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Reset authority, also called "reset ratio", is the ratio of the
effect of the reset sensor compared to the primary sensor.
Figure 23 shows the effect of authority on a typical reset
schedule. The authority can be set from 10 to 300 percent.
55
0
-20
OUTDOOR AIR TEMPERATURE ( C)
Fig. 23. Typical Reset Schedule for
Discharge Air Control.
The integral control reset time determines how quickly the
PI controller responds to a change in the controlled variable.
Proportional correction occurs as soon as the controlled
variable changes. The integral function is timed with the reset
time adjustment. The reset time adjustment is calibrated from
30 seconds to 20 minutes. The proper setting depends on system
response time characteristics.
PNEUMATIC SENSORS
Pneumatic sensors typically provide a direct acting 20 to 100 kPa
pneumatic output signal that is proportional to the measured
variable. Any change in the measured variable is reflected as a
change in the sensor output. Commonly sensed variables are
temperature, humidity, and differential pressure. The sensors use
the same sensing elements and principles as the sensors in the
controllers described earlier, but do not include setpoint and
throttling range adjustments. Their throttling range is the same as
their span.
A gage connected to the sensor output can be used to indicate
the temperature, humidity, or pressure being sensed. The gage
scale is calibrated to the sensor span.
Temperature sensors may be vapor-filled, liquid-filled,
averaging capillary, or rod-and-tube. The controller usually
provides restricted air to the sensor.
Humidity sensors measure the relative humidity of the air in
a room (wall-mounted element) or a duct (insertion element).
Nylon is typically used as the sensing element. Humidity sensors
include temperature compensation and operate on a force-
balance principle similar to a wall thermostat.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
The low-pressure sensor measures duct static pressure and
differential pressure. When the duct static pressure or the
pressure differential increases, branchline pressure increases.
VELOCITY SENSOR-CONTROLLER
The velocity sensor-controller combines a highly sensitive
air velocity sensor with a pneumatic controller to detect and
control airflow regardless of system static pressure. It is used
in air terminal units and other air handling systems. Reverse-
and direct-acting models are available for normally closed and
15
normally open dampers.
COMPENSATION
START POINT
The velocity sensor measures actual velocity and does not
C4221
require the conversion of velocity pressure to velocity. Although
the sensor is typically used in duct air velocity applications, it
can accurately sense velocities as low as 5 meters per second.
Flow-limiting orifices inserted into the sensor sampling tube
can measure velocity ranges up to 17.8 meters per second.
Figure 24 shows the operation of a velocity sensor. A restrictor
supplies compressed air to the emitter tube located in the air
stream to be measured. When no air is flowing in the duct, the
jet of air from the emitter tube impinges directly on the collector
tube and maximum pressure is sensed. Air flowing in the duct
blows the air jet downstream and reduces the pressure on the
collector tube. As the duct air velocity increases, less and less
of the jet enters the collector tube. The collector tube is
connected to a pressure amplifier to produce a usable output
pressure and provide direct or reverse action.
A controller connected to the pressure amplifier includes
setpoints for maximum and minimum dual air velocity limits.
This allows the air volume to be controlled between the limits
by a thermostat or another controller.
Two models of the controller are available. One model
operates with a one-pipe, bleed-type thermostat, and the other
with a two-pipe thermostat. The two-pipe model also allows
sequencing for reheat applications.
73
PNEUMATIC CONTROL FUNDAMENTALS
M
EMITTER TUBE
AIR FLOW
GAP
COLLECTOR
TUBE
TO PRESSURE
AMPLIFIER
Fig. 24. Velocity Sensor Operation.
C2610
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