Operating Instructions

Controlling Laboratory Temperature
Siemens Building Technologies, Inc. 4-7
In the second scenario, the measured supply flow exceeds TEMP CTL VOL. This means that
the fume hoods are so far open that the supply flow needed to balance the room pressure is
more than the flow needed for cooling. Typically, this is not a stable condition and the
controller moves to the mixing mode. In the mixing mode, TEMP CTL VOL goes to the supply
minimum. The controller regulates the room temperature by modulating the mix of cold and
hot supply air. As the need for heat increases, the mix moves toward heat, until MIX DMPR
CMD reaches 100%. At that point, the controller shifts into a heating mode. Notice that TEMP
CTL VOL rises above the minimum. If the need for heat is great enough, TEMP CTL VOL
increases until it exceeds the quantity needed to balance the fume hoods. When this
happens, then the supply volume tracks the TEMP CTL VOL and the room gets more hot air
to regulate the temperature.
Table 4-2 describes the points discussed above.
Table 4-2.
Descriptor Number Function
ROOM TEMP 4 Value read from the room temperature sensor.
ROOM STPT 13 Desired value of the room temperature.
MIX DMPR CMD 49 Position of the mixing damper. 100% means hot, 0% means cold.
MIX AO1 48 Voltage that drives the mixing damper actuator through a pneumatic
transducer.
TEMP CTL VOL 9 Amount of supply flow that the controller calculates is necessary to regulate
the room temperature.
SUP AIR VOL 35 Measured value of the airflow through the supply terminal.
TEMP LOOPOUT 79 Value calculated by the room temperature PID algorithm. It indicates the
thermal load on the room.
ROOM P GAIN 63 Proportional feedback gain used to tune the room temperature control.
ROOM I GAIN 64 Integral feedback gain used to tune the room temperature control.
ROOM D GAIN 65 Derivative feedback gain used to tune the room temperature control.
ROOM BIAS 66
Starting point where the value of TEMP LOOPOUT goes when the controller
turns on.
LOOP TIME 98
Time interval controls how often the LRC measures the room temperature and
calculates a new output value.