Vaisala MMT330 - Manual (Page 121)

Chapter 4 _________________________________________________________________ Operation
VAISALA _______________________________________________________________________ 121
Hysteresis
Hysteresis function is to prevent the relay switching back and forth when
the measured value is near to the setpoint values.
Relay is activated when the measured value passes the exact value of the
setpoint. When returning and passing the setpoint again relay is not
released before the value reaches the setpoint increased/decreased by the
hysteresis value.
Hysteresis should be smaller than difference of the setpoints.
Example: When the 'active above' value aw is 0.6 and the hysteresis
value is 0.05, relay activates when the aw reaches 0.60. As the humidity
decreases, relay releases at 0.55.
NOTE If both setpoints are specified and "above" setpoint is lower than
"below" setpoint, the hysteresis works in the opposite direction, that is,
relay is released when the measured value passes the exact value of the
setpoint.
Relay Indicating Transmitter Error Status
You can set a relay to follow the operation status of the device. By
selecting FAULT/ONLINE STATUS for output quantity a relay changes
state on the basis of the operation status as follows:
FAULT STATUS
Normal operation: relay active (C and NO outputs are closed)
Not measuring state (error state or power off): relay released (C and NC
outputs are closed)
ONLINE STATUS
Live measurement (data available): relay active (C and NO outputs are
closed)
No live data (for example: error state, chemical purge or adjustment
mode): relay released (C and NC outputs are closed)
See Figure 60 on page 122 for illustrative examples of the
FAULT/ONLINE STATUSrelay output modes.