AA (arithmetic average) A numerical measure of metal surface roughness.
ABS Acrylonitrile butadiene styrene.
absorption The soaking up of a gas or liquid into a solid substance.
ac Alternating current.
acfm Actual cubic feet per minute for compressed or vacuum air.
actuator A movable component of a valve that when operated, causes the closure element
to open or close.
adsorption The condensation of gas or liquid onto the surface of a solid.
AEC Architect, engineer, and constructor.
aerobic Bacteria requiring free oxygen for their growth.
aerosol Particles, solid or liquid, suspended in air.
In compressed-air service, the aftercooler is a device used to lower the temper-
ature of the compressed air immediately after the compression process.
AI Aggressiveness index (a guideline parameter to find the corrosive tendency of potable
air gap An unobstructed separation between a source of potable water and any source of
AISI American Iron and Steel Institute.
anaerobic Bacteria living in the absence of free oxygen, deriving it instead from breaking
down complex substances. Bacteria requiring no free oxygen for their growth.
anchor A pipe support that restrains a pipe against all movement.
anion A negatively charged atom attracted to an anode electrode.
ANSI American National Standards Institute.
API American Petroleum Institute.
approach A term, expressed in degrees Fahrenheit (F), used for fluid system dryers to
indicate how close the outlet temperature of the water being heated comes to, or approaches
the fluid heating medium to the cooling medium.
approved Accepted for the intended purpose, as an appropriate design or for installation
into a piping system, by a responsible code official or other agency having jurisdiction for a
APTE American pipe thread external.
APTI American pipe thread internal.
aquifer A water-bearing formation or stratum capable of storing or transmitting water in
sufficient quantities t