Vaisala WS425FG - Manual (Page 65)

Chapter 5 _________________________________________________________ Operating Principle
VAISALA_______________________________________________________________________ 63
Verifier Associated Operations
The sensor measures elapsed time for sound to travel from one
transducer to the other. The only two parameters that affect the accuracy
of the sensor are time and distance. The distance that affects the
accuracy of the sensor is the distance between transducers. This
distance is mechanically validated by the verifier. The verifier has to
slide over the sensor without force. The zero-point calibration is
ensured if the wind speed is 0.4 knot or less with the WA command.
Together with the verifier the reduced acoustic signal can be applied to
emulate severe conditions. The reduced signal level is generated with
the combination of the sensor firmware and hardware. This tests any
deterioration in the following areas:
- Transmitter output power
- Transducers (transmitting and receiving)
- Receiver sensitivity
- Acoustic crosstalk
- Electrical crosstalk
- Automatic gain threshold detector
The timing reference for the time of flight measurement is controlled by
a quartz crystal. The same crystal controls the baud rate for serial
communications. If the crystal drifts more than a few percent, the sensor
will not communicate any more.
This test gives a very high confidence that the sensor will work over its
measurement range.