Download Model 8670 SureFlow

Appendix B
Theory of Operation
Introduction to
SUREFLOW
The Model 8670 SUREFLOW Room Pressure Controller continuously controls room
pressure by measuring the velocity of the air flow between two rooms. The room
pressure in inches of water is calculated from the velocity reading as follows:
Pressure = 0.62 x 10-7 [(V/C)2/D]
V = measured velocity in ft/min
C = the discharge coefficient used to correct
Bernoulli's equation for additional energy loss
due to friction between the air and the sensor.
D = density correction factor that takes into
account the current elevation setting.
Normal/Caution Indication
The SUREFLOW features a yellow caution light and a green normal light for quick and
easy identification of the controller status and laboratory safety.
Normal Light Operation
If the green Normal light is on, the SUREFLOW is operating at the programmed control
setpoint and the room should be safe to use. If the yellow Caution light is also on, then
the controller is operating at the remote setpoint. If both the Alarm and Normal lights
are on together, the controller is operating at setpoint, but a previous alarm condition is
latched. Exactly which alarm occurred is indicated by a status message on the top line
of the display. When the Normal light is on, pressing the RESET key will turn off the
Alarm light and clear the alarm message from the display.
Caution Light Operation
The yellow Caution light is used to indicate several different caution conditions to the
user:
Remote Setpoint--If the Caution light is accompanied by a REMOTE SETP status
message on the system display, then the controller is operating at the remote setpoint.
Steady-State Caution--This occurs when the controller is unable to eliminate a steadystate error between the measured pressure and the control setpoint within 20 seconds.
This could happen for a variety of reasons. Please see Chapter 11, "Troubleshooting,"
to determine the cause of the steady-state caution.
Alarms
The SUREFLOW controller constantly compares the measured room pressure to the
programmed alarm setpoints and indicates the current condition via an Alarm light and
displayed message, an audible alarm, and alarm contacts.
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