Vaisala PTU300 - Manual (Page 135)

Chapter 4 _________________________________________________________________ Operation
VAISALA _______________________________________________________________________ 133
1102-007
Figure 60 Relay Output Modes
Mode 4 is usually used if an alarm needs to be triggered when the
measured value exceeds a safe range. The relay is active when
measurement is in range, and is released if the value goes out of range or
the measurement fails.
NOTE If the measurement of the selected quantity fails or the transmitter loses
its power, the relay is released.