Campbell CR10 PROM Instruction Manual download pdf (Page 12)

CSAT3 THREE DIMENSIONAL SONIC ANEMOMETER

TABLE 6-2. Calibration for Analog Outputs

Slope

Low Range

(m s

-1

)

Slope

High Range

(m s

-1

)

Offset

(m s

-1

)

6.5536 13.1072 0

1.6384 1.6384 0

c 6.5536 6.5536 340

When the analog outputs are used, the

anemometer uses its internal timer to trigger the

wind measurements. Thus, the wind

measurements can be skewed in time ±1/2 the

data acquisition system’s scan interval from

other analog measurements, e.g. temperature

or humidity. Recall that the CSAT3 analog

outputs also have a two measurement pipeline

delay.

7. OPERATIONAL PRINCIPLES

The CSAT3 measures wind speeds and the

speed of sound by determining the time of flight

of sound between pairs of transducers, as

described in Appendix C. However, unlike

many other commercial anemometers it does

not use simple threshold detection to determine

the ultrasonic times of flight. Instead, it uses

advanced, digital signal processing techniques

which, in comparison to other systems, results

in more accurate, lower noise measurements.

These techniques also allow the system to

quickly detect and indicate a degradation of

signal quality and system performance.

The CSAT3 signal processing techniques result

in slight differences in operation when

compared to simpler sonic anemometers.

These differences include a start-up delay

before the system is fully functional plus a

requirement for the correct triggering of the

sensor to ensure a constant flow of

synchronized data. The general principles of

operation are described in the following section.

After the CSAT3 transmits an ultrasonic signal

from one of its transducers, it then records the

received signal from the opposing transducer. To

optimize system performance, the CSAT3 only

looks for the received signal during a limited

window in time. The exact start and end time of

the window are determined not only by the spacing

between transducers but also by the current wind

speed and speed of sound conditions.

After power-up or loss of signal, the CSAT3

enters a mode where it tests the forward and

return paths for each of the pairs of ultrasonic

transducers. This is called the "acquire" mode

and takes about 10 seconds before normal

output is available from the anemometer. This

delay is caused in part because the

anemometer has no recent knowledge of the

responses of the transducer pairs, and in

particular no past history of the of wind speed

and speed of sound. During this acquire mode

it has to search across quite wide windows in

time to find the true arrival time of each

received signal.

Once it has acquired the signal it then uses an

tracking algorithm to adjust the start and end

times of its search window based both on the

Execution Parameter (see Section 8) and the

last measured ultrasonic time of flight. The

basis of this adjustment is that the dynamics of

air movement prevent the time of flight

changing by more than a certain amount in a

given time. The result of using this algorithm is

that the width of this window in time can be kept

to a minimum and measurements can be made

at a higher frequency for an overall lower

consumption of power when compared to using

wider fixed windows.

The main operational consequence of this

tracking algorithm is that the Execution

Parameter must be set to match the frequency

at which the anemometer is triggered, because

the Execution Parameter is used as an

important input to the tracking algorithm. The

setting of this parameter and matching it to the

trigger frequency are discussed in the following

section.

8. CSAT3 TRIGGER

The CSAT3 takes a measurement of wind

speed and the speed of sound when triggered

from one of three different sources. The trigger

sources are the internal CSAT3 timer, SDM

communications, or RS-232 communications.

In the case of the CSAT3 internal timer the

Execution Parameter exactly defines the trigger

rate. The Execution Parameter is set either via

an SDM or RS232 command, or using the

Windows based support software. Once set,

this value is stored in non-volatile memory and

is used thereafter. If the CSAT3 is set to use its

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