Accessory List

Glossary
388
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Sequencing: A control arrangement in which several
actuators move through their stroke in succession as the
signal from the controller changes.
Example: Electric; This arrangement is derived by using
actuators with different voltage ranges (such as 3 to 6 Vdc and
6 to 9 Vdc) or by the use of a sequencing adaptor.
Pneumatic; This arrangement is derived by using actuators
with different spring ranges (such as 3 to 8 psi and 8 to 13 psi)
or by the use of a pneumatic relay.
Setpoint: The desired value assigned to a controller.
Example: The setpoint dial on a thermostat indicates the
desired occupied condition.
Short-Cycling: When equipment is turned on and off at
frequent intervals. Normally associated with two-position
control. (Short-cycling is an undesirable condition).
SPDT (Single-Point, Double-Throw): An electromechanical
switch, which makes one circuit immediately upon breaking
the other.
Spring Return: The movement of an actuator as a result of
a decreasing voltage signal and therefore the force is supplied
by a coiled or compressed spring. Upon a power interruption
the spring will drive the actuator to a known position.
Squirrel Cage Fan: See Centrifugal fan.
Staged Heating/Cooling: A temperature control technique
in which heating or cooling is turned in stages. For example,
the farther away the temperature is from the setpoint, the more
stages of heating or cooling are turned on.
Staging: A method of control in which the total capacity of a
two-position mode of control application is divided into several
levels of capacity so as to match the capacity to the load more
evenly.
Stand-Alone: A device, such as a controller or computer,
that does not require support from another device or system.
Stand-Alone Operation: Performance independent of
direction of any other component in the system.
Start Point (Pneumatic): The pressure necessary to begin
compressing the spring of an actuator, therefore, causing the
actuator to begin its stroke. Example: An actuator with a 5 to
10 psi spring range, “5” is the start point.
Static Pressure Control: The process of regulating the air
pressure inside a duct or a room in relation to a reference
pressure. Usually controlled in terms of in. W. C.
Status: The state of a contact input that indicates the
position of the field device. Examples include: on/off, stop/run,
enable/disable, etc.
Strap-On Thermostat: A Controller designed for mounting
on and sensing the temperature of a surface. Example: the
surface of a pipe.
Stratification: Layers of air at different temperatures of
different velocities flowing through a duct or plenum.
Stroke Length: The linear distance the actuator shaft
moves.
Summer/Winter: A combination of a direct acting and a
reverse acting thermostat. The term heating/cooling is
synonymous.
Supply or Main Pressure (Pneumatic): The force per unit
area (psi) of the compressed air supplied to a controller. It is
usually constant at 15 to 20 psig, but may have some other
value in special cases.
TAC System 8000: This is an analog Electronic Component
Controls.
Thermistor: A semiconductor whose resistance is extremely
temperature sensitive. Like carbon, thermistors have negative
temperature coefficients; that is, their resistance increases as
temperature decreases. They are used to compensate for
temperature variations in other parts of a circuit and are also
used as transducers. Thermistors are not used in TAC System
8000.
Thermostat: An instrument which measures temperature
and controls device(s) for maintaining a desired temperature.
Throttling Range (Controller): Throttling range is the
change in measured variable (temperature, pressure, liquid
level, etc.) required to cause the controller output to vary a
pre-defined range. In TAC System 8000 this range is 6 to 9
volts. In Pneumatics, this range may be 3 to 13 psig or 3 to 15
psig.
Throttling Range (System): The amount of change of the
variable necessary for the controller to drive the actuator(s)
through their complete stroke(s).
Total Pressure: The sum of static and velocity pressure.
Transducer: A device which converts one form of energy
into another form of energy.
Transmitter (Pneumatic): A sensor element located remote
from the controller which bleeds off branch line air to create a
varying pressure signal as input to the controller.
Transmitter (TAC System 8000): A component added to a
control system which allows temperature or relative humidity
indication.
Tubeaxial Fan: An airfoil (propeller) fan within a cylinder and
including driving mechanism supports for belt drive or direct
connection.
Two-Position Control: A method of control in which the
control device is either 100% open or closed; therefore, the
controlled medium is flowing at these respective rates. Also
called On-Off control.
VA: Volt ampere.
VAC: Volts alternating current.
Vaneaxial Fan: An airfoil (propeller) fan within a cylinder and
equipped with air guide vanes before or after the wheel and
including driving mechanism supports for belt drive or direct
connection. Blades may have adjustable or controllable pitch.
Variable Air Volume (VAV): A system that controls space
temperature by varying the quantity of supply air rather than by
varying the temperature of the supply air.