The PEM operates on the principal of inertial separation of airborne particles using an impactor. Particle-laden air is accelerated into the sampler through the round nozzles located in a circle around the outer edge of the cover. The exiting airstreams impinge upon an impaction ring. Due to inertia, the larger particles cross the airstreams, impact, and are retained on the ring. The smaller particles are carried along the airstreams flowing around the ring and are collected on the after-filter. Oil must be applied to the impaction ring to keep particles from bouncing off this surface. Any type of oil can be used including olive oil, machine oil, vegetable oil, and silicone grease.