Campbell CR23X Dokumentacja Pobierz pdf (Strona 208)

SECTION 13. CR23X MEASUREMENTS

13-8

, the source resistance, is not constant

because R

varies from 0 to 10 kohms over the

0 to 360 degree wind direction range. The

source resistance is given by:

= R

+(R

-R

)/(R

)) =

+(R

(20k-R

)/20k) [13.3-12]

Note that at 360 degrees, R

is at a maximum

of 6k (R

=10k) and at 0 degrees, R

is 1k

=0). It follows that settling errors are less at

lower direction values.

The value of R

for any direction D (degrees) is

given by:

(kohms) = (10k)(D)/360 [13.3-13]

Equation 13.3-6 can be rewritten to yield the

settling error of a rising signal directly in units of

degrees.

Error (degrees) = De

-t/(R

L))

[13.3-14]

Equation 13.3-12, -13 and -14 can be combined

to estimate the error directly in degrees at

various directions and lead lengths (Table 13.3-

3). Constants used in the calculations are given

below:

= 3.3nfd

= 41 pfd/ft., p/n 9721 wire

t = 450µs

Example calculation at 360° with 1000 ft lead:

(all variables converted to ohms, farads,

seconds, or feet)

error =

360° *

fd fd ft ft))))

(/((.(-45 0* * * * + * *

-6 -9 -1 2 -1

10 610 3310 4110 1000

s Ω

error = 66°

TABLE 13.3-3. Settling Error, in Degrees, for

024A Wind Direction Sensor vs. Lead Length

Wind Error

Direction L=1000 ft. L=500 ft.

360° 66° 15°

270° 45° 9°

180° 21° 3°

90° 4° 0°

The values in Table 13.3-3 show that significant

error occurs at large direction values for leads

in excess of 500 feet. Instruction 4, Excite,

Delay, and Measure, should be used to

eliminate errors in these types of situations.

Using a 10 ms delay, settling errors are

eliminated up to lengths that exceed the drive

capability of the excitation channel (~ 2000 ft.).

13.3.3 TRANSIENTS INDUCED BY SWITCHED

EXCITATION

Figure 13.3-6 shows a typical half bridge

resistive sensor, such as Campbell Scientific's

Model 107 Temperature Probe, connected to

the CR23X. The lead wire is a single-shielded

pair, used for conducting the excitation (V

) and

signal (V

) voltages. When V

is switched on, a

transient is capacitively induced in V

, the signal

voltage. If the peak transient level, V

, is less

than the true signal, V

, the transient has no

effect on the measurement. If V

is greater

than V

, it must settle to the correct signal

voltage to avoid errors.

TABLE 13.3-4. Measured Peak Excitation Transients for 1000 Foot Lengths of Lead Wires Used

by Campbell Scientific

-----------------------V

(mV) -----------------------

Vx(mV) R

=1 kohm R

=10 kohm

### # ##

9661 7359 9721 9661 7359 9721

2000

50 100 60 100 140 80

1000

25 65 40 60 90 40

1 2 ... 203 204 205 206 207 208 209 210 211 212 213 ... 373 374

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