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Article Index

Absolute Pressure (psia): The sum of both atmospheric pressure (14.7) and gauge pressure (psig). Example: If a pneumatic gauge indicates 8 psig, the absolute pressure will 22.7 psia (8+14.7).

Action: The direction of magnitude change of the output of a controller with respect to the change in the variable that is being sensed. Example: Direct Action (D.A.): Variable increases, output increases. Reverse Action (R.A.): Variable increases, output decreases.

Actuator: A device which is mechanically linked to a damper and positions the damper to regulate the flow of air; or is mounted on a valve and repositions the valve to regulate the flow of steam or water. Actuators are sometimes referred to as operators or motors.

Air Handling Unit (AHU): A mechanical system usually consisting of an enclosure housing a supply-air fan (or fans), heating and/or cooling coils, filters, and outdoor air and return air dampers. May include return air fan(s) and relief air damper(s). May deliver air to a single space, to several zones, or to numerous constant-volume or variable-volume air terminal units.

Analog: A proportional type of signal whose level varies smoothly and continuously in amplitude or frequency.

Averaging Element: A sensing device that can extend across the entire duct and sense the average temperature.

Boiler: A closed vessel in which fuel is burned to generate steam or to heat water.

Branch Lines (Pneumatic): The tubing in a pneumatic control system which carries the output signal from controller to auxiliary devices or actuators.

Btu (British thermal unit): The energy or heat required to raise the temperature of one pound of water 1°F under standard pressure.

Butterfly Valve: A cylindrical flanged-end body with an internal, rotatable disc serving as a fluid flow regulating device.

Chiller: A machine, usually centrifugal or reciprocating, that chills the water used to cool a building. Heat removed from the water is rejected to a remote air-cooled condensing unit, or to a water-cooled condenser that is usually an integral part of the chiller.

Close-Off: The maximum allowable pressure drops to which a valve may be subjected while fully closed.

Comfort Zone: The range of temperatures and humidities over which most people feel comfortable. Generally, between 60°F and 70°F and 20% to 60% relative humidity.

Control Point: The actual value of the controlled variable which the controller operates to maintain (under any fixed set of conditions).

Controlled Device (C/D): An apparatus that receives the signal from a controller and positions the damper or valve to match the capacity to the load. Example: Motorized damper or valve.

Controller: A device that monitors a controlled variable and changes the position of final control devices (such as valves, dampers, or contacts) to maintain the value of the controlled variable at or near the controllers setpoint.

Cubic Feet Per Minute (cfm): A rate of air volume delivery. Standard measure for HVAC ducted systems.

Damper: A valve used to regulate the flow of air or some other gas.

Degree Day, Heating: A unit, based upon temperature and time, used in estimating fuel consumption and specifying nominal heating load of a building in winter. One heating degree day is given for each degree that the daily mean temperature is below 65°F 18°C).

Dew Point: The temperature at which a given mixture of air and water vapor is saturated.

Digital: An On/Off or two-position signal.