Vaisala WS425 - Manual (Page 71)

Chapter 5 ______________________________________________________________ Maintenance
VAISALA _______________________________________________________________________ 71
Periodic Testing
Section Measuring Principle on page 15 explains that the sensor
measures how long it takes for an ultrasonic signal to travel from
transmitter to receiver. Therefore, the accuracy of the sensor depends on
the accuracy of two factors:
- The distance between the ultrasonic transmitter and receiver. This
requires a measurement of the transducer arm trueness.
- The time-of-flight measurement circuit, which uses a crystal oscillator
for its time reference.
NOTE The crystal oscillator is used by the communications circuit for the bit
rate generator. If you use the serial communication modes and the
oscillator loses accuracy, the sensor stops communicating. Thus, the
sensor cannot send erroneous values produced by faulty timing.
Perform periodic testing to detect slow deterioration of the sensor before
it significantly affects accuracy. Perform the test either in the field or in a
laboratory. The periodic test uses the verifier, which is a small echo-free
chamber . The verifier is shown in Figure 14 on page 72.
The test consists of the following steps:
1. Slip the verifier over the three transducers (see Figure 14 on page
72).
2. In outdoor conditions, secure the zero wind flow by covering the
sensor and verifier.
3. The sensor must read less than 0.5 miles per hour
(0.22 m/s) with the verifier in place.
NOTE Some random data samples may be lost during the zero verifier test.
This, however, does not indicate that the sensor is faulty.