(n.) Drilling Fluids. A specialized apparatus used in the particle-plugging test. The PPA is used to determine the ability of particles in the drilling fluid to effectively bridge the pores in the filter medium and, therefore, the ability of the mud to reduce formation damage in the reservoir. The apparatus resembles a high-pressure, high-temperature filtration cell that has been modified to operate upside down (to remove the effects of gravity) and to accept filter media of different permeabilities (sintered metal, which is chosen for higher temperature conditions, aloxite, which is a porous ceramic material, or rock). The medium is selected to match the permeability of the reservoir to be drilled. The filter medium is at the top so that sediment will not affect the filtercake. Pressure is applied hydraulically from below. Alt. form: particle-plugging apparatus. See: Sintered