N - No Entry
O-Probe - A method used to test HEPA and ULPA filters for leaks utilizing either an aerosol photometer or discrete-particle counter. A sampling probe is moved in a series of parallel, slightly overlapping strokes across the test area at a distance of approximately 2.5cm (1 inch) from the filter face.
Outgassing - The passive liberation of a gas from any material.
Particle - An object that is solid, liquid or both, usually between 1 nanometer and 1 millimeter in size.
Particle Concentration - The number of particles per unit volume of air.
Particle Count - The number of particles detected (or reported) in a given volume of air.
Particle Counter, Airborne - An instrument for continuous counting of airborne particles larger than a given threshold size. The sensing means may be optical, electrical, aerodynamic, etc.
Particle Counter, Optical - A light scattering instrument with display and/or recording means to count and size discrete particles in air.
Particle Size - The apparent maximum linear dimension of a particle in the plane of observation, as observed with an optical microscope, or the equivalent diameter of a particle detected by automatic instrumentation. The equivalent diameter is the diameter of a reference sphere having known properties and producing the same response in the sensing instrument as the particle being measured.
Particulate - An adjective referring to particles, e.g., particulate matter.
Penetration - The exit concentration of a given particulate or gas from an air cleaning device, expressed as percentage of inlet concentration, i.e. 99.99% efficiency = .010% penetration (100% - 99.99% = .010%).
Photometer - an instrument used to measure light intensity, also see Aerosol Photometer
Plenum - An enclosure in which air or other gas is at a pressure greater than that outside the enclosure.
Prefilter - A filter unit installed to protect a second filter from high dust concentration or other environmental conditions. The prefilter usually has a lower efficiency than the filter it protects.
Pressure Drop (Differential) - Also referred to as resistance, the difference of the air pressure entering the filter and the air pressure exiting the filter at a stated flow and under given conditions. The pressure drop of a filter is a measure of its resistance to airflow through it. Typically expressed in inches of water column (gauge) or in Pascals.
Pressure, Gauge - The amount by which the total absolute pressure exceeds the ambient atmospheric pressure.
Pressure, Total - The sum of static pressure and velocity pressure at the point of measurement.
Pressure, Velocity - The impact pressure associated with the velocity of a flowing fluid. It is related to the kinetic energy per unit volume of the flow.
PSL (Polystyrene Latex Spheres) - Uniformly sized latex particles used as a challenge agent in some filter tests when suspended in an aerosol.
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