a cement silo was discharging into transfer pods for dense-phase transfer.  The plant routinely recorded the back-pressure from these pods as a measure of their transfer efficiency (basically, the higher the pressure could be maintained from each 2 min blow-down cycle the more cement was being transfered.

Inventory transfer rates are evidenced by the pressure logs. As shown below prior to the PMT trial had slowed to a trickle and essentially stopped discharging. This is “empty condition” is evidenced by looking at the Plant’s back-pressure graph below, where the pressure mostly drags along the 1 psi baseline.
As seen on the next graph  --  Multiple attempts on subsequent days confirm the silo was no discharging the bin-bottoms.

See below  --  What a difference PMT makes !
with PMT

Good pumping rates are indicated, by equipment operating in the range of 2.5 to 5 psi. 
Baseline pressures typically from equipment changeovers or high-level signals from down stream silos.