Particle Counter: A device for measuring the number and size distribution of particles in a fluid.

Pascal: The SI standard for measuring low gas pressures. By definition it is N/m₂ (N = Newtons, m²=square meters). 1 in. w.g.=248.8 Pa.

Passive Electrostatic Filter: See Electrostatic Filter (passive).

Penetration: A measure, in percent, of the material passing through a filter. Mathematically penetration is 100 - Efficency (percent). If a filter is 98% efficient, its penetration is 2% (100 - 98). Penetration is used to measure the performance of very high efficiency filters.

Photometer: A device which measures the mass concentration of an aerosol by the amount of light the aerosol scatters.

Pilot Tube: A device used to measure the velocity pressure of an airstream by simulataneously measuring its static and total pressures. Velocity pressure is the total pressure less the static pressure. Velocity of air at standard conditions can be calculated by using the formula V (fpm)=4005 VP (in w.g.) where: V = velocity of air and VP = velocity pressure from pilot tube readings.

Plenum Chamber: An air compartment maintained under positive or negative pressure and connected to one or more distributing ducts.

Polar Gases: Gases with unsymmetrical charges. One part of the molecule has more electrons than the rest.

Potassium permanganate (KMnO₄): An oxidizing agent impregnated on activated alumina. See Chemisorbtion.

PPI: Pores per inch. A measurement of the porosity of foam materials.

PPM: Parts per million.

Pressure Drop: The resistance of a device to the flow of a fluid through it. The pressure drop of a filter is a measure of its resistance to air flow through it. Resistance is measured in inches w.g. in the Inch-Pound system of measurement (1" w.g. = 0.036 psi). It is measured in Pascals in the SI system.

PSL: A suspension of polystyrene latex spheres all of which are uniform size. They are used to calibrate some test instruments or used as a challange agent in some filter tests. They are available in a variety of micro-sizes.

ABCDEFGHIJKLMNOPQRSTUVWXYZ