Campbell FW05 Manuel télécharger pdf (Page 63)

C-1

Appendix C. CSAT3 Measurement

Theory

C.1 Theory of Operation

C.1.1 Wind Speed

Each axis of the CSAT3 pulses two ultrasonic signals in opposite directions.

The time of flight of the first signal (out) is given by:

(1)

and the time of flight of the second signal (back) is given by:

(2)

where t

is the time of flight out along the transducer axis, t

is the time of

flight back, in the opposite direction, u

is the wind speed along the transducer

axis, d is the distance between the transducers, and c is the speed of sound.

The wind speed, u

, along any axis can be found by inverting the above

relationships, then subtracting Eq. (2) from (1) and solving for u

=−

⎡

⎣

⎢

⎤

⎦

⎥

(3)

The wind speed is measured on all three non-orthogonal axis to give u

, u

, and

, where the subscripts a, b, and c refer to the non-orthogonal sonic axis.

The non-orthogonal wind speed components are then transformed into

orthogonal wind speed components, u

, u

, and u

with the following:

⎡

⎣

⎢

⎤

⎦

⎥

⎡

⎣

⎢

⎤

⎦

⎥

(4)

where A is a 3 x 3 coordinate transformation matrix, that is unique for each

CSAT3 and is stored in ROM memory.

C.1.2 Temperature

The sonically determined speed of sound can be found from the sum of the

inverses of Eq. (1) and (2). The CSAT3 corrects online for the effect of wind

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Campbell FW05 Manuel Page 63

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