Vaisala WMT700 - Manual (Page 41)

Chapter 4 _______________________________ Operating WMT700 in WS425 Analog Output Mode
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Missing Readings and Error Indication
If WMT700 is not able to measure the wind, it indicates a missing
reading in the output. Most common reasons for measurement problems
are foreign objects (such as ice, birds, or other foreign object) on the line
of measurement or sound reflections from nearby objects (such as wind
tunnel walls).
The default error indication is an out-of-range signal that is more than
10 V or 20 mA but other error settings can also be configured. You can
also disable error indication if necessary.
For more details, see Vaisala WINDCAP ® Ultrasonic Wind Sensor
Series WMT700 User's Guide.